February, Wed 15th

Venue: C4 #02-01 multi-purpose room

Located above the air-con canteen @ Frontier, Science Drive 2, Faculty of Science

 

Presentation of Thales activities in quantum technologies

• Daniel Dolfi (with Arnaud Brignon and Frédéric Van Dau), Thales Research & Technology

Title: Quantum technologies for sensing, navigation, communications and signal processing

Abstract: Quantum technologies will have a strong impact on future navigation, sensing (for both RF and Optronics domains) and communication systems. Considering the field of “Positioning, navigation and timing”, these technologies will bring the capability, through different techniques, potentially hybridized, for any military platform, whatever the medium, to know its position with a better precision than GNSS, without GNSS. In the domain of “radio frequency sensing”, quantum technologies will provide radars and E.W systems with improved capabilities in terms of sensitivity, probability of intercept, dynamic range and frequency coverage/agility. In all cases, gains of at least one order of magnitude are targeted in order to perform longer range detection/identification/classification of “small” targets. 

Ultimately, quantum technologies will improve optronic systems in terms of sensitivity and resolution. Sensitivity down to photon-counting level together with operation in severe atmospheres are targeted in order to perform longer range detection, identification and classification functions.

In this presentation, we will review how these technologies can contribute to:

–    electromagnetic spectrum dominance through the use of SHB (spectral hole burning) based spectral holography and of NV (nitrogen vacancy) centers in diamond

–    highly sensitive RF detection thanks to SQIFs (superconducting quantum interference filters) and Rydberg atoms

–    highly accurate navigation in GNSS denied environments and to multi-static and distributed radar systems thanks to compact and precise atomic clocks and thanks to ultra-precise cold-atom based inertial sensors

–    neuromorphic processing of RF signals providing detection, identification and classification capabilities

–    secure communications

Extension of these concepts to magnetic anomaly detection (MAD), to fiber sensing (for both hydrophones and gyros) and to optronic systems (both active and passive) will also be detailed.

 

Quantum Communications & Cryptography

 

• Alex Ling, Centre for Quantum Technologies

Title: Updates on the space activity of SpooQyLab at CQT

Abstract: I will give an update on the latest analysis of scientific data observed on the SpooQy-1 mission, as well as discuss some of the next steps forward, such as the construction of an optical ground receiver on the NUS campus.

• Mathias Van den Bossche, Thales Research and Technology

Title: TBA

Abstract: TBA

• Divesh Aggarwal, Centre for Quantum Technologies

Title: Research trends in post-quantum cryptography

Abstract: In this talk, I will give a brief overview of how research in post-quantum cryptography has evolved over the last few decades. This includes both implementing standard primitives to make them more efficient, and to build more advanced cryptographic primitives. Then I will continue with a discussion on the emerging research directions in post-quantum cryptography, and whether it is important to transition to post-quantum cryptography right now.

• Christof Hirche, Centre for Quantum Technologies

Title: Benefits and Detriments of Noise in Quantum Classification

Abstract: Classification of data is a fundamental task in machine learning. While high accuracy is generally the main goal in classification, also other properties, such as good robustness, are often required. Intuitively, adding noise may sound like a questionable idea whenever one aims for high accuracy, but it was also shown that, in the classical case, it can lead to provable robustness guarantees at a manageable cost. In particular, adding any differentially private preprocessing can give such results. This approach has subsequently also been transferred to quantum classification, i.e. the task of classifying quantum states. Here, the depolarizing channel, as an epsilon-differentially private channel, was shown to give provable robustness guarantees. In this work, we revisit the question by significantly extending the previous results and also by discussing their limitations.  

Concretely, we give robustness bounds in terms of the Hockey-Stick divergence and Renyi divergences. These imply robustness against several types of adversarial noise and give rise to robustness guarantees when applying general (epsilon,delta)-differentially private or (epsilon,alpha)-Renyi differentially private noise. We then discuss the particular case of depolarizing noise and show that the robustness guarantees that can be given provide generally no improvement over the noiseless setting. However, such noise can still be used to improve the robustness of particular data subsets, which we call targeted smoothing. Additionally we discuss robust encoding of classical data.  

