Seminars

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.

In quantum theory general measurements are described by so-called Positive Operator-Valued Measures (POVMs). In this work we show that in d-dimensional quantum systems an application of depolarizing noise with constant (independent of d) visibility parameter makes any POVM simulable by a randomized implementation of projective measurements that do not require any auxiliary systems to be realized. This result significantly limits the asymptotic advantage that POVMs can offer over projective measurements in various information-processing tasks, including state discrimination, shadow tomography or quantum metrology. We also apply our findings to questions originating from quantum foundations. First, we asymptotically improve the range of parameters for which Werner and isotropic states have local models for generalized measurements (by factors of d and log(d) respectively). Second, we give asymptotically tight (in terms of dimension) bounds on critical visibility for which all POVMs are jointly measurable. On the technical side we use recent advances in POVM simulation, the solution to the celebrated Kadison-Singer problem, and a method of approximate implementation of a class of “nearly rank one” POVMs by a convex combination of projective measurements, which we call dimension-deficient Naimark extension theorem. The talk will be based on upcoming joint work with Michał Kotowski (MIM UW)

The combined action of a DC bias and a microwave drive on the transport characteristic of a superconductor-quantum dot-superconductor junction is investigated. To cope with time dependent non-equilibrium effects and interactions in the quantum dot, we develop a general formalism for the dynamics of the density operator based on a particle conserving approach to superconductivity. Without invoking a broken U (1) symmetry, we identify a dynamical phase connected to the coherent transfer of Cooper pairs across the junction. In the weak coupling limit, we show that besides quasiparticle transport, proximity induced superconducting correlations manifest in anomalous pair tunneling involving the transfer of a Cooper pair. The resulting generalized master equation in presence of the microwave drive showcases the characteristic bichromatic response due to the combination of the AC Josephson effect and an AC voltage. Analytical expressions for all harmonics in the driving frequency of both the current and the reduced dot operator are given for arbitrary driving strength. For the net DC current the resulting photon assisted processes give rise to rich current-voltage characteristics. In addition to photon assisted subgap transport we find regions of total current inversion in the stability diagram. There, the junction acts as a pump with the net DC current flowing against the applied DC bias. The first harmonic of the current, being closely related to the nonlinear dynamic susceptibility of the junction, is discussed at finite applied DC bias.

The transfer of quantum information between different locations is key to many quantum information processing tasks. Whereas, the transfer of a single qubit state has been extensively investigated, the transfer of a many-body system configuration has insofar remained elusive. We address the problem of transferring the state of n interacting qubits [1]. Both the exponentially increasing Hilbert space dimension, and the presence of interactions significantly scale-up the complexity of achieving high-fidelity transfer. By employing tools from random matrix theory and using the formalism of quantum dynamical maps, we derive a general expression for the average and the variance of the fidelity of an arbitrary quantum state transfer protocol for n interacting qubits. We find that the average fidelity decreases with the amount and the type of entanglement in the sender state [2]. Finally, by adopting a weak-coupling scheme in a spin chain, we obtain the explicit conditions for high-fidelity transfer of 3 and 4 interacting qubits.
[1] Tony J G Apollaro et al, Quantum transfer of interacting qubits, 2022 New J. Phys. 24 083025
https://iopscience.iop.org/article/10.1088/1367-2630/ac86e7
[2] Tony J G Apollaro et al, Entangled States Are Harder to Transfer than Product States, Entropy 2023, 25(1), > 46;
https://doi.org/10.3390/e25010046

What does it mean for a causal structure to be “unknown”? Can we even talk about “repetitions” of an experiment without prior knowledge of causal relations? And under what conditions can we say that a set of processes are independent and identically distributed (i.i.d.)? Similar questions for classical probabilities, quantum states, and quantum channels are beautifully answered by “de Finetti theorems”, which connect a simple and easy-to-justify condition—symmetry under exchange—to a very particular multipartite structure: a mixture of identical states/channels. Practically, they provide the foundations for principle-based Bayesian methods, e.g., in tomography. Apart from the foundational relevance, de Finetti representations for general causal structures would be useful in the analysis of multi-time, non-Markovian processes, with applications to state-of-the-art quantum devices.

At face value, it appears that each causal structure or assumption on causal structure requires its own de Finetti theorems. Fortunately, I will show that each scenario can be mapped to a linear constraint on quantum states. By proving a de Finetti representation for states subject to a sufficiently large class of constraints, we can derive all the desired results for a broad class of processes.