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Quantum sensing based on solid-state spin defects provides a uniquely versatile platform for imaging physical properties at the nanoscale under diverse environmental conditions. Operation of most sensors used to-date is based on projective measurement along a single axis combined with computational extrapolation. Here, we show that the individually addressable carbon-related spin defect in hexagonal boron nitride is a multi-axis spin system for vectorial nanoscale magnetometry. We demonstrate how its low symmetry and strongly spin-selective direct and reverse intersystem crossing dynamics provide sub-$\mu$T/$\sqrt{\text{Hz}}$ magnetic-field sensitivity for both on and off-axis bias magnetic field exceeding 50 mT. Alongside these features, the room-temperature operation and the nanometer-scale proximity enabled by the van der Waals host material further consolidate this system as an exciting quantum sensing platform.

Quantum sensors that use solid state spin defects have emerged as effective probes of weak alternating magnetic signals. By recording the phase of a signal relative to an external clock, these devices can resolve signal frequencies to a precision orders of magnitude longer than the spin state lifetime. However, these quantum heterodyne protocols suffer from sub-optimal sensitivity, as they are currently limited to pulsed spin control techniques, which are susceptible to cumulative pulse-area errors, or single continuous drives which offer no protection of the spin coherence. Here, we present a control scheme based on a continuous microwave drive that extends spin coherence towards the effective $T_2 \approx \frac{1}{2}T_1$ limit and can resolve the frequency, amplitude and phase of GHz magnetic fields. The scheme is demonstrated using an ensemble of boron vacancies in hexagonal boron nitride, and achieves an amplitude sensitivity of $\eta \approx 3-5 \:\mathrm{\mu T \sqrt{Hz}}$ and phase sensitivity of $\eta_{\phi} \approx 0.076 \:\mathrm{rads \sqrt{Hz}}$. By repeatedly referencing the phase of a resonant signal against the coherent continuous microwave drive in a quantum heterodyne demonstration, we measure a GHz signal with a resolution $<$1 Hz over a 10 s measurement. Achieving this level of performance in a two-dimensional material platform could have broad applications, from probing nanoscale condensed matter systems to integration into heterostructures for quantum networking.

The future of conversational agents will provide users with personalized information responses. However, a significant challenge in developing models is the lack of large-scale dialogue datasets that span multiple sessions and reflect real-world user preferences. Previous approaches rely on experts in a wizard-of-oz setup that is difficult to scale, particularly for personalized tasks. Our method, LAPS, addresses this by using large language models (LLMs) to guide a single human worker in generating personalized dialogues. This method has proven to speed up the creation process and improve quality. LAPS can collect large-scale, human-written, multi-session, and multi-domain conversations, including extracting user preferences. When compared to existing datasets, LAPS-produced conversations are as natural and diverse as expert-created ones, which stays in contrast with fully synthetic methods. The collected dataset is suited to train preference extraction and personalized response generation. Our results show that responses generated explicitly using extracted preferences better match user's actual preferences, highlighting the value of using extracted preferences over simple dialogue history. Overall, LAPS introduces a new method to leverage LLMs to create realistic personalized conversational data more efficiently and effectively than previous methods., Comment: Accepted at SIGIR 2024 (Full Paper)

We present the second version of the Open Assistant Toolkit (OAT-v2), an open-source task-oriented conversational system for composing generative neural models. OAT-v2 is a scalable and flexible assistant platform supporting multiple domains and modalities of user interaction. It splits processing a user utterance into modular system components, including submodules such as action code generation, multimodal content retrieval, and knowledge-augmented response generation. Developed over multiple years of the Alexa TaskBot challenge, OAT-v2 is a proven system that enables scalable and robust experimentation in experimental and real-world deployment. OAT-v2 provides open models and software for research and commercial applications to enable the future of multimodal virtual assistants across diverse applications and types of rich interaction.

Quantum networks and sensing require solid-state spin-photon interfaces that combine single-photon generation and long-lived spin coherence with scalable device integration, ideally at ambient conditions. Despite rapid progress reported across several candidate systems, those possessing quantum coherent single spins at room temperature remain extremely rare. Here, we report quantum coherent control under ambient conditions of a single-photon emitting defect spin in a a two-dimensional material, hexagonal boron nitride. We identify that the carbon-related defect has a spin-triplet electronic ground-state manifold. We demonstrate that the spin coherence is governed predominantly by coupling to only a few proximal nuclei and is prolonged by decoupling protocols. Our results allow for a room-temperature spin qubit coupled to a multi-qubit quantum register or quantum sensor with nanoscale sample proximity.