Finally, we discuss the effect of noise on the training process. In particular, we show that stable channels, a special case of differential privacy, show good generalization properties.

Quantum Interfaces

 

• Loïc Lanco, Centre for Nanosciences and Nanotechnologies

Title: Interactions between single spins and single photons

Abstract: The quest for efficient spin-photon interfaces is at the core of many proposals in quantum optics. On the one hand, it would allow developping deterministic spin-photon and photon-photon gates for quantum computing, as well as multi-photon « cluster states » for quantum communications. On the other hand, it would allow implementing interesting fundamental studies on quantum measurement, whereby each photon can be used as a « meter » of the spin system.

In this presentation, I will discuss the recent progress made in C2N, using spin-photon interfaces based on charged quantum dots in pillar-based cavities. I will show how one can entangle successively-emitted photons with the same electron spin, and how can use use an electron spin to conditionally  reverse the polarization state of reflected photons. 

 

• Steven Touzard, Centre for Quantum Technologies

Title: Building quantum networks of superconducting circuits mediated by telecom photons

Abstract: Quantum networks consist of several nodes of quantum processors that can communicate over long-distances, for example through commercial telecom fibers. Superconducting circuits are one of the most advanced technologies to construct the quantum processors at the nodes. However, these processors operate at microwave frequencies and cannot directly make use of the existing telecommunication infrastructure to be linked to one another. The only piece still missing for quantum networks to become a reality is a device that entangles superconducting circuits with traveling telecom photons.

In this talk, I will first review the working principles of superconducting circuits as well as recent progress towards making practical quantum processors. I will then show how my simultaneous expertise in solid-state physics, atomic physics, and quantum control led to a practical design for this desired device. I will explore the challenges of fabricating such a device, as well as state-of-the-art techniques to overcome them.

 

Quantum Photonics

 

• Nadia Belabas, Centre for Nanosciences and Nanotechnologies

Title: Quantumness in a tight space

Abstract: Quantum-dot based semiconductor sources offer unprecedented brightness.The achieved brightness enables the implementation of multiphoton protocols when feeding them into integrated reconfigurable photonic chips. We demonstrated a way to measure multi-photon indistinguishability on chip and generated a 4-photon GHZ state with high fidelity. Owing to the high photon rate of our demonstration, we could perform a 4-photon state full tomography on chip. Harnessing quantumness on chip also comes at the cost of the sacrifice of space-like separation that is typically relied upon for certification of randomness. By introducing a dedicated protocol and contextuality metrics, we could for the first time certify quantum randomness in a tight space.

 

• Kwek Leong Chuan, Centre for Quantum Technologies

Title: Photonic Integrated Chip: An Emerging Industry for Quantum and Classical Applications

Abstract: In the last twenty years, there has been a steady effort to miniaturize quantum devices on photonic chips.  This movement extends from semiconductor chips (GaAs, InP and others) to silicon substrate or silicon on silica. One of the most widely used applications has been quantum cryptography but it has since been applied extensively to quantum metrology and optical neural networks. We discuss some of the recent developments in chip design in Singapore.

 

• Alexia Auffèves, MajuLab

Title: TBA

Abstract: TBA

February, Thur 16th

Venue: S15-05-14, Seminar room 5th floor, S15 building 

CQT, National University of Singapore, Block S15, 3 Science Drive 2, 117543

Quantum Many-Body Systems

 

• Chris Westbrook, Institute of Optics Graduate School

Title Quantum atom optics using pair correlation measurements

Abstract: Since the demonstration of Bose-Einstein condensation almost 30 years ago, the study of interacting degenerate quantum gases in various configurations has occupied many researchers. The nature of pair correlations in such systems has played a central role and recent years have seen many experiments on this topic. In this talk, I will discuss the use of spatially and temporally resolved detectors to observe 2 particle and even multi particle correlations. The talk will cover the Hanbury Brown-Twiss effect, the Hong-Ou Mandel effect and the investigation of quantum depletion in a Bose-Einstein condensate.

