Events

Abstract: To achieve a fault-tolerant quantum computer, it is crucial to increase the coherence time of quantum bits. In this work, we theoretically investigate a system consisting of a series of superconducting qubits that alternate between XX and YY ultrastrong interactions. By considering the two-lowest energy eigenstates of this system as a logical qubit, we demonstrate that its coherence is significantly enhanced: both its pure dephasing and relaxation times are extended beyond those of individual physical qubits. Specifically, we show that by increasing either the interaction strength or the number of physical qubits in the chain, the logical qubit's pure dephasing rate is suppressed to zero, and its relaxation rate is reduced to half the relaxation rate of a single physical qubit. Single qubit and two-qubit gates can be performed with high intrinsic fidelity.   Reference: "Noise protected logical qubit in an open chain of superconducting qubits with ultrastrong interactions." npj Quantum Information (2026).

Abstract: Carmen de Matos (1999) is a visual artist based in Badajoz (Spain). She explores the concepts of memory, identity and heritage through multidisciplinary work. She combines traditional techniques such as painting and ceramics with audiovisual elements (photography and video). During the talk, she will show us some of her projects and some of her pieces.

Abstract: Quantum resource theory has sharpened our understanding of the intrinsic complexity of quantum systems, particularly their classical simulability. However, it remains unclear which quantum resource governs the classical learnability of quantum circuits, especially beyond the regime of efficient classical simulation. Here we close this knowledge gap by studying the expectation-value functions of families of tunable quantum circuits, with many applications in digital quantum simulation, quantum metrology, and quantum-system characterization. Specifically, we introduce a new resource measure, the dynamical stabilizer entropy (DSE), which quantifies how broadly an expectation-value function is distributed across its frequency modes. By relating DSE to operator stabilizer entropy, we establish a computational phase diagram that compares classical simulators with quantum-data-assisted classical surrogates. We first determine the DSE-dependent learnability boundary of this diagram by deriving bounds on the sample complexity and runtime of classical surrogates, and by developing a DSE-guided surrogate. We then complete the diagram by proving, under standard complexity-theoretic assumptions, the existence of circuit families that can be efficiently learned by this surrogate but cannot be efficiently emulated from their circuit descriptions alone. Numerical experiments on random and structured circuits with up to 80 qubits support the predicted DSE-dependent computational landscape. These results establish a quantitative resource-theoretic framework for delineating the boundary between classical simulation and learning, motivate resource measures linking quantum resources to learnability, and guide the design of learning-based algorithms for scalable quantum systems beyond the reach of direct classical simulation.

Abstract: Bayesian inference is central across the sciences, from statistical mechanics to machine learning, and is distinguished by its explicit use of prior knowledge. In quantum information theory, this has motivated extensive study of quantum generalizations of Bayes’ rule, with the Petz recovery map as a prominent example. However, the nature of the underlying quantum prior—the object being updated—has received comparatively little attention. It is commonly thought that the reduced density matrix fully represents a system’s state and thus its prior belief. We challenge this convention. While ρS suffices for prediction regarding the system (S), it does not capture all information relevant for Bayesian updating. In particular, correlations with inaccessible degrees of freedom can affect inference—an effect that is invisible in classical probability theory. To address this, we represent a quantum belief as a joint state of the system and a hidden reference system, and develop a framework for quantum Bayesian inference based on this structure. We also characterize when two such beliefs are operationally equivalent. Finally, we briefly extend the notion of belief equivalence to repeated evidence and multi-step updating.

The ninth edition of the Workshop on Quantum Technologies on Small Satellites will take place 19-20 October 2026 in Singapore. The two-day event will focus on (small) satellite technologies and quantum experiments in space – including both space-based and ground architectures, from component through to mission design. Participants are given the opportunity to share ideas, directions, and concerns to advance the state of the field. The workshop is co-hosted by the QTX research forum led by Alexander Ling of NUS and Markus Krutzik of Humboldt-Universität zu Berlin and Ferdinand-Braun-Institut, Leibniz-institut für Höchstfrequenztechnik. The forum met previously in Germany (2018, 2024), UK (2019), United Arab Emirates (2023) and online (2020, 2021).