Sensors based on spin qubits in 2D crystals offer the prospect of nanoscale sensing volumes, where the close proximity of the sensor and source could provide access to otherwise inaccessible signals. For AC magnetometry, the sensitivity and frequency range is typically limited by the noise spectrum, which determines the qubit coherence time. This poses a problem for III-V materials, as the non-zero spin of the host nuclei introduces a considerable source of magnetic noise. Here, we overcome this with a sensing protocol based on phase modulated continuous concatenated dynamic decoupling, which extends the coherence time towards the $T_1$ limit at room temperature and enables tuneable narrowband AC magnetometry. We demonstrate the protocol with an ensemble of negatively charged boron vacancies in hexagonal boron nitride, detecting in-plane AC fields within $\pm 150~\mathrm{MHz}$ of the electron spin resonance, and out-of-plane fields in the range of $\sim10-150~\mathrm{MHz}$. We measure an AC magnetic field sensitivity of $\sim1~\mathrm{\mu T/\sqrt{Hz}}$ at $\sim2.5~\mathrm{GHz}$, for a sensor volume of $\sim0.1~\mathrm{\mu m^3}$, and demonstrate that the sensor can reconstruct the AC magnetic field from a wire loop antenna. This work establishes the viability of spin defects in 2D materials for high frequency magnetometry, demonstrating sensitivities that are comparable to nitrogen vacancy centres in diamond for microscopic sensing volumes, and with wide-ranging applications across science and technology.

For widespread adoption, public security and surveillance systems must be accurate, portable, compact, and real-time, without impeding the privacy of the individuals being observed. Current systems broadly fall into two categories -- image-based which are accurate, but lack privacy, and RF signal-based, which preserve privacy but lack portability, compactness and accuracy. Our paper proposes mmSense, an end-to-end portable miniaturised real-time system that can accurately detect the presence of concealed metallic objects on persons in a discrete, privacy-preserving modality. mmSense features millimeter wave radar technology, provided by Google's Soli sensor for its data acquisition, and TransDope, our real-time neural network, capable of processing a single radar data frame in 19 ms. mmSense achieves high recognition rates on a diverse set of challenging scenes while running on standard laptop hardware, demonstrating a significant advancement towards creating portable, cost-effective real-time radar based surveillance systems., Comment: Accepted by ICASSP 2023

Spin defects in foils of hexagonal boron nitride are an attractive platform for magnetic field imaging, since the probe can be placed in close proximity to the target. However, as a III-V material the electron spin coherence is limited by the nuclear spin environment, with spin echo coherence time of $\sim100~\mathrm{ns}$ at room temperature accessible magnetic fields. We use a strong continuous microwave drive with a modulation in order to stabilize a Rabi oscillation, extending the coherence time up to $\sim4~\mathrm{\mu s}$, which is close to the 10-$\mathrm{\mu s}$ electron spin lifetime in our sample. We then define a protected qubit basis, and show full control of the protected qubit. The coherence times of a superposition of the protected qubit can be as high as $0.8~\mathrm{\mu s}$. This work establishes that boron vacancies in hexagonal boron nitride can have electron spin coherence times that are competitive with typical NV-centers in small nanodiamonds under ambient conditions.

We report optically detected magnetic resonance (ODMR) measurements of an ensemble of spin-1 negatively charged boron vacancies in hexagonal boron nitride. The photoluminescence decay rates are spin-dependent, with inter-system crossing rates of $1.02~\mathrm{ns^{-1}}$ and $2.03~\mathrm{ns^{-1}}$ for the $m_s=0$ and $m_s=\pm 1$ states, respectively. Time-gating the photoluminescence enhances the ODMR contrast by discriminating between different decay rates. This is particularly effective for detecting the spin of the optically excited state, where a zero-field splitting of $\vert D_{ES}\vert=2.09~\mathrm{GHz}$ is measured. The magnetic field dependence of the time-gated photoluminescence exhibits dips corresponding to the Ground (GSLAC) and excited-state (ESLAC) anti-crossings. Additional dips corresponding to anti-crossings with nearby spin-1/2 parasitic impurities are also observed. The ESLAC dip is sensitive to the angle of the external magnetic field. Comparison to a model suggests that the anti-crossings are mediated by the interaction with nuclear spins, and allow an estimate of the ratio of the spin-dependent relaxation rates from the singlet back into the triplet ground state of $\kappa_0/\kappa_1=0.34$. This work provides important spectroscopic signatures of the boron vacancy, and information on the spin pumping and read-out dynamics.

We outline the role of forward and inverse modelling approaches in the design of human--computer interaction systems. Causal, forward models tend to be easier to specify and simulate, but HCI requires solutions of the inverse problem. We infer finger 3D position $(x,y,z)$ and pose (pitch and yaw) on a mobile device using capacitive sensors which can sense the finger up to 5cm above the screen. We use machine learning to develop data-driven models to infer position, pose and sensor readings, based on training data from: 1. data generated by robots, 2. data from electrostatic simulators 3. human-generated data. Machine learned emulation is used to accelerate the electrostatic simulation performance by a factor of millions. We combine a Conditional Variational Autoencoder with domain expertise/models experimentally collected data. We compare forward and inverse model approaches to direct inference of finger pose. The combination gives the most accurate reported results on inferring 3D position and pose with a capacitive sensor on a mobile device.