 

• Huanqian Loh, Centre for Quantum Technologies

Title: Scaling up atom arrays for quantum simulation

Abstract: Arrays of singly trapped neutral atoms are a promising platform for the scalable and programmable control of individual qubits. These atom arrays host tunable interactions and can be arranged to form arbitrary geometries, making them attractive candidates for quantum simulation. Scaling up atom arrays while maintaining high fidelity control would allow one to simulate larger and more complex quantum many-body systems. In this talk, I will discuss the limits of scaling up atom arrays and present two solutions to overcome these limits: the first involving a new class of “magic” wavelengths for optically trapping atoms, and the second involving a novel atom-sorting algorithm based on multiple tweezers, with which we achieved a defect-free array of up to 225 atoms.

 

• Maxime Richard, MajuLab 

Title: Many-body elementary excitations in a quantum fluid of light

Abstract: The elementary excitations in weakly interacting quantum fluids have a hybrid particle-hole nature as described in Bogoliubov theory. This many-body character is particularly appealing for photon-like particles trapped in a microcavity, due to their driven-dissipative character, and to their potential in the context of quantum simulation, and for the preparation of many-body quantum states of light in a compact and scalable system. But to fully achieve this potential, the Bogoliubov excitations must arise from the quantum fluctuations and not from a classical fluctuations source.

Here, we drive a CW steady-state condensate of photon-like particles in a solid-state microcavity (“exciton-poalritons”), and we demonstrate a regime in which all extrinsic fluctuations, such as intentional optical probes, or the undesired presence of an incoherent reservoir, are suppressed. In this regime, the only remaining fluctuations the condensate is coupled to are solid-state lattice phonons: an intrinsic source of fluctuations in solid-state environment, which is thermal in nature and thus easily controllable via temperature. We characterize the Bogoliubov excitations created by thermal, and find a quantitative agreement with our driven-dissipative Bogoliubov theory, which allows us determining a cross-over temperature to reach the full-fledge quantum regime.

 

• Shaffique Adam, YaleNUS

Title: Universal hydrodynamic semiconductors†

Abstract: Hydrodynamic electronic transport occurs when carrier-carrier collisions constitute the dominant scattering mechanism. This regime has attracted intense recent interest with its discovery in two-dimensional materials, for which interactions are intrinsically strong and disorder plays a minimal role.  In this talk I discuss the physics of a hydrodynamic semiconductor where we show that the conductivity is given by the sum of two Drude-like terms: the first is universal, dominates at charge neutrality, and describes the friction between electrons and holes, while the second is dissipative and obtained by collective scattering of the electron-hole plasma on the residual impurities [1].  Since bilayer graphene is a superior platform to observe hydrodynamic behavior [2], we study such ambipolar hydrodynamic transport in ultra-clean dual-gated bilayer graphene devices.  As predicted, the measured conductivity of gapless, charge-neutral bilayer graphene is sample- and temperature-independent over a wide range. Away from neutrality, the electron-hole conductivity collapses to a single curve, and a set of just four fitting parameters provides quantitative agreement between theory and experiment at all densities, temperatures, and gaps measured. This work validates recent theories for dissipation-enabled hydrodynamic conductivity and creates a link between semiconductor physics and the emerging field of viscous electronics.

[1] Cheng Tan, Derek Ho, Lei Wang, Jia Li, Indra Yudhistira, Daniel Rhodes, Takashi Taniguchi, Kenji Watanabe, Kenneth Shepard, Paul McEuen, Cory Dean, Shaffique Adam, and James Hone “Dissipation-enabled hydrodynamic conductivity in a tunable bandgap semiconductor” Science Advances, 8, eabi8481 (2022)

[2] Derek Ho, Indra Yudhistira, Nilotpal Chakraborty, and Shaffique Adam “Microscopic  theory for electron hydrodynamics in monolayer and bilayer graphene” Physical Review B, Rapid Communication, 97, 121404 (2018).

