top of page
QSL Seminars
26 May 2026
QSL Seminar
Fault-Tolerant Quantum Computers: From Surface Codes to LDPC Architectures
Michael Beverland (IBM)
Fault-tolerant quantum computers will not simply run today’s quantum programs on more reliable hardware. Error correction imposes constraints that propagate across the stack, from hardware and classical control to compilation and program design. I will describe a layered framework for understanding how different error-correcting codes shape the design of fault-tolerant architectures. Comparing concrete surface-code and quantum-LDPC-code architectures, I will explain how quantum LDPC codes can substantially reduce physical-qubit requirements, but can also increase algorithm runtime due to constraints of the fault-tolerant logical instruction set. The resulting comparison points toward a broader theoretical programme: understanding how code structure, fault-tolerant logical operations, compilation, and hardware constraints jointly determine the resources required for scalable quantum computation.
21 May 2026
QSL Seminar
Crises in and out of Physics: the Cold War and Quantum Research
Susannah Glickman ( Stony Brook)
Quantum technology is far from new. In this lecture, Susannah Glickman, Assistant Professor of History at Stony Brook University, traced the field back to its prioritization for funding during the Cold War, exploring how this has shaped the field’s perceived historic and future value.
14 May 2026
QEC Journal Club
Scalable Neural Decoders for Practical Fault-Tolerant Quantum Computation
Andi Gu (Harvard University)
Quantum error correction (QEC) is essential for scalable quantum computing. However, it requires classical decoders that are fast and accurate enough to keep pace with quantum hardware. While quantum low-density parity-check codes have recently emerged as a promising route to efficient fault tolerance, current decoding algorithms do not allow one to realize the full potential of these codes in practical settings. Here, we introduce a convolutional neural network decoder that exploits the geometric structure of QEC codes, and use it to probe a novel “waterfall” regime of error suppression, demonstrating that the logical error rates required for large-scale fault-tolerant algorithms are attainable with modest code sizes at current physical error rates, and with latencies within the real-time budgets of several leading hardware platforms. For example, for the [144,12,12] Gross code, the decoder achieves logical error rates up to ∼17x below existing decoders - reaching logical error rates ∼10^−10 at physical error p=0.1% - with 3-5 orders of magnitude higher throughput. This decoder also produces well-calibrated confidence estimates that can significantly reduce the time overhead of repeat-until-success protocols. Taken together, these results suggest that the space-time costs associated with fault-tolerant quantum computation may be significantly lower than previously anticipated.
7 May 2026
QSL Seminar
What is concurrency?
Priyaa Varshinee Srinivasan (Tallinn University of Technology)
In this talk, we will demonstrate a fundamental difference between parallelism and concurrency with the aid of a functional-style concurrent language called CaMPL. Categorical Message Passing Language, CaMPL in short, implements a type system given by the Cockett-Pastro equivalence (concurrent analogue of the sequential functional Curry-Howard-Lambek equivalence). CaMPL’s type system ensures that processes can never be connected in a cycle. This guarantees that CaMPL programs never deadlock. Additionally, livelock-freedom is guaranteed in the absence of general recursion. In this talk, we will use code examples to showcase the type system of CaMPL which enables parallel computation. Then, to be able to write concurrent programs, one requires controlled non-determinism. Finally, we will discuss how CaMPL’s type system can be extended to quantum realms to describe quantum message passing.
30 Apr 2026
QSL Seminar
Can we coherently control decoherence?
Louis Lemonnier
An introduction to coherent control, followed by an exploration of whether, and how, decoherence fits into this framework.
29 Apr 2026
QEC Journal Club
Generating the logical Clifford group from fold-transversal gates on high-rate codes
Tim Chan (University of Oxford)
Our work focuses on a high-rate self-dual code family called quantum Reed–Muller codes; for such high-rate codes, finding addressable logical operations is challenging. We construct a fold-transversal generating set of the logical Clifford group, thus enabling the implementation of any addressable Clifford without ancilla qubits. To our knowledge, this is the first such construction of the logical Clifford group for a code family whose logical qubit count grows near-linearly in physical qubit count. The accompanying paper is [https://arxiv.org/abs/2602.09788] and the accompanying Python code is available in the PyPI package ‘qrmfold’.
29 Apr 2026
QEC Journal Club
Erasure conversion for singlet-triplet spin qubits enables high-performance shuttling-based quantum error correction
Adam Siegel
No abstract provided.
19 Mar 2026
QSL Seminar
Quantum Data Centres in the Presence of Noise
Kenny Campbell (Heriot-Watt University)
Distributed quantum computing is a promising way of overcoming the scalability challenges of quantum computers, but establishing quantum links over large distances remains difficult. In this talk, we will explore quantum data centres (QDCs), in which multiple quantum processing units (QPUs) are housed together within the same warehouse and linked together over short links. By considering such a setting, we avoid the challenges of long-distance quantum communications while retaining the scalability of distributed architectures. In particular, we will focus on the impact of different types of noise in the QDC setting, as well exploring how we might combat the most impactful forms of noise. A novel simulation tool for distributed quantum computing, which is now open source, will also be briefly introduced. Finally, some current and future research directions will be outlined
12 Mar 2026
QSL Seminar
Optimizing unitary coupled cluster wave functions on quantum hardware : theoretical guarantees and resource-efficient optimizer
Martin Plazanet (École Polytechnique)
Simulating quantum many-body physics is the prime motivation behind the inception of quantum computing. To this end, many methods for quantum chemistry calculations have been developed, some of which with noisy intermediate scale quantum (NISQ) devices in mind. The Unitary Coupled Cluster (UCC) ansatz remains a leading candidate for capturing electronic correlation on near-term quantum devices. While the Variational Quantum Eigensolver (VQE) is the standard optimization framework, the Projective Quantum Eigensolver (PQE) introduced in[1] aims at overcoming some of VQE’s shortcomings by using projections of the Schrödinger equation instead of direct energy minimization. In this talk, we provide a mathematical analysis of the PQE algorithm. We present new theoretical bounds relating the Hamiltonian’s off-diagonal residues to the total energy error and wave function overlap, providing formal convergence guarantees for the algorithm. Furthermore, we analyze the classical optimization landscape and introduce a novel residue-based optimizer. Numerical simulations of our approach significantly outperform both the original PQE formulation and BFGS-optimized VQE.
12 Mar 2026
QSL Seminar
Approximate combinatorial optimization with Rydberg atoms: the barrier of interpretability
Corentin Bertrand (Bull SAS)
Analog quantum computing with Rydberg atoms is seen as an avenue to solve hard graph optimization problems, because they naturally encode the Maximum Independent Set (MIS) problem on Unit Disk (UD) graphs, a problem that admits rather efficient approximation schemes on classical computers. Going beyond UD-MIS to address generic graphs requires embedding schemes, typically with chains of ancilla atoms, and an interpretation algorithm to map results back to the original problem. However, interpreting approximate solutions obtained with realistic quantum computers proves to be a difficult problem. As a case study, we evaluate the ability of two interpretation strategies to correct errors in the recently introduced Crossing Lattice (CL) embedding. We find that one strategy, based on finding the closest embedding solution, leads to very high qualities, albeit at an exponential cost. The second strategy, based on ignoring defective regions of the embedding graph, is polynomial in the graph size, but it leads to a degradation of the solution quality which is prohibitive under realistic assumptions on the defect generation. Moreover, more favorable defect scalings lead to a contradiction with well-known approximability conjectures. Therefore, it is unlikely that a scalable and generic improvement in solution quality can be achieved with Rydberg platforms—thus moving the focus to heuristic algorithms.
