Advancing the frontiers of quantum science and engineering

Abstract: In this talk, I will introduce quantum states over time (QSOT), a formalism for describing quantum systems over space-time. I will begin by reviewing how QSOT has emerged in the literature. While conventional density operator formalism has been effective across many areas of quantum information theory, QSOT was developed to meet more specialized research needs— most notably, as a key ingredient to develop a quantum version of Bayes’ theorem. I will end the first part of my talk by comparing various QSOT that have been proposed. In the second part, I will discuss the causal compatibility problem as application of QSOT. I will focus on the temporal compatibility problem, which asks the following: from correlations in measurement outcomes alone, can two otherwise isolated parties establish whether such correlations are atemporal (i.e., temporally incompatible)? That is, can they rule out that they have been given the same system at two different times? I will first explain how characterizing measurement statistics in a causal agnostic scenario is equivalent to characterizing a specific type of QSOT, known as pseudo-density operators. I will then present our recent findings obtained by analyzing pseudo-density operators; In particular, we demonstrate that atemporality is distinct from entanglement, though they appear to be equivalent at first glance. Specifically, we show atemporality implies entanglement, but not vice versa, thus revealing that atemporality is a strictly stronger form of quantum correlations than entanglement. Nevertheless, we also find that sufficiently strong entanglement does imply atemporality.

Abstract: Classical cryptography, which often involves pre-shared secret keys, is vulnerable to malicious parties who employ various methodologies to gain unauthorized access to the keys, underlying the safety of our network. Quantum cryptography, such as Quantum Key Distribution (QKD), leverages the no-cloning property of quantum mechanics to improve the security level without the need for pre-distributed secret keys. Extending beyond secret keys sharing, special relativity can also be incorporated to authenticate the geographical location of the node within the network, a method called Quantum Position Verification (QPV). In this talk, I focus on the development of hardware platforms specifically designed for free-space photon-polarization realization of QPV at Harvey Mudd College. Our objective is to provide a proof-of-concept demonstration of a QPV network operating over a kilometer range. The proposed demonstration includes two verifier nodes and one prover node, the position of which is verified using QPV. We utilize a free-space polarization photonic qubit link, accompanied with wavelength-multiplexed classical communication pathways, to perform BB84-type transactions. In addition, we employ the Xilinx Zynq FPGA platform to control and maintain the timing of these transactions to verify the prover node’s position relative to each verifier node. Our photon source has demonstrated a qubit polarization basis transition in under 20 ns, while the classical communication link has achieved a bit-error-rate below one-in-a-million at 10 MHz over a 10-meter separation. We are currently working on refining the polarization qubit generation and measurement methodologies to obtain a higher fidelity BB84-type qubit. At the same time, we are extending the range of our classical link moving closer to the kilometer range while examining potential higher data transmission speeds up to 25 MHz.

Abstract: I give an overview of several recent and ongoing works, all unified by taking information-theoretic perspectives on many-body quantum systems. I first discuss our work on memory loss in quantum computers; wherein the uncharacterized native dynamics of a device frustrates it's ability to locally retain information over time. We develop a protocol for quantifying information loss, and deploy it in a comprehensive analysis of IBM devices. Secondly I discuss our random-matrix treatment of quantum state tomography errors; a novel toolkit which allows for direct evaluation of mean error rates and complexity bounds. Finally, I discuss how these developments feed forward into my ongoing work on causation and manipulation, with an eye towards developing practical quantum counterfactual accounts of causation for deployment in experimental and technological setttings.

Abstract: Quantum memory based on ensembles of two-level systems is a promising platform for scalable quantum information processing and communication. However, achieving high efficiency requires operating in the regime of high collective cooperativity, which can also lead to increased noise in the output state due to amplified spontaneous emission. Recently, experiments have observed that the use of chirped pulses, which are widely employed for high-fidelity adiabatic population inversion, can significantly suppress this noise.   In this work, we develop a clean theoretical model of photon-echo-based quantum memory protocols using chirped pulses, focusing in particular on the revival of silenced echo (ROSE) protocol and the rephased amplified spontaneous emission (RASE) protocol. By taking into account both inhomogeneous broadening and inhomogeneity in the coupling strengths, we provide a complete explanation of the observed noise suppression. Our theoretical modelling clarifies the dynamics of these protocols and provides guidance for the design of future protocols.

Abstract: Quantum error correction offers a path to practical quantum computing by combining multiple physical qubits into a single logical qubit. The central promise of this approach relies on the "threshold theorem": if physical error rates can be reduced below a critical threshold value, increasing the number of physical qubits (the code distance) should result in an exponential suppression of the logical error rate. In this talk, I will describe the first experimental demonstration of this foundational principle using Willow, a new generation of superconducting quantum processors from Google Quantum AI. As we scale the surface code from distance 3 to 7, the logical error rate is exponentially suppressed, culminating in a logical memory that persists longer than the coherence time of all constituent physical qubits. I will also discuss the engineering challenges required to achieve this performance. More information about this work can be found in Nature 638, 920–926 (2025).

ACTI will bring together the growing community of ion-trappers from Asia and across the globe. As an inaugural conference, this year will highlight various scientific fields covered by trapped ion research within Asia and facilitate meaningful exchange between junior and senior members of the different groups.

The goal of this workshop is to bring the active research community together to discuss the progress, challenges and new possible directions for quantum repeaters and networks. We invite researchers working on key enabling technologies and system integration, protocols for connecting repeaters across network links with high fidelity, architectures for large-scale networks, and applications of distributed quantum entanglement. The WQRN has been and continues to be a limited-attendance, few-day workshop, in which participants are given the opportunity to share ideas, directions, and concerns to advance the state of the field. Talks are by invitation only and catered to broadly cover the applications, theory, components, and worldwide directions in quantum repeaters and networks. These talks frame group discussion sessions for which all participants are welcome to contribute; participants are also welcome to contribute to a poster session.