† This work is supported by the Singapore National Science Foundation Investigator Award (NRF-NRFI06-2020-0003)

 

Quantum Material Science

 

• Qu Tingyu, Centre for Advanced 2D Materials, NUS

Title: Microscopic Spin-polarized Superconductivity in Two-dimensional Niobium Diselenide

Abstract: Endowing superconductors with spin degrees of freedom is an ultimate goal towards non-dissipative spintronics and topological quantum computation. Such pursuit dates back to the late 1950s. Yet, spin-dependent characterization for ferro- magnetic order in a superconducting medium remains to be seen. Here, we intercalate dilute magnetic atoms into atomically-thin niobium diselenide and leverage its unconventional superconductivity to trigger ferromagnetism . We construct a magnetic tunnel junction that consists of cobalt and cobalt-doped niobium diselenide as two ferromagnets, with an ultra-thin boron nitride as the tunneling barrier. Our results show an induced ferromagnetic order inside the superconducting gap, as evidenced by both the Yu-Shiba-Rusinov states in differential conductance and the field-switchable tunneling magnetoresistance. This in-gap ferromagnetism suggests an unusual Ruderman–Kittel–Kasuya–Yosida exchange coupling mediated by non-singlet superconducting carriers, in contrast to pre-existing systems showing co-existing superconductivity and magnetism. We resolve the triplet components in a few-layer niobium diselenide by non-local spin injection, and provide direct transport proof for spin-polarized supercurrents diffusing through micron-scale superconducting channels. The discovery in long-range superconductivity-mediated ferromagnetism presents a new recipe for tunable superconducting spintronics. 

 

• Eda Goki, Department of Physics NUS

Title: Dilute impurity doping and bound excitons in 2D semiconductors

Abstract: Owing to reduced screening, point defects in 2D semiconductors such as monolayer transition metal dichalcogenides (TMDs) can serve as optically addressable quantum dots. However, the density of common defects in TMDs are excessively high, making it challenging to address individual defect states for quantum operations. Accordingly, the physical origin of the localized states also remains elusive, preventing strategic quantum defect engineering. We introduce and optically probe a variety of atomic defects in monolayer TMDs in the dilute limit with the aim of accessing their quantum nature. One example is Nb-doped monolayer WS2, which is found to exhibit bright sub-gap emission even at ppm concentrations. We show that such emission arises from ionized-acceptor-bound excitons, a three-body charge complex analogous to a negative trion. These bound exciton complexes exhibit sizable valley selectivity reflecting their partial free exciton character. We further discuss a scanning probe approach to rapidly quantifying impurities in the ultra-dilute limit (<1010 cm-2) in ambient conditions.

 

Quantum Sensing 

 

• Jean-François Roch, ENS Paris-Saclay

Title: Quantum diamond magnetometry at extreme pressures

Abstract: Diamond anvil cell technology has been used for many years to study materials under high pressure. These materials can exhibit exotic phases of matter such as superconductivity with exceptionally high critical temperatures. The main difficulty lies in the challenge to measure magnetic properties at high pressure (> 100 GPa) due to the minute size of the sample that is allowed by the pressurized chamber of the diamond anvil cell.

I will describe the implementation of high-pressure magnetometry based on the optical detected magnetic resonance (ODMR) of nitrogen-vacancy (NV) centers [1]. Due to their electronic spin properties, these atomic-like systems are highly sensitive magnetic probes and their atomic size can allow for sub-micrometer spatial resolution. Using a customized optical microscope, we use the spin dependent luminescence of NV centers to perform a mapping of the magnetic field at the diamond anvil tip. Expulsion of magnetic field lines due to the Meissner effect in a superconductor results in a clear drop of the magnetic field in the close vicinity of the sample, where the NV sensors are located. This direct identification provides an unambiguous diagnosis of superconductivity that does not rely on questionable electrical contacts or indirect probes. This measurement can be performed on any magnetic sample and is compatible with synchrotron X-ray diffraction for structural characterization [2].