19 Feb 2026
QSL Seminar
Conditioning in Generative Quantum Denoising Diffusion Models
Gabriele de Chiara (Universitat Autònoma de Barcelona)
Quantum denoising diffusion models have recently emerged as a powerful framework for generative quantum machine learning. In this seminar, I will give a survey of the recent developments of these techniques for preparing ensemble of resourceful quantum states. I will then explain our new contribution [1], in which we extend these models by introducing a conditioning mechanism that enables the generation of quantum states drawn from multiple target distributions. By sharing parameters across distinct classes of quantum states, our approach avoids the need to train separate models for each distribution. We validate our method through numerical simulations that span single-qubit generation tasks, entangled state preparation, and many-body ground state generation. Across these tasks, conditioning significantly reduced the error of targeted state generation by up to an order of magnitude. Finally, we perform an ablation study to quantify the effect of key hyperparameters on the model performance.
12 Feb 2026
QSL Seminar
Generalized contextuality via equirank NMF
Farid Shahandeh (Royal Holloway)
Generalized contextuality is a hallmark of nonclassical theories like quantum mechanics. It characterizes the fact that describing nonclassical phenomena using classical probabilistic models requires an overhead in the number of parameters. A theory is thus noncontextual if it admits a probabilistic model without an overhead. We present a reformulation of generalized contextuality as a matrix decomposition problem with a specific condition on the rank of its components, which we call equirank nonnegative matrix factorization (ENMF). We provide examples of theories which admit and do not admit such a decomposition. We briefly discuss the complexity of finding ENMFs
12 Feb 2026
QSL Seminar
Variational Quantum Circuits for Vision Language Understanding
Mehrnoosh Sadrzadeh (UCL)
Vision language models (VLMs) align images with text and were made popular by Open AI's CLIP transformer architecture in 2021. Soon after, this architecture was challenged with a barrage of compositional challenges, revealing that it cannot understand meaning. Inspired by a decade of work in compositional natural language processing, we replace CLIP's text transformer with a parser + a semantic analyzer. The parser uses the CCG grammar. The semantic analyzer works with tensors. We translate the tensors to tensor networks and quantum circuits and introduce QuLIP, a Quantum VLM. QuLIP is tested on compositional benchmarks and its results are better than CLIP, but at the same time due to its use of tensor networks and VQC's, it uses significantly less parameters (~100k vs ~100m). This is joint work with Tilen Liback-Stokin and Kin Ian Lo
9 Feb 2026
QSL Seminar
The Computational Advantage of MIP* Vanishes in the Presence of Noise
Penghui Yao (Nanjing University)
The class MIP* of quantum multiprover interactive proof systems with entanglement is much more powerful than its classical counterpart MIP: while MIP = NEXP, the quantum class MIP* is equal to RE, a class including the halting problem. This is because the provers in MIP* can share unbounded quantum entanglement. However, recent works have shown that this advantage is significantly reduced if the provers’ shared state contains noise. We attempt to exactly characterize the effect of noise on the computational power of quantum multiprover interactive proof systems. We investigate the quantum two-prover one-round interactive system MIP*[poly, O(1)], where the verifier sends polynomially many bits to the provers and the provers send back constantly many bits. We show noise completely destroys the computational advantage given by shared entanglement in this model. Specifically, we show that if the provers are allowed to share arbitrarily many EPR states, where each EPR state is affected by an arbitrarily small constant amount of noise, the resulting complexity class is equivalent to NEXP = MIP. This improves significantly on the previous best-known bound of NEEEXP (nondeterministic triply exponential time). We also show that this collapse in power is due to the noise, rather than the O(1) answer size, by showing that allowing for noiseless EPR states gives the class the full power of RE = MIP*[poly, poly]. Along the way, we develop two technical tools of independent interest. First, we give a new, deterministic tester for the positivity of an exponentially large matrix, provided it has a low-degree Fourier decomposition in terms of Pauli matrices. Secondly, we develop a new invariance principle for smooth matrix functions having bounded third-order Fréchet derivatives or which are Lipschitz continous
9 Feb 2026
QSL Seminar
No Practical Quantum Broadcasting: Even Virtually
Yunlong Xiao (A*Star)
Quantum information cannot be broadcast — an intrinsic limitation imposed by quantum mechanics. However, recent advances in virtual operations offer new insights into the no-broadcasting theorem. Here, we focus on the practical utility and introduce sample efficiency as a fundamental constraint, requiring any practical broadcasting protocol perform no worse than the naive approach of direct preparation and distribution. We prove that no linear process — whether quantum or beyond — can simultaneously uphold sample efficiency, unitary covariance, permutation invariance, and classical consistency. This leads to a no-practical-broadcasting theorem, which places strict limits on the practical distribution of quantum information. By applying Schur-Weyl duality, we establish the uniqueness of the canonical 1-to-N virtual broadcasting map that satisfies the latter three conditions, provide its construction, and determine its sample complexity through semidefinite programming. Finally, we explore the interplay between virtual broadcasting and a quantum spacetime framework, known as the pseudo-density operator, showing that their correspondence holds only in the 1-to-2 case, underscoring the fundamental asymmetry between spatial and temporal statistics in the quantum world
9 Feb 2026
QSL Seminar
Fullqubit alchemist: Quantum algorithm for alchemical free energy calculations
Po-Wei (George) Huang (Oxford
Accurately computing the free energies of biological processes is a cornerstone of computer-aided drug design but it is a daunting task. The need to sample vast conformational spaces and account for entropic contributions makes the estimation of binding free energies very expensive. While classical methods, such as thermodynamic integration and alchemical free energy calculations, have significantly contributed to reducing computational costs, they still face limitations in terms of efficiency and scalability. We tackle this through a quantum algorithm for the estimation of free energy differences by adapting the existing Liouvillian approach and introducing several key algorithmic improvements. We directly implement the Liouvillian operator and provide an efficient description of electronic forces acting on both nuclear and electronic particles on the quantum ground state potential energy surface. This leads to super-polynomial runtime scaling improvements in the precision of our Liouvillian simulation approach and quadratic improvements in the scaling with the number of particles. Second, our algorithm calculates free energy differences via a fully quantum implementation of thermodynamic integration and alchemy, thereby foregoing expensive entropy estimation subroutines used in prior works. Our results open new avenues towards the application of quantum computers in drug discovery
6 Feb 2026
QEC Journal Club
Verifying Fault Tolerance for QEC Codes
Wang Fang
6 Feb 2026
QSL Seminar
A bit of freedom goes a long way: classical and quantum algorithms for reinforcement learning under a generative mode
Debbie Lim (University of Lativa)
