Probabilistic Interpolation of Quantum Rotation Angles.

Quantum computing requires a universal set of gate operations; regarding gates as rotations, any rotation angle must be possible. However a real device may only be capable of B bits of resolution, i.e., it might support only 2^{B} possible variants of a given physical gate. Naive discretization of an algorithm's gates to the nearest available options causes coherent errors, while decomposing an impermissible gate into several allowed operations increases circuit depth. Conversely, demanding higher B can greatly complexify hardware. Here, we explore an alternative: probabilistic angle interpolation (PAI). This effectively implements any desired, continuously parametrized rotation by randomly choosing one of three discretized gate settings and postprocessing individual circuit outputs. The approach is particularly relevant for near-term applications where one would in any case average over many runs of circuit executions to estimate expected values. While PAI increases that sampling cost, we prove that (a) the approach is optimal in the sense that PAI achieves the least possible overhead and (b) the overhead is remarkably modest even with thousands of parametrized gates and only seven bits of resolution available. This is a profound relaxation of engineering requirements for first generation quantum computers where even 5-6 bits of resolution may suffice and, as we demonstrate, the approach is many orders of magnitude more efficient than prior techniques. Moreover we conclude that, even for more mature late noisy intermediate-scale quantum era hardware, no more than nine bits will be necessary.

Stability of Near-Surface Nitrogen Vacancy Centers Using Dielectric Surface Passivation.

We study the photophysical stability of ensemble near-surface nitrogen vacancy (NV) centers in diamond under vacuum and air. The optically detected magnetic resonance contrast of the NV centers was measured following exposure to laser illumination, showing opposing trends in air compared to vacuum (increasing by up to 9% and dropping by up to 25%, respectively). Characterization using X-ray photoelectron spectroscopy (XPS) suggests a surface reconstruction: In air, atmospheric oxygen adsorption on a surface leads to an increase in NV- fraction, whereas in vacuum, net oxygen desorption increases the NV0 fraction. NV charge state switching is confirmed by photoluminescence spectroscopy. Deposition of ∼2 nm alumina (Al2O3) over the diamond surface was shown to stabilize the NV charge state under illumination in either environment, attributed to a more stable surface electronegativity. The use of an alumina coating on diamond is therefore a promising approach to improve the resilience of NV sensors.

Q-band EPR cryoprobe.

Following the success of cryogenic EPR signal preamplification at X-band, we present a Q-band EPR cryoprobe compatible with a standard EPR resonator. The probehead is equipped with a cryogenic ultra low-noise microwave amplifier and its protection circuit that are placed close to the sample in the same cryostat. Our cryoprobe maintains the same sample access and tuning which is typical in Q-band EPR, as well as supports high-power pulsed experiments on typical samples. The performance of our setup is benchmarked against that of existing commercial and home-built Q-band spectrometers, using CW EPR and pulsed EPR/ENDOR experiments to reveal a significant sensitivity improvement which reduces the measurement time by a factor of about 40× at 6 K temperature at reduced power levels.

Probing spin dynamics of ultra-thin van der Waals magnets via photon-magnon coupling.

Layered van der Waals (vdW) magnets can maintain a magnetic order even down to the single-layer regime and hold promise for integrated spintronic devices. While the magnetic ground state of vdW magnets was extensively studied, key parameters of spin dynamics, like the Gilbert damping, crucial for designing ultra-fast spintronic devices, remains largely unexplored. Despite recent studies by optical excitation and detection, achieving spin wave control with microwaves is highly desirable, as modern integrated information technologies predominantly are operated with these. The intrinsically small numbers of spins, however, poses a major challenge to this. Here, we present a hybrid approach to detect spin dynamics mediated by photon-magnon coupling between high-Q superconducting resonators and ultra-thin flakes of Cr2Ge2Te6 (CGT) as thin as 11 nm. We test and benchmark our technique with 23 individual CGT flakes and extract an upper limit for the Gilbert damping parameter. These results are crucial in designing on-chip integrated circuits using vdW magnets and offer prospects for probing spin dynamics of monolayer vdW magnets.

X- and Q-band EPR with cryogenic amplifiers independent of sample temperature.

Inspired by the success of NMR cryoprobes, we recently reported a leap in X-band EPR sensitivity by equipping an ordinary EPR probehead with a cryogenic low-noise microwave amplifier placed closed to the sample in the same cryostat [Simenas et al. J. Magn. Reson.322, 106876 (2021)]. Here, we explore, theoretically and experimentally, a more general approach, where the amplifier temperature is independent of the sample temperature. This approach brings a number of important advantages, enabling sensitivity improvement irrespective of sample temperature, as well as making it more practical to combine with ENDOR and Q-band resonators, where space in the sample cryostat is often limited. Our experimental realisation places the cryogenic preamplifier within an external closed-cycle cryostat, and we show CW and pulsed EPR and ENDOR sensitivity improvements at both X- and Q-bands with negligible dependence on sample temperature. The cryoprobe delivers signal-to-noise ratio enhancements that reduce the equivalent pulsed EPR measurement time by 16× at X-band and close to 5× at Q-band. Using the theoretical framework we discuss further improvements of this approach which could be used to achieve even greater sensitivity.

