Events

Abstract: We investigate two recently proposed witnesses of non-Markovianity -- one based on temporal correlations in the pseudo-density matrix (PDM) and the other on temporal steering -- to assess their ability to detect information backflow. Using counterexamples from existing literature, we demonstrate that both witnesses can reveal non-Markovian behavior even when traditional measures, such as trace distance and entropic distinguishability, fail. We further show that the PDM, being directly related to the Choi matrix via partial transposition, can be extended to quantify the total quantum memory in any indivisible processes. Interestingly, we find that the temporal steering-based measure may fail to capture eternal non-Markovianity and is not necessarily proportional to Choi matrix-based methods. In contrast, the PDM-based measure introduced here faithfully captures such memory effects which may not induce information backflow. Our results highlight a fundamental distinction between weak and strong forms of quantum direct cause in the context of information backflow in open quantum system dynamics.

References:

1. Heinz-Peter Breuer, Elsi-Mari Laine, Jyrki Piilo, and Bassano Vacchini., Rev. Mod. Phys, 88(2):021002, 2016.

2. Shin-Liang Chen, Neill Lambert, Che-Ming Li, Adam Miranowicz, Yueh-Nan Chen, and Franco Nori, Physics Review Letters, 116(2):020503, 2016.

3. Huan-Yu Ku, Shin-Liang Chen, Neill Lambert, Yueh-Nan Chen, and Franco Nori, Physical Review A, 98(2):022104, 2018.

4. S. Utagi, Physics Letters A, 386:126983, 2021.

5. U. Shrikant and P. Mandayam, Frontiers in Quantum Science and Technology, 2, 2023.

6. S. Utagi and P. Chawla, arXiv:2506.23267 (2025).

Abstract: Quantum nonlocality—once a challenge to local realism—has become central both to the foundations of physics and to emerging quantum technologies. In this talk, I will discuss manifestations of quantum nonlocality in three distinct settings. The first is the well-explored domain of spatial correlations, where nonlocality has long been recognized as a useful resource. Focusing on quantum steering, we build upon previous results and show that all pure bipartite maximally entangled states can be robustly certified in a semi-device-independent framework, establishing steering as a practical approach for entanglement verification. The second setting shifts from space to time, examining correlations between sequential measurements on the same system. When such correlations violate macrorealism, they reveal a temporal form of non-classicality—yet practical applications in this domain have remained scarce. Here, a protocol is developed to certify unitarity of quantum channels using temporal correlations alone, expanding the reach of nonlocality-based certification methods beyond spatial scenarios. Finally, the third setting turns to many-body chaotic dynamics, where I describe a previously unknown manifestation of nonlocality that emerges in the phenomenon of deep thermalization. Considering a maximally chaotic system, the rate at which a small subsystem deep thermalizes is shown to depend on boundary conditions lying outside the Lieb-Robinson light cone, i.e., beyond the region accessible by any finite-speed information propagation. This establishes deep thermalization as a qualitatively new, intrinsically nonlocal process, distinct from conventional equilibration. Together, these results chart how nonlocality extends across spatial, temporal, and many-body dynamical contexts, broadening its role as both a fundamental concept and a practical resource in quantum theory.

Abstract: Non-local quantum computation (NLQC) replaces a localinteraction between two systems with a single round of communication and sharedentanglement. It arises naturally in the analysis of attacks on quantumposition verification (QPV). Despite many partial results, it is known that acharacterization of entanglement cost in at least certain NLQC tasks wouldimply significant breakthroughs in complexity theory. Here, we avoid theseobstructions and take an indirect approach to understanding resource requirementsin NLQC, which mimics the approach used by complexity theorists: we study therelative hardness of different NLQC tasks by identifying resource efficientreductions between them. Most significantly, we prove that f-measure (the taskof measuring in a certain BB84 basis according to a classical function f) andf-route (the task of routing a qubit according to a classical function f), thetwo best studied NLQC tasks, are in fact equivalent under O(1) overheadreductions. This result simplifies many existing proofs in the literature asresults were often proved separately for the two, it also extends several newproperties to f-measure that were only known for f-routing. For instance, weobtain sub-exponential upper bounds on f-measure for all functions.This talk is based on joint work with the same title withAndreas Bluhm, Simon Höfer, Alex May, Mikka Stasiuk and Henry Yuen https://arxiv.org/abs/2505.23893

Abstract: Implementing a quantum circuit on specific hardware with a reduced available gate set is often associated with a substantial increase in the length of the equivalent circuit. This process is also known as transpilation and due to decoherence, it is mandatory to keep quantum circuits as short as possible, without affecting functionality. In this work we propose three different transpilation approaches, based on a localized term-replacement scheme, to substantially reduce circuit lengths while preserving the unitary operation implemented by the circuit. The first variant is based on a stochastic search scheme, and the other variants are driven by a database retrieval scheme and a machine learning based decision support. We show that our proposed methods generate short quantum circuits for restricted gate sets, superior to the typical results obtained by using various qiskit and BQSKit optimization levels. Our method can be applied to different gate sets and scales well with an arbitrary number of qubits.

Abstract: Coherent states conveniently represent the classically meaningful wavelike states of light, as opposed to the corpuscular number states, but superposingcoherent states is mind-bogglingly unclassical, including as examples "cat states", "comb states" and "compass states". I present a history of superposing coherent states, especially for oscillators (including electromagnetic field states) and for spin, and extend to entangled coherent states. I then show that superposed coherent states are a viable path for photonic quantum computing and for reaching towards the ultimate limits of sensing. Finally, I discuss our experimental realisations (at University of New South Wales) of superposed coherent states in a spin-7/2 Antimony nucleus in a silicon substrate.

Abstract: This talk traces the development of quantum physics throughout the 20th century, emphasizing its intricate entanglement with economic, political, and cultural contexts. Unexpected associations—linking streetlights to Planck’s quantum of action, lasers to ballet dancers, and timekeeping to ion traps—serve as narrative threads illustrating the multifaceted nature of quantum discoveries. These examples, drawn from the “Light and Matter” exhibition at the Deutsches Museum in Munich, Germany, offer historical vantage points from which to reflect on the evolving landscape of quantum technologies today.

The workshop concerns both the theoretical and experimental aspects of this subject. This workshop aims to build connections between young scientists and promotes open discussions between participants. As such, 30-minute talks by all participants will be held at the beginning of the week, leaving two full days open for discussions on the presented work (and beyond).

Please note that participation in this workshop is by invitation only.

Quantum Techniques in Machine Learning (QTML) is a leading international conference at the forefront of quantum science and machine learning. Held annually, it brings together researchers and industry experts to explore how quantum computing can transform learning, optimisation, and data-driven discovery. CQT is proud to support planning for the ninth edition of this international conference, QTML 2025, to take place in Singapore 16-21 November. Registration will open from 11 August.