We propose novel classical and quantum online algorithms for learning finite-horizon and infinite-horizon average-reward Markov Decision Processes (MDPs). Our algorithms are based on a hybrid exploration-generative reinforcement learning (RL) model wherein the agent can, from time to time, freely interact with the environment in a generative sampling fashion, i.e., by having access to a "simulator". By employing known classical and new quantum algorithms for approximating optimal policies under a generative model within our learning algorithms, we show that it is possible to avoid several paradigms from RL like "optimism in the face of uncertainty" and instead compute and use optimal policies directly, which yields better regret bounds compared to previous works. For finite-horizon MDPs, our quantum algorithms obtain regret bounds which only depend logarithmically on the number of time steps $T$, thus breaking the $\tilde O(\sqrt T)$ classical barrier. This matches the time dependence of the prior quantum works of Ganguly et al. (arXiv'23) and Zhong et al. (ICML'24), but with improved dependence on other parameters like state space size $S$ and action space size $A$. For infinite-horizon MDPs, our classical and quantum bounds still maintain the $\tilde O(\sqrt T)$ dependence but with better $S$ and $A$ factors. Nonetheless, we propose a novel measure of regret for infinite-horizon MDPs with respect to which our quantum algorithms have polylog $T$ regret, exponentially better compared to classical algorithms. Finally, we generalise all of our results to compact state spaces
3 Feb 2026
QSL Seminar
Group theoretical analysis of higher-order quantum transformations
Satoshi Yoshida & Ryotaro Niwa (The University of Tokyo)
This talk provides a group theoretical perspective on the analysis of higher-order quantum transformations. These transformations are categorized into three hierarchies: mapping channels to classical data to quantum states and to other channels (unitary inversion and related transformations ). By exploiting the inherent symmetries of these tasks, representation theory allows for a simplified analysis of their optimal performance. Furthermore, I will introduce a canonical decomposition of quantum combs with group symmetry. This structure offers significant advantages for optimizing higher-order quantum algorithms in a variational manner and simulating random unitaries in quantum circuits
15 Jan 2026
QSL Seminar
Arbitrary Polynomial Separations in Trainable Quantum Machine Learning
Eric Anschuetz, (Caltech)
Recent theoretical results in quantum machine learning have demonstrated a general trade-off between the expressive power of quantum neural networks (QNNs) and their trainability; as a corollary of these results, practical exponential separations in expressive power over classical machine learning models are believed to be infeasible as such QNNs take a time to train that is exponential in the model size. We here circumvent these negative results by constructing a hierarchy of efficiently trainable QNNs that exhibit unconditionally provable, polynomial memory separations of arbitrary constant degree over classical neural networks -- including state-of-the-art models, such as Transformers -- in performing a classical sequence modeling task. This construction is also computationally efficient, as each unit cell of the introduced class of QNNs only has constant gate complexity. We show that contextuality -- informally, a quantitative notion of semantic ambiguity -- is the source of the expressivity separation, suggesting that other learning tasks with this property may be a natural setting for the use of quantum learning algorithms
11 Dec 2025
QEC Journal Club
Decoding in FPGA Land
Michael Beverland (IBM Quantum)
Real-time decoding is crucial for fault-tolerant quantum computing but likely requires specialized hardware such as field-programmable gate arrays (FPGAs), whose parallelism can alter relative algorithmic performance. We analyze FPGA-tailored versions of three decoder classes for quantum low-density parity-check (qLDPC) codes: message passing, ordered statistics, and clustering. For message passing, we analyze the recently introduced Relay decoder and its FPGA implementation; for ordered statistics decoding (OSD), we introduce a filtered variant that concentrates computation on high-likelihood fault locations; and for clustering, we design an FPGA-adapted generalized union-find decoder. We design a systolic algorithm for Gaussian elimination on rank-deficient systems that runs in linear parallel time, enabling fast validity checks and local corrections in clustering and eliminating costly full-rank inversion in filtered-OSD. Despite these improvements, both remain far slower and less accurate than Relay, suggesting message passing is the most viable route to real-time qLDPC decoding. Based on arxiv:2511.21660 with Satvik Maurya, Thilo Maurer, Markus Bühler and Drew Vandeth
9 Dec 2025
QSL Seminar
Dynamic Parameterised Quantum Circuits
Christa Zoufal (IBM Quantum)
Classical optimization of parameterized quantum circuits is a widely studied methodology for the preparation of complex quantum states, as well as the solution of machine learning and optimization problems. However, it is well known that many proposed parameterized quantum circuit architectures suffer from drawbacks which limit their utility, such as their classical simulability or the hardness of optimization due to a problem known as "barren plateaus". We propose and study a class of dynamic parameterized quantum circuit architectures. These are parameterized circuits containing intermediate measurements and feedforward operations. In particular, we show that these architectures:
1. Provably do not suffer from barren plateaus. 2. Are expressive enough to describe arbitrarily deep unitary quantum circuits. 3. Are competitive with state of the art methods for preparing ground states and facilitating the representation of nontrivial thermal states.
These features make the proposed architectures promising candidates for a variety of applications
4 Dec 2025
QSL Seminar
Nonlocal Games with Many Realisations
Dominik Leichtle
No abstract provided.
9 Oct 2025
QSL Seminar
A Mixed Quantum/Classical Language via Relative Adjunctions
Anders Miltner (Simon Fraser University)
A Mixed Quantum/Classical Language via Relative Adjunctions In this work, we aim to build a mixed quantum/classical language that describes physically realizable circuits. We have a few core goals in this language: (1) We want this language to enable circuits to be interpreted within FdHilb, (2) We want to be able to describe transformations via transformations of basis elements, and (3) we want to be able to duplicate (duplicable) circuits, and use these circuits as inputs to other circuits. Inspired by Benton's linear/nonlinear logic, we try to build as two languages that communicate via adjunctions. In this talk, I'll go through some of the design decisions and snags we are hitting as we progress in this work
2 Oct 2025
QSL Seminar
Qwerty: A Basis-Oriented Quantum Programming Language
Austin Adams (Georgia Institute of Technology)
Quantum computers have leaped from the theoretical realm into a race to large-scale implementations. This is due to the promise of revolutionary speedups, where achieving such speedup requires designing an algorithm that harnesses the structure of a problem using quantum mechanics. Yet many quantum programming languages today require programmers to reason at a low level of physics notation and quantum gate circuitry. This presents a significant barrier to entry for programmers who have not yet built up an intuition about quantum gate semantics, and it can prove to be tedious even for those who have. In this paper, we present Qwerty, a new quantum programming language that allows programmers to manipulate qubits more expressively than gates and trace programs without bra-ket notation. Due to its novel basis type and easy interoperability with Python, Qwerty is a powerful framework for high-level quantum-classical computation.
