Seminars

The usual explanation of the quantum computing speedup is parallel calculation, inserting a superposition of all possible input values into the computation. The picture painted is that the machine performs a calculation for every input, perhaps in multiple parallel worlds, and then combines the outcomes of each individual calculation by interference to obtain the desired answer. This explanation has become standard, but does not give any guidance on how to identify new mathematical problems that could be solved, and how to design algorithms to solve them. New developments seem to need another explanatory model.

An alternative is tracing the calculation in phase space, a standard tool in classical mechanics but more challenging to use in the quantum realm. The reason is that the Liouville distribution, the probability distribution over classical phase space, becomes the Wigner function in quantum mechanics, a quasi-probability distribution that is not always positive. A negative Wigner function has been linked to presence of quantum contextuality, a behavior only seen in quantum systems.

This presentation will introduce a description of the phase space of a restriction of quantum mechanics, that generates a positive distribution but still reproduces the contextual behavior of quantum systems. The description is Einstein-complete, but distinct from Bohmian mechanics. We will briefly see how this can be used to give a better explanation of the quantum speedup, but also use this new description as a tool for reasoning about more generic interpretational issues within the foundations of quantum mechanics.

extended abstract (pdf file)

What is the dimension of a quantum ensemble? The standard answer is to count how many classical states are superposed, but that depends entirely on the reference frame of the observer. I discuss a different approach to the dimensionality of quantum systems which is independent of the frame in which we view the quantum state. I then take these ideas further and address the question of when a set of quantum states can be considered classical. The standard answer is if they commute. I propose a stronger (frame-independent) notion of classicality and discuss its consequences for the quantumness of noisy ensembles.

The intersection between quantum mechanics and gravitational physics has been providing challenging puzzles for quite some time. In this presentation I discuss the dynamics of an open quantum system coupled with a bath of gravitons, the quanta of the gravitational field in the linear limit of general relativity. The focus is on two main aspects. First, I analyze the decoherence induced by gravitons when considering the open system to be described by both external and internal degrees of freedom. Since gravity is universal, the internal variables also interact with the gravitons, and it is shown that this interaction cannot be neglected in gravitational decoherence models as it turns out to be responsible for greater contributions to the decoherence rate once we treat both gravitons and internal degrees of freedom as environments. I then proceed to the second main aspect, which is the entropy production that arises when an external agent drives a quantum system through the graviton bath. This irreversibility comes from quantum fluctuations of spacetime itself and, as such, has a fundamental universal aspect.

Photonics is a key enabler for quantum systems, providing both core quantum functionality and the means by which quantum effects can be achieved, expressed, combined and utilized. In this talk, long distance quantum communications and silicon based photonic quantum process research activities of ETRI will be presented with short video introduction of ETRI organization.

Quantum money is the first invention in quantum information science, promising advantages over classical money by simultaneously achieving unforgeability, user privacy, and instant validation. However, standard quantum money relies on quantum memories and long-distance quantum communication, which are technologically extremely challenging. Quantum “S-money” tokens eliminate these technological requirements while preserving unforgeability, user privacy, and instant validation. Here, we report the first full experimental demonstration of quantum S-tokens, proven secure despite errors, losses and experimental imperfections. The heralded single-photon source with a high system efficiency of 88.24% protects against arbitrary multi-photon attacks arising from losses in the quantum token generation. Following short-range quantum communication, the token is stored, transacted, and verified using classical bits. We demonstrate a transaction time advantage over intra-city 2.77 km and inter-city 60.54 km optical fibre networks, compared with optimal classical cross-checking schemes. Our implementation demonstrates the practicality of quantum S-tokens for applications requiring high security, privacy and minimal transaction times, like financial trading and network control. It is also the first demonstration of a quantitative quantum time advantage in relativistic cryptography, showing the enhanced cryptographic power of simultaneously considering quantum and relativistic physics. Based on arXiv:2408.13063. Work in collaboration with Yang-Fan Jiang, Adrian Kent, Xiaochen Yao, Xiaohan Chen, Jia Huang, George Cowperthwaite, Qibin Zheng, Hao Li, Lixing You, Yang Liu, Qiang Zhang and Jian-Wei Pan. If there is time I will also briefly discuss the multiphoton attacks loophole (PRX Quantum 2, 030338, (2021)) that applies to various previous implementations of mistrustful quantum cryptography, which we have closed in our quantum tokens implementation.

Quantifying entanglement between two regions is particularly elusive in the context of quantum field theory (QFT), mainly because the Hilbert space of a QFT does not factor as a tensor product of Hilbert spaces associated with different regions of spacetime. In this talk, I will adapt tools from Gaussian quantum information theory to analyze entanglement in subsystems made of finitely many field degrees of freedom, in a free scalar theory in D + 1-dimensional Minkowski spacetime. Applying these techniques, I will argue that while entanglement between localized individual modes is limited, bipartite multimode entanglement in quantum field theory is ubiquitous.

The ability to characterise and discern quantum channels is a crucial aspect of noisy quantum technologies. In this work, we explore the problem of distinguishing quantum channels when limited to sub-exponential resources, framed as von Neumann (projective) measurements. We completely characterise equivalence classes of quantum channels with different Kraus ranks that have the same marginal distributions under compatible projective measurements. In doing so, we explicitly identify gauge freedoms which can be varied without changing those compatible marginal outcome distributions, opening new avenues for quantum channel simulation, variational quantum channels, as well as novel adversarial strategies in noisy quantum device certification. Specifically, we show how a Sinkhorn-like algorithm enables us to find the minimum admissible Kraus rank that generates the correct outcome marginals. For a generic d-dimensional quantum system, this lowers the Kraus rank from d^2 to the theoretical minimum of d. For up to d = 20, we numerically demonstrate our findings, for which the code is available and open source. Finally, we provide an analytic algorithm for the special case of spoofing Pauli channels.