Advancing the frontiers of quantum science and engineering

Abstract: We present the National Quantum-Safe Network (NQSN) Testbed, a resilient, interoperable quantum-safe network built on a star-mesh topology with over 1500 km of fiber infrastructure. The testbed supports multi-protocol quantum key distribution (QKD), post-quantum cryptography (PQC) and accommodates diverse applications from multiple vendors. In this talk, we discuss how interoperability is enabled by a centralized key and network management system, ensuring seamless integration across heterogeneous technologies. Next, we highlight reference applications such as quantum-secured critical infrastructure, multi-hops multi-vendors key-relay, QKD traffic switching, hybrid QKD–PQC encryption, and multi-layer OSI integration. The testbed also supports field research in quantum networks, showcasing the practical implementation of quantum-safe technologies within complex environments. We also present briefly other testbed activities, including local & international standardisation efforts, ecosystem building and other related quantum-safe communication initiatives in Singapore.

Abstract: Atomtronics is the science of manipulating atoms in complex potentials much like electrons in electronic circuits. Mattewaves consisting of atoms are promising candidates for the realization of extremely sensitive sensors for rotation or acceleration. They could also form the basis for complex atom-circuit quantum computers. Indeed, some of the most sensitive and precise measurements to date of gravity, inertia, and rotation are carried out using matter-wave interferometry albeit with free-falling atomic clouds. A critical requirement to achieve these very high sensitivities is the long interaction times, which consequently leads to experimental apparatus up to a hundred meters tall or the requirement for experiments to be performed in microgravity in space. To tackle this problem, the gravitational acceleration must be cancelled, e.g. by manipulating atomic waves in time-changeable traps and waveguides. In the past, we demonstrated (near) perfect smooth and controllable matter-wave guides by transporting Bose-Einstein condensates (BECs) over macroscopic distances without any observable heating or decoherence their internal quantum states. A neutral-atom accelerator ring was utilized to bring BECs to very high speeds (up to 40 times their sound velocity) and transport them in a magnetic matter-wave guide for 15 centimeters whilst fully preserving their internal coherence. If this represents a ‘perfect’ waveguide, then what is a non-perfect waveguide? How much imperfection can a waveguide tolerate before it starts to harm quantum-transport? In this presentation, we will introduce the basics of TAAP waveguides and explore experimentally how strong an obstacle has to be to disturb the travelling matterwave. We propose a simple fundamental quantum limit, which depends only on the transverse confinement of the atoms.

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 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.