25 Sept 2025
QSL Seminar
Compiling higher-order quantum programming languages
Kostia Chardonnet (INRIA Nancy)
In the standard model of quantum computation, called the QRAM model, a classical computer is linked to a quantum processor. In this model, the classical computer builds a quantum circuit that is to be executed by the quantum processor, which then returns a classical result with some probability: the outcome of the quantum measurement. Then, a computation consists of a succession of exchanges between the classical computer and quantum processor until an end result is obtained. Obviously, the classical computer and quantum processor do not follow the same rules: the quantum processor has to obey the law of quantum physics. In particular, it means that no quantum data can ever be erased or duplicated and that all the operations inside the quantum processor are unitary operations over some Hilbert space of finite dimension. In order to manipulate this model, multiple quantum programming languages have been developed, in particular the quantum lambda-calculus is an extension of the well known lambda-calculus. This language features higher-order functions, linear types and classical control-flow (i.e., if-then-else). However, in this language, it can be difficult to know what kind of quantum operations, and in which order, will be applied to the quantum memory. The following question then arises naturally: given a program written in the quantum lambda-calculus, what is the underlying circuit that is executed ? In order to answer this question, we base our work on Girard's Geometry of Interaction, which allows one to go from a higher-order computation to a first-order one. The framework of GoI has already been used in the context of quantum computing, but never as a tool to compile programs.
18 Sept 2025
QSL Seminar
All you need is love controlled-V: universality of a standard two-qubit gate by catalytic embedding
Robin Kaarsgaard (University of Southern Denmark)
We present an encoding that renders the controlled-V gate (also known as controlled-√X) computationally universal in isolation. Specifically, we show that this gate can simulate the universal Clifford+Toffoli gate set with at most two clean auxiliary qubits and a constant overhead in gate count, and that an additional auxiliary qubit suffices to simulate Clifford+T. Our result settles an open question on the expressiveness of De Vos’ gate set based on Negators, and shows that the two-qubit gate S_U(τ) due to Sleator and Weinfurter is capable of universal quantum computation even for rational choices of τ. See https://arxiv.org/pdf/2509.07578
21 Aug 2025
QSL Seminar
One-Bit Addition with the Smallest Interesting Colour Code
Ben Criger (Quantinuum)
There are fault-tolerant quantum computing techniques available to enhance the success probability of algorithms we wish to run, but the overheads associated with these techniques make most of them impossible to execute on available devices. In addition, fault-tolerant procedures that inflict large overheads also induce larger-than-necessary logical error rates. The dominant source of overhead in current fault-tolerant circuits is magic state distillation, which is necessary to perform non-Clifford gates in many fault-tolerant protocols using stabilizer codes. By contrast, there exist some quantum codes possessing transversal non-Clifford gates, facilitating fault-tolerant universal quantum computation without state distillation. In this work, we focus on an eight-qubit code, the "Smallest Interesting Colour Code", demonstrating low-overhead procedures for Clifford operations and Pauli measurements, culminating in a fault-tolerant implementation of one-bit addition with 10 qubits and 26 CNots, suppressing the probability of classically detectable errors by a factor of more than 20.
7 Aug 2025
QSL Seminar
Exploring Research Frontiers in Quantum Software Engineering and Securing the Quantum Software Stack
Majid Haghparast (University of Jyväskylä)
I will present some of the ongoing research activities of our group and highlight opportunities for collaboration in the areas of quantum software engineering, and quantum software security. Our current efforts include investigating challenges and solutions for securing the quantum software stack, advancing quantum error correction (QEC), and enhancing the developer experience in quantum software development environments.
17 Jul 2025
QSL Seminar
Towards Distributed Quantum Error Correction for Distributed Quantum Computing
Chunming Qiao ( University at Buffalo)
Quantum computing as a promising technology can utilize stochastic solutions instead of deterministic approaches for complicated scenarios for which classical computing is inefficient, provided that both the concerns of the error-prone nature of qubits and the limitation of the number of qubits are addressed carefully. In order to address both concerns, a new qubit-based Distributed Quantum Error Correction (DQEC) architecture is proposed in which three physical qubits residing on three Quantum Processing Units (QPU) are used to form a logical qubit. This paper illustrates how three QPUs collaboratively generate a joint quantum state in which single bit-flip and phase-flip errors can be properly resolved. By reducing the number of qubits required to form a logical qubit in the proposed architecture, each QPU with its limited number of physical qubits can accommodate more logical qubits than when it has to devote its three physical qubits for each logical qubit. The functional correctness of the proposed architecture is evaluated through the Qiskit tool and stabilizer generators. Moreover, the fidelity of input and output quantum states, the complexity of the proposed designs, and the dependency between error probability and correctness of the proposed architecture are analyzed to prove its effectiveness.
15 May 2025
QSL Seminar
Contemporary approaches to surgery with quantum LDPC codes
Alexander Cowtan (Xanadu)
Lattice surgery is a well-known method of fault-tolerant quantum computation with surface codes. In recent years, substantial effort has gone into generalising lattice surgery to quantum low-density parity check codes. I will give an overview of this body of work: what it is, why it is important, and the various contemporary approaches. Time permitting, I will also discuss interesting connections between surgery and aspects of algebra, such as mapping cones, pushouts and the ZX-calculus.
10 Apr 2025
QEC Journal Club
Leveraging symmetries for quantum error correction
Stergios Koutsioumpas
Advances in quantum hardware are pushing error correction to evolve beyond just protecting qubits, with the next milestones being efficient logical operations and fast decoding. I’ll explain how we’re using symmetries to tackle these challenges. Focusing on quantum LDPC codes, I’ll share three projects where classical error correction and graph theory-based insights led to fast and accurate decoders, fault-tolerant gates through automorphisms, and quantum circuit optimisation techniques. Finally, I will discuss future directions and extensions, as well as the corresponding open-source implementations of our methods.
This talk is based on the following three papers:
- Heuristic and Optimal Synthesis of CNOT and Clifford Circuits, https://arxiv.org/abs/2503.14660
- Automorphism Ensemble Decoding of Quantum LDPC Codes, https://arxiv.org/abs/2503.01738
- Fault-Tolerant Logical Clifford Gates from Code Automorphism, https://arxiv.org/abs/2409.18175
8 Apr 2025
QSL Seminar
"SIS-with-hints" assumptions, reductions and lattice-based polynomial commitments
Oleksandra Lapiha
In this talk we will discuss a family of assumptions that has recently appeared in lattice-based cryptography called the “SIS-with-hints” family. We will build reductions for some of these assumptions and discuss how they can be used to build more advanced commitment schemes.
The talk will be based on the following paper:
https://eprint.iacr.org/2023/1469
27 Mar 2025
QEC Journal Club
Towards Real-Time QEC Decoding System
Yue Wu
Real-time quantum error correction decoding is a crucial component for achieving utility-scale fault-tolerant quantum computation. We developed fast, accurate and generic decoders with algorithm, software, and hardware designs. Fusion Blossom introduced a method for efficiently decoding dynamic logical circuits with a global MWPM solution. Micro Blossom achieved sub-microsecond decoding latency for MWPM decoding using automatically configured FPGAs the Minimum-Weight Parity Factor (MWPF) a new, generic decoding algorithm that addresses decoding problems for arbitrary qLDPC codes and noise models. These works build foundations to the real-time QEC decoding system and are publicly available on GitHub (https://github.com/yuewuo/mwpf).