I will discuss how this method can be implemented at extreme pressures above 100 GPa by fabricating pillars on the anvil tip to create a quasi-hydrostatic stress environment for the NV centers [3]. This is quantified using the pressure dependence of the diamond Raman shift, the NV ODMR dependence on applied magnetic field, and NV photoluminescence spectral shift. In these conditions the sensitivity of NV micro-sensing is at 130 GPa almost as if at ambient pressure. This result enables the undisputable detection of the Meissner effect of super-hydrides that is currently under stifling debate.  

[1] M. Lesik, T. Plisson, L. Toraille, J. Renaud, F. Occelli, M. Schmidt, O. Salord, A. Delobbe, T. Debuisschert, L. Rondin, P. Loubeyre, J.-F. Roch, Magnetic measurements on micrometer-sized samples under high pressure using designed NV centers, Science 366, 1359–1362(2019).

[2] L. Toraille, A. Hilberer, T. Plisson, M. Lesik, M. Chipaux, B. Vindolet, C. Pépin, F. Occelli, M. Schmidt, T. Debuisschert, N. Guignot, J.-P. Itie, P. Loubeyre, J.-F. Roch, Combined synchrotron X-ray diffraction and NV diamond magnetic microscopy measurements at high pressures, New J. Phys. 22, 103063 (2020).

[3] A. Hilberer, L. Toraille, C. Dailledouze, M.-P. Adam, L. Hanlon, G. Weck, M. Schmidt, P. Loubeyre, J.-F. Roch, NV center magnetometry up to 130 GPa as if at ambient pressure, arXiv:2301.05094 (2023).

• Thierry Debuisschert, Thales Research & Technology

Title: Quantum sensing with ensembles of NV centers in diamond

Abstract: The ability of quantum technologies to control matter on the scale of a single quantum object opens up entirely new possibilities for many applications. Quantum sensing has been identified by Thales as a leading-edge technology with a high potential impact on future navigation, detection and communication systems. We are investigating platforms such as spin impurities in diamond, rare-earth doped crystals, cold atoms, quantum photonics, with a particular focus on practical operational considerations.

Among them, nitrogen-vacancy centers in diamond have very attractive features for various applications. They consist of a nitrogen atom substituted to a carbon atom in the diamond lattice coupled to a vacancy in its immediate vicinity. They behave like an atom in the solid-state with well-defined spin properties and they naturally couple to an external magnetic field. They can be operated at room temperature combining microwave excitation and optical detection of the magnetic resonance. We have used ensembles of NV centers to image the magnetic field distribution of various objects, which has allowed us, first, to characterize the distribution of the current density in an electronic circuit. A following development is based on the use of NV centers implanted in diamond anvil cells for the characterization of phase transitions in materials under high pressure. The versatility of NV centers also makes possible the conversion of a microwave signal into an optical signal. Applying a controlled magnetic field gradient over an ensemble of NV centers, we have demonstrated the real-time spectrum analysis of a microwave field over a broad range covering the whole spectrum of radio-frequency communications. More recently, we have demonstrated an efficient and all-carbon electrical readout of a NV based quantum sensor. Besides those examples, numerous new applications of NV centers in diamond are expected in the future. Furthermore, this technology can be further extended to new detection techniques or to new defects or host materials.

 

• Jiang Zhengzhi, Centre for Quantum Technologies

Title: Quantum sensing of radio-frequency signal with NV centers in SiC

Abstract: Silicon carbide is an emerging platform for quantum technologies that provides wafer scale and low-cost industrial fabrication. The material also hosts high-quality defects with long coherence times that can be used for quantum computation and sensing applications. Using an ensemble of NV centers and an XY8-2 Correlation Spectroscopy approach, we demonstrate a room temperature quantum sensing of an artificial AC field centered at ~ 900 kHz with a spectral resolution of 10 kHz. Implementing the novel Synchronized Readout technique, we further extend the frequency resolution of our sensor to 0.1 kHz. These results pave the first steps for Silicon Carbide quantum sensors towards low-cost Nuclear Magnetic Resonance (NMR) spectrometers with a wide range of practical applications in medical, chemical, and biological analysis.