Near-Surface

Impurity spins in crystal matrices are promising components in quantum technologies, particularly if they can maintain their spin properties when close to surfaces and material interfaces. Here, we investigate an attractive candidate for microwave-domain applications, the spins of group-VI ^{125}Te^{+} donors implanted into natural Si at depths as shallow as 20 nm. We show that surface band bending can be used to ionize such near-surface Te to spin-active Te^{+} state, and that optical illumination can be used further to control the Te donor charge state. We examine spin activation yield, spin linewidth, and relaxation (T_{1}) and coherence times (T_{2}) and show how a zero-field 3.5 GHz "clock transition" extends spin coherence times to over 1 ms, which is about an order of magnitude longer than other near-surface spin systems.

Functional basis of electron transport within photosynthetic complex I.

Photosynthesis and respiration rely upon a proton gradient to produce ATP. In photosynthesis, the Respiratory Complex I homologue, Photosynthetic Complex I (PS-CI) is proposed to couple ferredoxin oxidation and plastoquinone reduction to proton pumping across thylakoid membranes. However, little is known about the PS-CI molecular mechanism and attempts to understand its function have previously been frustrated by its large size and high lability. Here, we overcome these challenges by pushing the limits in sample size and spectroscopic sensitivity, to determine arguably the most important property of any electron transport enzyme - the reduction potentials of its cofactors, in this case the iron-sulphur clusters of PS-CI (N0, N1 and N2), and unambiguously assign them to the structure using double electron-electron resonance. We have thus determined the bioenergetics of the electron transfer relay and provide insight into the mechanism of PS-CI, laying the foundations for understanding of how this important bioenergetic complex functions.

A sensitivity leap for X-band EPR using a probehead with a cryogenic preamplifier.

Inspired by the considerable success of cryogenically cooled NMR cryoprobes, we present an upgraded X-band EPR probehead, equipped with a cryogenic low-noise preamplifier. Our setup suppresses source noise, can handle the high microwave powers typical in X-band pulsed EPR, and is compatible with the convenient resonator coupling and sample access found on commercially available spectrometers. Our approach allows standard pulsed and continuous-wave EPR experiments to be performed at X-band frequency with significantly increased sensitivity compared to the unmodified setup. The probehead demonstrates a voltage signal-to-noise ratio (SNR) enhancement by a factor close to 8× at a temperature of 6 K, and remains close to 2× at room temperature. By further suppressing room-temperature noise at the expense of reduced microwave power (and thus minimum π-pulse length), the factor of SNR improvement approaches 15 at 6 K, corresponding to an impressive 200-fold reduction in EPR measurement time. We reveal the full potential of this probehead by demonstrating such SNR improvements using a suite of typical hyperfine and dipolar spectroscopy experiments on exemplary samples.

Spin-enhanced nanodiamond biosensing for ultrasensitive diagnostics.

The quantum spin properties of nitrogen-vacancy defects in diamond enable diverse applications in quantum computing and communications1. However, fluorescent nanodiamonds also have attractive properties for in vitro biosensing, including brightness2, low cost3 and selective manipulation of their emission4. Nanoparticle-based biosensors are essential for the early detection of disease, but they often lack the required sensitivity. Here we investigate fluorescent nanodiamonds as an ultrasensitive label for in vitro diagnostics, using a microwave field to modulate emission intensity5 and frequency-domain analysis6 to separate the signal from background autofluorescence7, which typically limits sensitivity. Focusing on the widely used, low-cost lateral flow format as an exemplar, we achieve a detection limit of 8.2 × 10-19 molar for a biotin-avidin model, 105 times more sensitive than that obtained using gold nanoparticles. Single-copy detection of HIV-1 RNA can be achieved with the addition of a 10-minute isothermal amplification step, and is further demonstrated using a clinical plasma sample with an extraction step. This ultrasensitive quantum diagnostics platform is applicable to numerous diagnostic test formats and diseases, and has the potential to transform early diagnosis of disease for the benefit of patients and populations.

Self-Stimulated Pulse Echo Trains from Inhomogeneously Broadened Spin Ensembles.

We show experimentally and describe theoretically how a conventional magnetic resonance Hahn echo sequence can lead to a self-stimulated pulse echo train when an inhomogeneously broadened spin ensemble is coupled to a resonator. Effective strong coupling between the subsystems assures that the first Hahn echo can act as a refocusing pulse on the spins, leading to self-stimulated secondary echoes. Within the framework of mean field theory, we show that this process can continue multiple times leading to a train of echoes. We introduce an analytical model that explains the shape of the first echo and numerical results that account well for the experimentally observed shape and strength of the echo train and provides insights into the collective effects involved.