20 Mar 2025
QSL Seminar
A Crash Course in Condensed Matter Physics for Quantum Computing Scientists
Steven Thomson
It's easy to claim quantum advantage if you don't know what classical computers are capable of. In this informal talk, we'll discuss the state-of-the-art in quantum condensed matter physics. We'll talk about what sort of problems physicists find interesting, what sort of techniques have been developed to address these problems on classical computers (or even - shock horror - analytically), and where we hope quantum computers may one day deliver real transformative advantage.
11 Mar 2025
QEC Journal Club
Quantum Error Correction for Neutral Atom Quantum Computing
Liam Veeder-Sweeney
In this journal club, we will explore recent advances in applying quantum error correction (QEC) codes to neutral atom quantum processors, considering both theoretical proposals and experimental implementations. Neutral atom systems offer dynamic reconfigurability and long-range connectivity, allowing them to go beyond strictly 2D QEC codes, such as the surface code, while maintaining low error rates. This makes them a promising platform for implementing quantum low-density parity-check (qLDPC) codes, which require a degree of non-locality to achieve high encoding rates and favourable distance scaling. As a result, neutral atom quantum computers provide a compelling route toward fault-tolerant quantum computing.
Key references:
- Constant-overhead fault-tolerant quantum computation with reconfigurable atom arrays
- Logical quantum processor based on reconfigurable atom arrays
20 Feb 2025
QSL Seminar
How to represent a quantum-classical program
Alex Rice
The problem of optimising classical programs has been well studied. The key idea underpinning modern optimising compilers is to convert the input program into many different forms known as intermediate
representations (IRs), each of which is suited for a different set of transformations. Many of these IRs are based on Static Single Assignment (SSA) form, where each variable is defined in a single location and
cannot be mutated. I will begin the talk by explaining this form and how it enables optimisation by allowing the program to be represented as a graph. The situation for quantum programs, and especially hybrid
quantum-classical programs, is less established. Commonly used toolkits often run optimisations directly on circuits. I will introduce our new quantum-classical SSA IR based on a concept we call "dynamic gates".
These allow gates in the circuit to be chosen at runtime, enabling complex interaction between classical and quantum components. I will demonstrate this IR by applying it to 3 case studies: representing quantum noise channels, quantum error correction, and measurement-based quantum computing.
13 Feb 2025
QSL Seminar
Automatic Quantum Uncomputation by Affine Types with Lifetime
Kengo Hirata
Uncomputation is a feature in quantum programming that allows the programmer to discard a value without losing quantum information, and that allows the compiler to reuse resources. Whereas quantum information has to be treated linearly by the type system, automatic uncomputation enables the programmer to treat it affinely to some extent. Automatic uncomputation requires a substructural type system between linear and affine, a subtlety that has only been captured by existing languages in an ad hoc way. We extend the Rust type system to the quantum setting to give a uniform framework for automatic uncomputation called Qurts (pronounced quartz). Specifically, we parameterise types by lifetimes, permitting them to be affine during their lifetime, while being restricted to linear use outside their lifetime. We also provide two operational semantics: one based on classical simulation, and one that does not depend on any specific uncomputation strategy.
10 Feb 2025
QEC Journal Club
Homological Algebra and its Applications in Quantum Error Correction
Abhishek Rajput
In this decade, homological algebra has played an essential role in important advancements in error correction and has continued to inspire novel ideas and techniques ranging from code constructions to fault-tolerant measurement schemes. Yet there remain numerous ideas from homological algebra that remain to be exploited for quantum error correction purposes. This talk will discuss key tools in homological algebra as it is used by mathematicians, summarize several results from last year that have used some of them, and highlight possible new avenues for their application in error correction.
6 Feb 2025
QEC Journal Club
Verifying Fault-Tolerance of Quantum Error Correction Codes
Wang Fang
Quantum computers have advanced rapidly in qubit number and gate fidelity. However, they would still lack practicalness without utilizing fault-tolerant quantum error correction code (QECC) implementations to suppress noise. Manually or experimentally verifying the fault-tolerance of complex QECCs is impractical due to the vast error combinations. This paper formalizes fault-tolerance within the language of quantum programs. By incorporating the techniques of quantum symbolic execution, we provide an automatic verification tool for quantum fault-tolerance. We evaluate and demonstrate the effectiveness of our tool on a universal set of logical operations across different QECCs.
Relevant papers: https://scirate.com/arxiv/2501.14380, https://dl.acm.org/doi/10.1145/3656419.
5 Feb 2025
QSL Seminar
Putting the Q in PQC: quantum theory in post-quantum cryptography
Christopher Battarbee
Perhaps the two most popular families of solutions to quantum-safe communication are those offered by Quantum Key Distribution (QKD) and by Post-Quantum Cryptography (PQC). In this talk I will recap the quantum threat to secure key establishment, explain the PQC response, and discuss its weaknesses, with particular reference to my own quantum cryptanalytic work. I will discuss how PQC and QKD can mutually address each other's shortcomings, motivating the nascent field of research into hybridisation of these approaches.
23 Jan 2025
QSL Seminar
A gentle introduction to Dynamical Mean-Field Theory from a quantum computing perspective
Pauline Besserve
Quantum many-body physics is often cited as one of the domains which could benefit first from quantum computing. In particular the study of strongly-correlated materials, described by the Fermi-Hubbard lattice model which captures a wide range of electronic correlation effects, is a use case considered in a large body of work. The way quantum advantage could be attained is often understood through the lens of quantum simulation: ultracold atoms optically-trapped in a tunable optical lattice can naturally mimic the behaviour of Fermi-Hubbard electrons.
In this talk I will present an alternative approach, the Dynamical Mean-Field Theory (DMFT) framework. DMFT strikes me as rather niche outside the many-body community but lends itself very well to quantum computation. It relies on a simpler, proxy model to the Fermi-Hubbard model, called an impurity model. I will give you a flavour of DMFT and its theoretical raison d'etre, and show you how quantum-classical hybrid DMFT can be implemented. Finally I will argue that DMFT is a very promising candidate for quantum advantage in the late NISQ era based on the fact that requirements over the quantum hardware to tackle impurity models are considerably milder than for the Fermi-Hubbard model.
18 Dec 2024
Colloquium
Interactive proofs for verifying (quantum) learning and testing
Matthias Caro
We consider the problem of testing and learning from data in the presence of resource constraints, such as limited memory or weak data access, which place limitations on the efficiency and feasibility of testing or learning. In particular, we ask the following question: Could a resource-constrained learner/tester use interaction with a resource-unconstrained but untrusted party to solve a learning or testing problem more efficiently than they could without such an interaction? In this work, we answer this question both abstractly and for concrete problems, in two complementary ways: For a wide variety of scenarios, we prove that a resource-constrained learner cannot gain any advantage through classical interaction with an untrusted prover. As a special case, we show that for the vast majority of problems in which quantum memory is a meaningful resource, a memory-constrained quantum algorithm cannot overcome its limitations via classical communication with a memory-unconstrained quantum prover. In contrast, when quantum communication is allowed, we construct a variety of interactive proof protocols, for specific learning and testing problems, which allow memory-constrained quantum verifiers to gain significant advantages through delegation to untrusted provers. These results highlight both the limitations and potential of delegating learning and testing problems to resource-rich but untrusted third parties.
