Quantum Information Theory Seminar

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We host seminars! These are held online and in the CS department at UCL. See below for details of the upcoming schedule. If you wish to be kept updated about future seminars, please contact uclqitseminars<@>gmail<.>com

Upcoming

6th September 2024: Mischa Woods (Inria – University Grenoble Alpes)

14:00 BST (UCL)

Quantum frequential computing: a quadratic run-time advantage for all algorithms

Abstract: We introduce a new class of computer called a quantum frequential computer. They harness quantum properties in a different way to conventional quantum computers to generate a quadratic computational run time advantage for all algorithms as a function of the power consumed. They come in two variants: type 1 can process classical algorithms only while type 2 can also process quantum ones. In a type-1 quantum frequential computer, only the control is quantum, while in a type 2 the logical space is also quantum. We also prove that a quantum frequential computer only requires a classical data bus to function. This is useful, because it means that only a relatively small part of the overall architecture of the computer needs to be quantum in a type-1 quantum frequential computer in order to achieve a quadratic run time advantage. As with classical and conventional quantum computers, quantum frequential computers also generate heat and require cooling. We also characterise these requirements.

11th July 2024: Roberto Campos and Gabriel Escrig (Universidad Complutense de Madrid)

A Quantum Walks Approach to Optimization Problems applied to Parameter Estimation of Gravitational Waves

Abstract: The efficient resolution of optimization problems is crucial in modern industry, often relying on classical algorithms that face scalability and processing limitations. Quantum computing offers a promising alternative to these challenges. In this talk, we present the Quantum Metropolis Solver (QMS), a quantum software tool based on the Metropolis-Hastings algorithm and quantum walks. We validate QMS through the use case of Parameter Estimation of Gravitational Waves (GW) events. Since the first GW detection in 2015, the field has rapidly advanced, yet current analysis techniques are bottlenecked by high computational demands. Our exploration demonstrates that quantum algorithms can overcome this obstacle. We implement QMS in a quantum environment on classical hardware, developing a metric for fair comparison between quantum and classical algorithms. Testing on real GW data from the GWTC-1 detection period reveals a polynomial advantage for quantum algorithms, establishing a foundation for future developments in this area.

7th June 2024: Lorenzo Leone (FU Berlin)

Magic-induced computational separation in entanglement theory

Abstract: Entanglement serves as a foundational pillar in quantum information theory, delineating the boundary between what is classical and what is quantum. The common assumption is that higher entanglement corresponds to a greater degree of ‘quantumness’. However, this folk belief is challenged by the fact that classically simulable operations, such as Clifford circuits, can create highly entangled states. The simulability of these states raises a question: what are the differences between ‘low-magic’ entanglement, and ‘high-magic’ entanglement? We answer this question in this work with a rigorous investigation into the role of magic in entanglement theory. We take an operational approach to understanding this relationship by studying tasks such as entanglement estimation, distillation and dilution. This approach reveals that magic has notable implications for entanglement. Specifically, we find an operational separation that divides Hilbert space into two distinct regimes: the entanglement-dominated (ED) phase and magic-dominated (MD) phase. Roughly speaking, ED states have entanglement that significantly surpasses their magic, while MD states have magic that dominates their entanglement. The competition between the two resources in these two phases induces a computational phase separation between them: there are sample- and time-efficient quantum algorithms for almost any entanglement task on ED states, while these tasks are provably computationally intractable in the MD phase. Our results find applications in diverse areas such as quantum error correction, many-body physics, and the study of quantum chaos, providing a unifying framework for understanding the behavior of quantum systems. We also offer theoretical explanations for previous numerical observations, highlighting the broad implications of the ED-MD distinction across various subfields of physics.

28th May 2024: Mircea Bejan (Cambridge)

Dynamical magic transitions in Clifford+T circuits

Abstract: Recently discovered measurement-induced transitions (MIPTs) in entanglement are phase transitions in classical simulability. However, some highly-entangling dynamics (e.g., integrable systems or Clifford circuits) are easy to classically simulate. Here, we study simulability transitions beyond entanglement by asking how the dynamics of magic competes with measurements. We find distinct MIPTs in magic, simulability, and entanglement. We identify dynamical “stabilizer-purification” as the mechanism driving the magic transition. En route, we use Pauli-based computation to distill the quantum essence of the dynamics to a set of measurements. We link stabilizer-purification to “magic fragmentation” wherein these measurements separate into disjoint, O(1)-weight blocks, and relate this to the spread of magic in the original circuit becoming arrested.