Abstract: The nitrogen vacancy (NV) center in diamond exhibits spin-dependent fluorescence and long spin coherence times under ambient conditions, enabling applications in quantum information processing and sensing. NV centers near the surface can have strong interactions with external materials and spins, enabling new forms of nanoscale spectroscopy. However, NV spin coherence degrades within 100 nanometers of the surface, suggesting that diamond surfaces are plagued with ubiquitous defects. I will describe our recent efforts to correlate direct materials characterization with single spin measurements to devise methods to stabilize highly coherent NV centers within nanometers of the surface. We deploy these coherent shallow NV centers for a new nanoscale sensing technique, whereby we use covariance measurements of two or more NV centers to measure two-point magnetic field correlators. We demonstrate measurements of correlated applied noise using spin-to-charge readout of two NV centers and implement a spectral reconstruction protocol for disentangling local and nonlocal noise sources. We also demonstrate massively multiplexed magnetometry with high fidelity spin readout of hundreds of NV centers simultaneously. Finally, we develop protocols to use optically unresolved NV center pairs, entangled states of NV center pairs, and nuclear spins as multi-qubit sensors for measuring correlated noise, enabling covariance magnetometry at nanometer length scales. This novel quantum sensing platform will allow us to measure new physical quantities that are otherwise inaccessible with current tools, particularly in condensed matter systems where two-point correlators can be used to characterize charge transport, magnetism, and non-equilibrium dynamics. Our approach for correlating surface spectroscopy techniques with single qubit measurements to realize directed improvements is generally applicable to many systems. Separately, I will describe our recent efforts to tackle noise and microwave losses in superconducting qubits. Building large, useful quantum systems based on transmon qubits will require significant improvements in qubit relaxation and coherence times, which are orders of magnitude shorter than limits imposed by bulk properties of the constituent materials. This indicates that loss likely originates from uncontrolled surfaces, interfaces, and contaminants. Previous efforts to improve qubit lifetimes have focused primarily on designs that minimize contributions from surfaces. However, significant improvements in the lifetime of 2D transmon qubits have remained elusive for several years. We have recently fabricated 2D transmon qubits that have both lifetimes and coherence times exceeding 0.3 milliseconds by using tantalum as the material in the capacitor. Following this discovery, we have parametrized the remaining sources of loss in state-of-the-art devices using systematic measurements of the dependence of loss on temperature, power, and geometry. This parametrization, complemented by direct materials characterization, allows for rational, directed improvement of superconducting qubits. We have used this playbook to tackle the dominant sources of loss and noise to realize 2D transmons with coherence times exceeding 1 millisecond and lifetimes up to 1.68 milliseconds. Because we have improved the underlying material system without alteration to the qubit architecture, these qubits are readily translated to existing control schemes, and we demonstrate single qubit gates with 99.994% fidelity.

The Quantum Information Processing Conference (QIP) is the largest quantum information processing technology conference, which annually gathers the world’s brightest researchers and industry leaders for a week long program to present and discuss the latest achievements in quantum computing, cryptography, information theory, mathematics and physics. Recent editions have drawn nearly 1000 participants. In 2027, CQT will host the 30th edition of this conference in Singapore. QIP2027 will take place 20-26 February 2027.

The International Conference on Women in Physics (ICWIP) of the International Union of Pure and Applied Physics (IUPAP) will take place in the National University Singapore (NUS) during 7-11 June 2027, hosted by the Physics Department, NUS and the Centre for Quantum Technologies, Singapore. The ICWIP2027 is the 9th in a series of triennial conferences by IUPAP since 2002 that brings together practitioners from all over the world in order to deliberate on both topics in physics and processes within physics practice, with the goal of transforming physics into a welcoming inclusive enterprise for everyone. The conference will feature inspiring keynote talks by eminent women scholars, sessions where participants can present their own research in physics and on equity and inclusion in physics, interactive workshops to engage with the barriers to equity and inclusion in physics, call-to-action sessions on recommendations for the profession and more...