18 Dec 2024
Colloquium
A Quantum Optical Support Vector Machine
Gerard Milburn
Quantum technologies are grounded on the ability to control highly non-classical probability distributions that are inefficient to simulate with conventional computers. I will describe a quantum hardware system to sample a quantum kernel for a support vector machine learning algorithm. The schema is analogue, not a qubit circuit, and suggests a path to harness quantum analogue computing for efficient machine learning.
11 Dec 2024
QEC Journal Club
Circuit Level Decoding of QEC
Tamas Noszko
We will take a closer look at fault-tolerance, focusing on so called “internal” errors, errors introduced by the correction process itself. We will show how we can investigate and solve this problem using the detector error model formalism - leading to the topic of circuit level decoding. The relevant paper is "Designing fault-tolerant circuits using detector error models".
5 Dec 2024
QSL Seminar
Can quantum computing change drug discovery?
Julien Michel
In this seminar I will aim to provide an accessible overview of current
industrial practices for drug discovery. I will then discuss how
computational chemistry methodologies are currently used to support the
preclinical stages of drug discovery, drawing on examples from my own
academic drug discovery activities.
I will then discuss opportunities quantum computers might offer to
improve the effectiveness of current drug discovery processes, and even
perhaps allow to break into new territories.
21 Nov 2024
QSL Seminar
Information locking and extraction protocols using local indistinguishability
Suchetana Goswami
Locally indistinguishable states are useful to distribute information
among spatially separated parties such that the information is locked.
This implies that the parties are not able to extract the information
completely via local operations and classical communication (LOCC),
while it might be possible via LOCC when the parties share entanglement
in a particular fashion. Based on this, we design an information locking
protocol to securely hide classical information in quantum states with
the possibility of extraction to the trusted parties, when needed. We
make our protocol resource efficient in terms of information extraction.
On the other hand, we use the basic property of indistinguishability of
quantum states as a resource in secure hybrid communication protocol.
Introduction of quantumness over the classical primitives in the form of
state discrimination ensures security for such protocols. We prescribe
entanglement-based communication protocols which are easily expandable
to multipartite domains.
14 Nov 2024
QSL Seminar
Fault-Tolerant Logical Clifford Gates from Code Automorphisms
Hasan Sayginel
We study the implementation of fault-tolerant logical Clifford gates on
stabilizer quantum error correcting codes based on their symmetries. Our
approach is to map the stabilizer code to a binary linear code, compute
its automorphism group, and impose constraints based on the Clifford
operators permitted. We provide a rigorous formulation of the method for
finding automorphisms of stabilizer codes and generalize ZX-dualities to
non-CSS codes. We provide a Python package implementing our algorithms
which uses the computational algebra system MAGMA. Our algorithms map
automorphism group generators to physical circuits, calculate Pauli
corrections based on the destabilizers of the code, and determine their
logical action. We discuss the fault tolerance of the circuits and
include examples of gates through automorphisms for the [[4,2,2]] and
perfect [[5,1,3]] codes, bivariate bicycle codes, and the best known
distance codes.
13 Nov 2024
QEC Journal Club
Magic States
Adithya Sireesh
We will explore the implementation of universal quantum computation on quantum error-correcting codes (QECCs) through the technique of Magic State Distillation (MSD) technique, originally introduced by Bravyi and Kitaev in 2005. MSD is pivotal for enabling the execution of non-Clifford gates, which are essential for achieving universal quantum computation. We will discuss the process of preparing and purifying magic states, detailing how these states can be utilized alongside Clifford operations to run quantum algorithms.
Relevant papers: Bravyi, S. and Kitaev, A., 2005. Universal quantum computation with ideal Clifford gates and noisy ancillas. Physical Review A—Atomic, Molecular, and Optical Physics, 71(2), p.022316.
Jochym-O'Connor, T., Yu, Y., Helou, B. and Laflamme, R., 2012. The robustness of magic state distillation against errors in Clifford gates. arXiv preprint arXiv:1205.6715.
30 Oct 2024
QSL Seminar
The Long Path towards Quantum Simulations of High-Energy Physics
Dorota Grabowska
The Standard Model of Particle Physics encapsulates the vast
majority of our understanding of the fundamental nature of our Universe.
Massive theoretical and algorithmic developments in classical computing,
as well as hardware advancements, have allowed us to make
first-principle predictions about many of its properties. Nevertheless,
there remain a plethora of open questions that do not seem amenable to
these methods. With a fundamentally different computational strategy,
quantum computers hold the potential to address these open questions. In
this talk, I will first give a casual overview of how I, as a particle
physicist, understand quantum computation. I will then give an
introduction to the particular physical systems in which I am
interested, namely Quantum Chromodynamics. I will then outline the
various hurdles, and some solutions, that must be overcome before
quantum simulations of the Standard Model become possible.
24 Oct 2024
QSL Seminar
Optimal trace distance and fidelity estimations for pure quantum states
Qisheng Wang
In this talk, we introduce optimal quantum algorithms that estimate both
the trace distance and the (square root) fidelity between pure states to
within additive error ε using Θ(1/ε) queries to their state-preparation
circuits. This approach quadratically improves the long-standing
folklore O(1/ε^2) based on the SWAP test, challenging the common belief
that the SWAP test is already optimal for testing pure states.
23 Oct 2024
QEC Journal Club
QEC lattice surgery: the applications of the ZX-Calculus to fault-tolerant quantum computing
Robert Booth
Relevant papers: https://quantum-journal.org/papers/q-2020-01-09-218/ (and bonus material: http://arxiv.org/abs/2204.14038).
17 Oct 2024
QSL Seminar
Semantics of Quantum Loops and Recursion
Louis Lemonnier
Denotational semantics is a cornerstone in the study of programming languages. It aims to translate syntax from a specific
language into mathematical objects. It helps abstract away, in order to
prove some properties of the language. When recursion or loops are
involved in the language, the mathematical interpretation must have
corresponding fixed points or a similar theory. This is the case for
classical and probabilistic languages and also quantum programming
languages with classical control. We briefly present the latter, and
then focus on recursion with quantum control, i.e. without measurement.
We show what problems arise on the mathematical side and why the usual
tools do not apply in the desired setting. We end with a positive note
with guarded quantum recursion.
4 Oct 2024
QSL Seminar
The Born Ultimatum: Simulability in Quantum Generative Models.
Mario Herrero Gonzalez
The main objective is to explore the simulability of the Quantum Circuit
Born Machine (QCBM) from the perspective of k-order correlators. This
ongoing work leverages the knowledge of classical shadows, the Lie
algebra of a parameterized quantum circuit, and the approximation of
loss functions when considering complex quantum and classical data. Our
quantum target data is based on the ground states of Hamiltonians in
one-dimensional and two-dimensional lattices, while the classical target
is constructed from a coherent superposition of the data. Additionally,
this k-order correlation approach allows us to evaluate the inductive
bias and generalization capabilities of the algorithm.
