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

11th April 2024: Nathanan Tantivasadakarn (Caltech)

15:00 BST (Zoom)

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.

16th April 2024: Zhi Li (Perimeter Institute)

15:00 BST (UCL)

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.

28th May 2024: Mircea Bejan (Cambridge)

13:00 BST (UCL)

Title and abstract TBC

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.