Meeting 4th September 2019, St Andrews

Day Schedule                   Room: Theatre B, Department of Physics

10.00–10.25 Arrivals (Coffee/tea available)
10.25–10.30 Welcome
10.30–10.50 Bernd Braunecker, Univ. St Andrews.
Coherent backaction between spins and an electronic bath:
Non-Markovian dynamics and low temperature quantum
thermodynamic electron cooling
10.50–11.20 Mehul Malik, Heriot-Watt Univ.
Unscrambling Entanglement through a Complex Medium
11.20–11.40 Anette Messinger, Univ. Glasgow.
Coherence in the Jaynes-Cummings model
11.40–11.50 Break (coffee/tea available)
11.50–12.20 Dale Scerri, Heriot-Watt Univ.
Fundamental Limits to Coherent Photon Generation with
Solid-State Atom-Like Transitions
12.20–13.05 Chris Hooley, Univ. St Andrews.
Many-body localisation and slow entanglement growth in
a disorder-free model
13.05–14.00 Lunch – Physics Foyer
14.00–14.35 Daniel Oi, Univ. Strathclyde.
Satellite Quantum Key Distribution
14.35–14.55 Matthew Thornton, Univ. St Andrews.
CV quantum digital signatures over insecure channels
14.55–15.15 Mahshid Delavar, Univ. Edinburgh.
Security analysis of Quantum Physical Unclonable Functions
15.15-15.30Sivert Aasnaess, Univ. Oxford
Contextuality as a resource for computation
Open QuestionsPresentations
Staff members who have not spoken yet are asked to say
a few words on future directions, ongoing work,
interesting future directions, open questions which
intrigue them
(5-10 min each roughly).
Open QuestionsDiscussion. (Coffee/tea available)
After brief presentations from group leaders (some
during the scheduled talks, some in the first part of
discussion session) we break up in 4 groups in 4 corners
of the room for an open discussions. We want to ensure
the good mixing of Universities and breaking of the usual
constellations. The idea is to get to know the overarching
themes, interesting challenges, open questions of common
interest. New ideas about possible collaborations, formal
or informal, may emerge. We also kindly ask not only staff,
but also research students to be active in these
discussions (please prepare: just think about
topics/questions/problems you find exciting and maybe
also thinking of working on). There will be tea/coffee to
make the discussions more fruitful.
To the Pub

Contact: Viktor Nordgren


Unscrambling entanglement through a complex medium
Mehul Malik, Heriot-Watt Univ.

We demonstrate how a high-dimensionally entangled state can be used to measure the transmission matrix of a complex medium, which in turn allows us to transmit entanglement through it.

Many-body localisation and slow entanglement growth in a disorder-free model
Chris Hooley, Univ. St Andrews

It has been known since the late 1950s that a single quantum particle subject to a Hamiltonian with a random element (called “disorder”) often becomes spatially localised.  In the past 13 years it has been realised that this phenomenon can also occur in many-body quantum systems.  In this case the localisation is in the system’s many-body Hilbert space, and is thus called many-body localisation (MBL).  It can be viewed as a failure of thermalisation; unlike other failures of thermalisation, however (e.g. integrability), it is robust to small changes in the Hamiltonian.

Even in the many-body localised state the entanglement entropy across a bipartition of the system continues to show slow (logarithmic) growth as a function of time [1].  This was neatly explained as a many-body dephasing effect by Serbyn, Papic, and Abanin [2].  However, there is nothing in their analysis that requires the localisation to arise from disorder.  Motivated by this, we have recently shown the existence of both many-body localisation and slow entanglement growth in a model with no random components [3].

In this talk I shall describe the conventional tests for localisation, including many-body localisation, and present the numerical evidence and analytic explanation for the slow growth of entanglement entropy in the many-body-localised phase.  I shall then present our disorder-free model, showing that it exhibits very similar signatures to those seen in the disordered case, and discuss what we should make of this fact.

[1] J. H. Bardarson, F. Pollmann, and J. E. Moore, Phys. Rev. Lett. 109, 017202 (2012).
[2] M. Serbyn, Z. Papić, and D. A. Abanin, Phys. Rev. Lett. 110, 260601 (2013).
[3] M. Schulz, C. A. Hooley, R. Moessner, and F. Pollmann, Phys. Rev. Lett. 122, 040606 (2019).

Fundamental limits to coherent photon generation with solid-state atom-like transitions
Dale Scerri (taken from the related paper submission) Heriot-Watt Univ.

(Taken from arXiv article with same title; to be published in PRL)
Coherent generation of indistinguishable single photons is crucial for many quantum communication and processing protocols. Solid-state realizations of two-level atomic transitions or three-level spin-Λ systems offer significant advantages over their atomic counterparts for this purpose, albeit decoherence can arise due to environmental couplings. One popular approach to mitigate dephasing is to operate in the weak excitation limit, where excited state population is minimal and coherently scattered photons dominate over incoherent emission. Here we probe the coherence of photons produced using two-level and spin-Λ solid-state systems. We observe that the coupling of the atomic-like transitions to the vibronic transitions of the crystal lattice is independent of driving strength and detuning. We apply a polaron master equation to capture the non-Markovian dynamics of the ground state vibrational manifolds. These results provide insight into the fundamental limitations for photon coherence from solid-state quantum emitters, with the consequence that deterministic single-shot quantum protocols are impossible and inherently probabilistic approaches must be embraced

CV quantum digital signatures over insecure channels
Matthew Thornton,
Univ. St Andrews

(Taken from PRA article with same title)
Digital signatures ensure the integrity of a classical message and the authenticity of its sender. Despite their far-reaching use in modern communication, currently used signature schemes rely on computational assumptions and will be rendered insecure by a quantum computer. We present a quantum digital signatures (QDS) scheme whose security is instead based on the impossibility of perfectly and deterministically distinguishing between quantum states. Our continuous-variable (CV) scheme relies on phase measurement of a distributed alphabet of coherent states and allows for secure message authentication against a quantum adversary performing collective beamsplitter and entangling-cloner attacks. Crucially, in the CV setting we allow for an eavesdropper on the quantum channels and yet retain shorter signature lengths than previous protocols with no eavesdropper. This opens up the possibility to implement CV QDS alongside existing CV quantum key distribution platforms with minimal modification.

Security analysis of Quantum Physical Unclonable Functions
Mahshid Delavar, Univ. Edinburgh

Physical Unclonable Functions (PUFs) are physical devices that have unique behavior which is hard to clone. These hardware structures are considered as an effective and feasible security primitive. The application of a wide variety of PUF structures for different security purposes such as identification and key generation has been widely studied in the context of Classical PUFs. In addition, the quantum-readout PUF (QR-PUF) has been studied as a proposition for a quantum version of classical PUFs. However, the formal definition of Quantum PUF and its security analysis in a standard security model are missing. In this project, we have filled this gap by formally defining the notion of Quantum PUF and presenting a quantum game-based framework for analyzing its security. We have focused on the most important security properties of QPUFs, i.e. unforgeability and shown although they do not provide strong unforgeability notions, they provide the required unforgeability property for being applied in different use cases. We also discuss the differences between the security of classical and quantum PUFs.