Seminars

The talk is based on the following preprints:

• arXiv:2301.10727 Contra Bellum: Bell’s theorem as a confusion of languages
• arXiv:2105.12728 Imitating quantum probabilities: Beyond Bell’s theorem and Tsirelson bounds
• arXiv:2010.03366 Unifying Aspects of Generalized Calculus
• arXiv:2004.04097 Arithmetic loophole in Bell’s theorem: An overlooked threat to entangled-state quantum cryptography

With the prospect of reaching hundreds of fault-tolerant qubits by the end of the decade, scaling up quantum computing infrastructure is as much of a theoretical as a practical challenge. Against the backdrop of progress in hardware, simple yet effective techniques are needed to address the question of how well our quantum computers can perform their intended task: implementing one, and two-qubit gates within a given computational model or architecture. In this talk, I will define the problem of fidelity estimation, and provide a common approach for solving it — the Direct Fidelity Estimation algorithm, originally devised for the Clifford + T model and recently extended for matchgate computations.

In the first part of the talk, I will outline the concept of Channel Fidelity and the closely-related Entanglement Fidelity of quantum processes. After introducing the necessary tools for estimating these quantities (via the Pauli-Liouville representation of quantum channels), I will present the Direct Fidelity Estimation (DFE) algorithm of Flammia & Liu [PRL 106, 230501], a commonly used technique for certifying quantum processes. In the second part I will also introduce Matchgates, a family of gates capable of universal quantum computation that is native to many quantum computing architectures. After modifying the DFE algorithm, we will arrive at Matchgate Fidelity Estimation [arXiv:2404.07974], a variant of DFE capable of benchmarking Matchgate circuits in a more favourable runtime.

This talk is based on arXiv:2404.07974, and will assume minimal knowledge of channel certification or benchmarking techniques.

The learner’s ability to generate a hypothesis that closely approximates the target function is crucial in machine learning. Achieving this requires sufficient data; however, unauthorized access by an eavesdropping learner can lead to security risks. Thus, it is important to ensure the performance of the “authorized” learner by limiting the quality of the training data accessible to eavesdroppers. Unlike previous studies focusing on encryption or access controls, we provide a theorem to ensure superior learning outcomes exclusively for the authorized learner with quantum label encoding. In this context, we use the probably-approximately-correct (PAC) learning framework and introduce the concept of learning probability to quantitatively assess learner performance. Our theorem allows the condition that, given a training dataset, an authorized learner is guaranteed to achieve a certain quality of learning outcome, while eavesdroppers are not. Notably, this condition can be constructed based only on the authorized-learning-only measurable quantities of the training data, i.e., its size and noise degree. We validate our theoretical proofs and predictions through convolutional neural networks (CNNs) image classification learning.

Gottesman-Kitaev-Preskill (GKP) states provide a promising route to fault-tolerant photonic quantum computing. In this talk, I will give a brief overview of GKP-based photonic quantum computing, highlighting its advantages and limitations. I will then discuss the challenges associated with classical simulations of multi-mode photonic systems and propose a potential solution to address these difficulties.

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.