23rd May 2024: Anna Biggs (Princeton)

Comparing the decoherence effects due to black holes versus ordinary matter

Abstract: Recently, a certain thought experiment was discussed (arXiv: 2301.00026, arXiv: 2205.06279) which involves the decoherence of a quantum system due to a black hole. Here we show how this phenomenon is consistent with standard ideas about quantum black holes. In other words, modeling the black hole as a quantum system at finite temperature one obtains the same answer. We demonstrate this by analyzing the problem in terms of an effective theory that can apply both for the black hole case and for an ordinary matter system, showing that the same qualitative effect is present for ordinary matter at finite temperature. Based on arXiv: 2405.02227

14th May 2024: Ángela Capel (Cambridge)

Abstract: Rapid thermalisation of quantum dissipative many-body systems

Quantum systems typically reach thermal equilibrium when in weak contact with a large external bath. Understanding the speed of this thermalisation is a challenging problem, especially in the context of quantum many-body systems where direct calculations are intractable. The usual way of bounding the speed of this process is by estimating the spectral gap of the dissipative generator, but this does not always yield a reasonable estimate for the thermalisation time. When the system satisfies instead a modified logarithmic Sobolev inequality (MLSI), the thermalisation time is at most logarithmic in the system size, yielding wide-ranging applications to the study of many-body in and out-of-equilibrium quantum systems, such as stability against local perturbations (in the generator), efficient preparation of Gibbs states (the equilibria of these processes), etc.

In this talk, I will present an overview on a strategy to prove that a system satisfies a MLSI provided that correlations decay sufficiently fast between spatially separated regions on the Gibbs state of a local, commuting Hamiltonian. This will allow us to conclude that any Davies or Schmidt dissipative generator converging to a 2-local, commuting Hamiltonian at high-enough temperature thermalises in a time logarithmic in the system size.

16th April 2024: Zhi Li (Perimeter Institute)

How entangled are quantum eror-correcting codes?

Abstract: Quantum error-correcting codes play a pivotal role in enabling fault-tolerant quantum computation. In quantum error-correcting codes, the quantum information is encoded globally via quantum entanglements: the knowledge of individual subsystems, even when combined, reveals nothing about the overall state.
In this talk, we explore the quantification of how entangled quantum error-correcting codes are, via a quantity we term “product overlap”, the maximal fidelity between any code state and any product state. We will show that the product overlap of a quantum error-correcting code must be exponentially small in the code distance if it (1) is a low-density parity check (LPDC) code, or (2) is a stabilizer code, or (3) has high code rate. On the opposite side, for fixing code distance, we construct a class of codes where the product overlap reaches one as the code length increases.

11th April 2024: Nathanan Tantivasadakarn (Caltech)

Quantum computation from dynamic automorphism codes

Abstract: We propose a new model of quantum computation comprised of low-weight measurement sequences that simultaneously encode logical information, enable error correction, and apply logical gates. These measurement sequences constitute a new class of quantum error-correcting codes generalizing Floquet codes, which we call dynamic automorphism (DA) codes. We construct an explicit example, the DA color code, which is assembled from short measurement sequences that can realize all 72 automorphisms of the 2D color code. On a stack of N triangular patches, the DA color code encodes N logical qubits and can implement the full logical Clifford group by a sequence of two- and, more rarely, three-qubit Pauli measurements. We also make the first step towards universal quantum computation with DA codes by introducing a 3D DA color code and showing that a non-Clifford logical gate can be realized by adaptive two-qubit measurements.

8th April 2024: Simon Langenscheidt (LMU Muenchen)

Channel-state duality with centers in random tensor network holography

Abstract: In the pursuit of understanding the AdS/CFT correspondence better from a quantum information perspective, the recent literature has produced a number of toy models using PEPS tensor networks with random vertex states, known as random tensor networks. This approach becomes more than a toy model in discrete approaches to quantum gravity, where these tensor networks naturally appear as basis states of quantum pregeometry. In this talk, I present the notion of transport superoperators between subsystems, which presents an interesting object from a quantum information perspective and is central to the formalisation of holography in discrete quantum geometries. I present furthermore the general framework of random tensor network holography and recent results for a new class of PEPS states featuring superposed bond dimensions.