Stimulated emission depletion, or STED microscopy is a well-established super-resolution technique, but is ultimately limited by the chosen flourophore. Here we demonstrate STED microscopy with color centers in nanoscale flakes of hexagonal boron nitride using time-gated continuous wave STED. For color centers with zero phonon line emission around 580 nm we measure a STED cross-section of (5.5 $\pm$ 3.2) x $10^{-17} \mbox{cm}^{2}$, achieve a resolution of $\sim$ 50 nm and resolve two color centers separated by 250 nm, which is less than the diffraction limit. The achieved resolution is limited by the numerical aperture of the objective lens (0.8) and the available laser power, and we predict that a resolution of sub-10 nm can be achieved with an oil immersion objective lens, similar to state-of-the-art resolution obtained with nitrogen vacancy centers in diamond., Comment: 20 pages, 6 figures, SI included in the main text

We demonstrate the use of Stimulated Emission Depletion (STED) spectroscopy to map the electron-optical-phonon sideband of the ground state of the radiative transition of color centers in hexagonal boron nitride emitting at 2.0-2.2 eV, with in-plane linear polarization. The measurements are compared to Photoluminescence of Excitation (PLE) spectra, that maps the electron-optical-phonon sideband of the excited state. The main qualitative difference is a red-shift in the longitudinal optical phonon peak associated with $E_{1u}$ symmetry at the zone center. We argue that this is consistent with recent findings for a carbon-based line defect with admixture of energetically similar excited states.

We report on multicolor excitation experiments with color centers in hexagonal boron nitride at cryogenic temperatures. We demonstrate controllable optical switching between bright and dark states of color centers emitting around 2eV. Resonant, or quasi-resonant excitation also pumps the color center, via a two-photon process, into a dark state, where it becomes trapped. Photoluminescence excitation spectroscopy reveals a defect dependent energy threshold for repumping the color center into the bright state of between 2.2 and 2.6eV. Photoionization and photocharging of the defect is the most plausible explanation for this behaviour, with the negative and neutral charge states of the boron vacancy potential candidates for the bright and dark states, respectively. Furthermore, a second zero phonon line, detuned by +0.4eV, is observed in absorption with orthogonal polarization to the emission, evidencing an additional energy level in the color center., Comment: 19 pages, 4 figures

We demonstrate an on-demand hole spin qubit initialization scheme meeting four key requirements of quantum information processing: fast initialization (1/e ~ 100 ps), high fidelity (F > 99%), long qubit lifetime $(2T_{1}>T_{2}^{*}\simeq10\:\mathrm{ns})$, and compatibility with optical coherent control schemes. This is achieved by rapidly ionizing an exciton in an InGaAs quantum dot with very low fine-structure splitting at zero magnetic field. Furthermore, we show that the hole spin fidelity of an arbitrary quantum dot can be increased by optical Stark effect tuning of the fine-structure splitting close to zero., Comment: Manuscript (6 pages, 5 figures) followed by supplemental materials (4 pages, 3 figures)

A scalable optical quantum information processor is likely to be a waveguide circuit with integrated sources, detectors, and either deterministic quantum-logic or quantum memory elements. With microsecond coherence times, ultrafast coherent control, and lifetime-limited transitions, semiconductor quantum-dot spins are a natural choice for the static qubits. However their integration with flying photonic qubits requires an on-chip spin-photon interface, which presents a fundamental problem: the spin-state is measured and controlled via circularly-polarised photons, but waveguides support only linear polarisation. We demonstrate here a solution based on two orthogonal photonic nanowires, in which the spin-state is mapped to a path-encoded photon, thus providing a blue-print for a scalable spin-photon network. Furthermore, for some devices we observe that the circular polarisation state is directly mapped to orthogonal nanowires. This result, which is physically surprising for a non-chiral structure, is shown to be related to the nano-positioning of the quantum-dot with respect to the photonic circuit.

Omni-directional light coupling to surface plasmon polariton (SPP) modes to make use of plasmon mediated near-field enhancement is challenging. We report possibility of near dispersion-less modes in structures with unpatterned metal-dielectric interfaces having 2-D dielectric patterns on top. We show that the position and dispersion of the excited modes can be controlled by the excitation geometry and the 2-D pattern. The anti-crossings resulting from the in-plane coupling of different SPP modes are also shown., Comment: 15 pages, 4 figures