2 Oct 2024
QSL Seminar
Fundamentals of lattice surgery of surface codes
Boren Gu
Lattice surgery is a crucial technique for enabling interactions between
logical qubits encoded in surface codes, paving the way for scalable
fault-tolerant quantum computing. This journal club will introduce the
key concepts and principles of lattice surgery for surface codes. We
will begin by reviewing the basics of surface code architecture and
logical qubit encoding. The core operations of lattice surgery - merging
and splitting code surfaces - will then be explained, demonstrating how
they allow controlled interactions between logically-encoded qubits
while maintaining error correction properties. Finally, we will see how
these fundamental operations can be used to implement logical gates like
CNOT.
26 Sept 2024
QSL Seminar
Peeking at the Theoretical Foundation of (Quantum) Machine Learning
Mina Doosti
In this talk, I will give a brief introduction to both classical and
quantum learning theory, covering key concepts from fundamental ideas to
the latest developments, including our work on learning quantum
processes using statistical queries. I will explore fundamental
questions such as: What does it mean to learn "quantum stuff"? And how
difficult is it to learn them? Finally, I will discuss why quantum
learning theory is a powerful framework that not only pushes us towards
a better understanding of machine learning but also offers valuable
insights into other fields like cryptography.
19 Sept 2024
QSL Seminar
Windowed Quantum Arithmetic
Adithya Sireesh
Quantum modular multiplication, a key subroutine in algorithms like
shor's algorithms for factoring and discrete log, if executed naively,
would be computationally demanding and memory-intensive. Windowed
quantum arithmetic [1] leverages precomputed lookup tables to reduce the
complexity of quantum modular multiplication, achieving asymptotic
speedups from O(n^2) to O(n^2/log^2 n). In this seminar, we will
explore how this technique optimizes quantum modular multiplication and,
if time permits, discuss further optimizations to windowing.
[1] Gidney, Craig. "Windowed quantum arithmetic." arXiv preprint
arXiv:1905.07682 (2019)
18 Sept 2024
QSL Seminar
Welcome to the Fault-Tolerant Era of Quantum Computing
Joschka Roffe
In what is undoubtedly the most significant quantum computing
experiment to date, the Google Quantum team has successfully
demonstrated the first logical qubit protected using quantum error
correction! Specifically, they have implemented a logical qubit and
shown that it operates below the breakeven point. This milestone
represents a key foundational step toward building arbitrarily scalable
quantum systems and paves the way for operationally useful quantum
computers that will vastly surpass the capabilities of today’s classical
supercomputers. In this journal club, I will provide an accessible
overview of Google’s technical paper
(arXiv: 2408.13687, Quantum error correction below the surface code
threshold), addressing key questions such as:
* Why is quantum error correction essential?
* What is the “surface code” protocol, and how has Google implemented it?
* What does it mean for a logical qubit to achieve performance below
the breakeven point?
I will also discuss the challenges that remain in developing full-scale
quantum computers.
12 Sept 2024
QSL Seminar
Classical simulation of Gaussian Boson Sampling
Tom Dodd and Julia Miklas
Gaussian Boson Sampling (GBS) is a non-universal quantum computation
that in recent years has been used to claim experimental quantum
advantage. The noise in these experiments may however be exploited to
create classical simulation methods that would invalidate these claims.
One such method, based on Fourier analysis of the probability
distributions of the samples, is currently being developed at the
University of Edinburgh.
In this talk, Julia will present an analysis of the viability of the
method by looking at the behaviour of Fourier coefficients of GBS sample
probabilities under specific system parameters. Then, Tom will present a
description of a class of classical simulators that make use of the
statistics of these Fourier coefficients and the approximation of
marginal probabilities.
27 Aug 2024
QSL Seminar
Exploring the potential of photonic quantum processors in machine learning
William Clements (ORCA Computing)
Quantum computers have progressed to the point where they
can now solve some specific mathematical problems that classical
computers cannot solve. This talk will present ongoing research on
potential applications to machine learning. I will first introduce
near-term photonic quantum processors. Although they are not universal
for quantum computation, they provide a scalable route to solving some
classes of hard sampling problems. I will then describe ways in which
current classical neural network architectures may harness these unique
computational capabilities for generative modelling and for
classification. This talk will highlight both the potential and the
limitations of these approaches, as well as opportunities for further
research.
26 Aug 2024
QSL Seminar
Quantum Error Suppression with Subgroup Stabilisation
Bo Yang
Quantum state purification is the functionality that, given multiple copies
of an unknown state, outputs a state with increased purity. This will be an
essential building block for near- and middle-term quantum ecosystems before
the availability of full fault tolerance, where one may want to suppress
errors not only in expectation values but also in quantum states. We propose
an effective state purification gadget with a moderate quantum overhead by
projecting M noisy quantum inputs to their symmetric subspace defined by
a set of projectors forming a symmetric subgroup with order M. Our method,
applied in every short evolution over M redundant copies of noisy states,
can suppress both coherent and stochastic errors by a factor of 1/M,
respectively. This reduces the circuit implementation cost M times smaller
than the state projection to the full symmetric subspace proposed by Barenco
et al. more than two decades ago. We also show that our gadget purifies the
depolarised inputs with probability p to asymptotically O(p^{2}) with an
optimal choice of M when p is small. The sampling cost scales O(p^{−1})
for small p, which is also shown to be asymptotically optimal. Our method
provides flexible choices of state purification depending on the hardware
restrictions before fully fault-tolerant computation is available.
20 Jun 2024
QSL Seminar
A tour of the zoo of tensor network simulators
Pablo Andres-Martinez
When are tensor networks (TN) simulators better than other methods? Which are their bottlenecks? Should I use GPUs? Can I implement an error model on top of a TN simulator? In this talk I will cover frequently asked questions about quantum circuit simulation using TNs. While doing so, I will describe the taxonomy of TN methods, along with their strengths and weaknesses.
13 Jun 2024
QSL Seminar
The problem of photon loss in DVLOQC
James Mills
I will introduce DVLOQC and describe how photon loss is an issue for
running computations using LO circuits. I will then present the notion
of recycled probabilities, describe their construction and methods by
which these may be used to construct loss-mitigated outputs. I'll argue
why postselection should be the benchmark against which the performance
of any such method should be measured and present analysis comparing the
performance of recycling mitigation to postselection. Finally I will
describe a photonic QCBM and present results indicating a performance
improvement when recycling mitigation is applied.
6 Jun 2024
QSL Seminar
Learning simple quantum channels
Chirag Wadhwa
Learning quantum channels is a fundamental problem in quantum information.
Unfortunately, this problem is provably hard for arbitrary quantum channels,
requiring an exponential amount of data in the system size. A lot of recent
work has instead focused on developing efficient algorithms when the channel
is guaranteed to be "simple" in some sense. The kinds of channels for which
such endeavours have been successful include those with underlying circuits
having low depth or few gates, or those affecting a small number of qubits. In
this talk, I will present the efficient algorithms for these channels and
focus on the currently relevant technical tools used in these works, such as
classical shadows, Pauli/Fourier spectrum analysis, and covering nets.
30 May 2024
QSL Seminar
Security and Efficiency of Delegated Quantum Computing
Dominik Leichtle
Quantum information promises to revolutionize our world, from the way in
which we communicate to the way in which we compute, deriving its power
directly from the laws that govern the behavior of nature on extremely
small scales - quantum mechanics. In the near future, the hardware of
possibly useful quantum computers is expected to remain very expensive
and thus out of reach for most interested end users. In such a world, it
is an important problem to provide security guarantees for customers who
wish to remotely instruct quantum servers, by keeping their data private
(blindness) and checking the correctness of the results (verification).
This functionality of secure delegated quantum computing received a lot
of attention during recent years, but still admits many open questions.
We explore the (im)possibility of securing delegated quantum
computations in different settings: what is the hardware that the client
needs trusted access to, what is the minimum hardware required by the
server, and how must the parties communicate? This work is driven by the
motivation to break down the barriers that keep us from securing and
verifying quantum computations in practice, by identifying and removing
unnecessary overheads.
14 Mar 2024
QSL Seminar
The Penrose tiling is a quantum error correcting code
Latham Boyle
I will begin by introducing Penrose tilings ("PTs") and quantum error
correcting codes ("QECCs"). A PT is a remarkable, intrinsically
non-periodic way of tiling the plane whose many beautiful and unexpected
properties have fascinated physicists, mathematicians, and geometry
lovers of all sorts, ever since its discovery in the 1970s. A QECC is a
fundamental way of protecting quantum information from noise, by
encoding the information with a sophisticated type of redundancy. Such
codes play an increasingly important role in physics: in quantum
computing (where they protect the delicate quantum state of the
computer); in condensed matter physics (where they underpin the notion
of topologically-ordered phases); and even in quantum gravity (where the
"holographic" or "gauge/gravity" duality may be understood as such a code).
Although PTs and QECCs might seem unrelated, I will explain how PTs
gives rise to (or, in a sense, *are*) a new type of QECC in which any
local errors or erasures in any finite region of the code space, no
matter how large, may be diagnosed and corrected. Variants of this code
(based on the cousins of the Penrose tiling, called the Ammann-Beenker
and Fibonacci tilings) can live in a finite space, in discrete spin
systems, and in an arbitrary number of spatial dimensions.
29 Feb 2024
QSL Seminar
Quantum programming and algorithms based on higher-order quantum operations
Mio Murao
No abstract provided.
29 Feb 2024
QSL Seminar
Characterising & Controlling Complex Quantum Processes with Classical Memory
Philip Taranto
No abstract provided.
22 Feb 2024
QSL Seminar
Qualitative equivalence between incompatibility and Bell nonlocality
Ravi Kunjwal
No abstract provided.
18 Feb 2024
QSL Seminar
Introduction to quantum learning theory
Chirag Wadhwa
In this seminar, we will introduce some quantum learning models along
with their classical counterparts. We will also discuss the sample
complexity of learners for specific problems in these models and ways of
characterizing the complexity.
14 Nov 2023
QSL Seminar
Harnessing quantum coherence for unlocking hybrid and multisource thermodynamic operations in autonomous thermal machines
Kenza Hammam
Due to the nanotechnological progress and the increasing interest in quantum systems, heat engines are no longer limited to the size of steam engines from the industrial revolution. Their
miniaturisation is currently leading to the emergence of thermal machines that harness quantum
effects to operate. Located at the interface between stochastic thermodynamics, quantum mechanics and quantum information, quantum thermodynamics offers an ideal platform to explore and
address several challenges, for instance, energy management at the nanoscale, the energetic cost of
measurement and quantum computing as well as the impact of quantum features on the operation
of quantum thermal devices such as heat engines and refrigerators. Answering these challenges
requires an adequate characterisation of heat and work at the quantum level which is essential for
the advancement of novel quantum technologies in the future.
In this talk, I will be focusing on highlighting the role of quantum resources, mainly quantum coherence, in the performance of quantum thermal machines that rely on the framework of collision
models (CM) to study their open quantum system dynamics. we demonstrate that the presence of even small amounts of coherence in the thermal reservoirs powering a three-terminal machine, constitutes an extra resource for allowing otherwise forbidden combined and hybrid modes of
operation, where either different resources are combined to perform a single thermodynamic task,
or more than one task is performed at the same time. In order to assess the performance of such
coherence-enabled modes of operation, we analyse their power and efficiency and discussing the
quantum advantage that coherence provides, as well as its detrimental effects.
14 Nov 2023
QSL Seminar
Gate-efficient fermionic quantum state preparation with a Natural-Orbitalizing variational scheme
Pauline Besserve
Quantum computing platforms have been drawing much attention over the past few years in quantum chemistry and condensed matter. In these domains a possibly large number of particles
interact and exhibit quantum effects which are hard to tackle classically. Whereas the first prototypical quantum devices that are being built now are still greatly error-prone, the development of
variational methods as well as error mitigation strategies has sparked the hope to leverage
such devices in the near term to make computations escaping the reach of classical methods.
In particular, strongly-correlated many-fermion systems in solids exhibit rich phase diagrams which
lend themselves to plenty of applications in the industry. They are now widely studied through the
celebrated Dynamical Mean-Field Theory (DMFT). DMFT consists in computing relevant observables regarding the ’spherical cow’ of strongly-correlated systems, namely the Fermi-Hubbard
lattice model, by mapping it to a simpler, auxiliary ’impurity’ model. The solving of this latter
model constitutes an exponential bottleneck for any classical algorithm to date.
The hybrid implementation of the DMFT scheme with near-term quantum computers typically
relies on the variational preparation of the impurity model’s ground state. Scaling the variational
circuits to tackle richer impurity models is a challenge due to the relatively low gate fidelities
and coherence times characterizing quantum hardware as for now, resulting in greatly degraded
performances.
This talk will be devoted to the presentation of an algorithmic strategy I have set forth during
my PhD research to be able to scale the impurity model. The algorithm, dubbed NaturalOrbitalization, greatly reduces the requirements over the variational circuit depth as the size of
the proxy impurity model is increased. It consists in interleaving variational runs with tailored
rewritings of the impurity model’s Hamiltonian, which enhance the expressivity of the circuit at
hand. We show that this strategy coupled to an adaptive state preparation strategy leads to
shorter depths.
13 Nov 2023
QSL Seminar
TKET 2 - Next Generation Quantum Compiler
Ross Duncan (Quantinuum)
No abstract provided.
2 Nov 2023
QSL Seminar
A gentle introduction to dagger category theory and categorical quantum mechanics
Matthew Di Meglio
I will give an introduction to some elementary concepts of category theory while motivating their application to the foundations of quantum mechanics and quantum information theory. I would also like to impart an intuition for why the categorical way of thinking can be so productive and for which kinds of problems it is useful.
19 Oct 2023
QSL Seminar
On quantum oracles and their role in cryptography and machine learning
Mina Doosti
No abstract provided.
bottom of page