Remote Capacitive Sensing in Two-Dimensional Quantum-Dot Arrays.

We investigate gate-induced quantum dots in silicon nanowire field-effect transistors fabricated using a foundry-compatible fully depleted silicon-on-insulator (FD-SOI) process. A series of split gates wrapped over the silicon nanowire naturally produces a 2 × n bilinear array of quantum dots along a single nanowire. We begin by studying the capacitive coupling of quantum dots within such a 2 × 2 array and then show how such couplings can be extended across two parallel silicon nanowires coupled together by shared, electrically isolated, "floating" electrodes. With one quantum dot operating as a single-electron-box sensor, the floating gate serves to enhance the charge sensitivity range, enabling it to detect charge state transitions in a separate silicon nanowire. By comparing measurements from multiple devices, we illustrate the impact of the floating gate by quantifying both the charge sensitivity decay as a function of dot-sensor separation and configuration within the dual-nanowire structure.

Distance Measurement of a Noncovalently Bound Y@C82 Pair with Double Electron Electron Resonance Spectroscopy.

Paramagnetic endohedral fullerenes with long spin coherence times, such as N@C60 and Y@C82, are being explored as potential spin quantum bits (qubits). Their use for quantum information processing requires a way to hold them in fixed spatial arrangements. Here we report the synthesis of a porphyrin-based two-site receptor 1, offering a rigid structure that binds spin-active fullerenes (Y@C82) at a center-to-center distance of 5.0 nm, predicted from molecular simulations. The spin-spin dipolar coupling was measured with the pulsed EPR spectroscopy technique of double electron electron resonance and analyzed to give a distance of 4.87 nm with a small distribution of distances.

Storing quantum information in spins and high-sensitivity ESR.

Quantum information, encoded within the states of quantum systems, represents a novel and rich form of information which has inspired new types of computers and communications systems. Many diverse electron spin systems have been studied with a view to storing quantum information, including molecular radicals, point defects and impurities in inorganic systems, and quantum dots in semiconductor devices. In these systems, spin coherence times can exceed seconds, single spins can be addressed through electrical and optical methods, and new spin systems with advantageous properties continue to be identified. Spin ensembles strongly coupled to microwave resonators can, in principle, be used to store the coherent states of single microwave photons, enabling so-called microwave quantum memories. We discuss key requirements in realising such memories, including considerations for superconducting resonators whose frequency can be tuned onto resonance with the spins. Finally, progress towards microwave quantum memories and other developments in the field of superconducting quantum devices are being used to push the limits of sensitivity of inductively-detected electron spin resonance. The state-of-the-art currently stands at around 65 spins per Hz, with prospects to scale down to even fewer spins.

Coherent storage of microwave excitations in rare-earth nuclear spins.

Interfacing between various elements of a computer--from memory to processors to long range communication--will be as critical for quantum computers as it is for classical computers today. Paramagnetic rare-earth doped crystals, such as Nd(3+):Y2SiO5(YSO), are excellent candidates for such a quantum interface: they are known to exhibit long optical coherence lifetimes (for communication via optical photons), possess a nuclear spin (memory), and have in addition an electron spin that can offer hybrid coupling with superconducting qubits (processing). Here we study two of these three elements, demonstrating coherent storage and retrieval between electron and (145)Nd nuclear spin states in Nd(3+):YSO. We find nuclear spin coherence times can reach 9 ms at ∼5 K, about 2 orders of magnitude longer than the electron spin coherence, while quantum state and process tomography of the storage or retrieval operation between the electron and nuclear spin reveal an average state fidelity of 0.86. The times and fidelities are expected to further improve at lower temperatures and with more homogeneous radio-frequency excitation.

Synthesis and investigation of donor-porphyrin-acceptor triads with long-lived photo-induced charge-separate states.

Two donor-porphyrin-acceptor triads have been synthesized using a versatile Suzuki-coupling route. This synthetic strategy allows the powerful donor tetraalkylphenylenediamine (TAPD) to be introduced into tetraarylporphyrin-based triads without protection. The thermodynamics and kinetics of electron transfer in the new triads are compared with a previously reported octaalkyldiphenyl-porphyrin triad exhibiting a long-lived spin-polarized charge separate state (CSS), from theoretical and experimental perspectives, in both fluid solution and in a frozen solvent glass. We show that the less favorable oxidation potential of the tetraaryl-porphyrin core can be offset by using C60 , as a better electron-acceptor than triptycenenaphthoquinone (TNQ). The C60 -porphyrin-TAPD triad gives a spin-polarized charge-separated state that can be observed by EPR-spectroscopy, with a mean lifetime of 16 ms at 10 K, which is longer than in the previously reported TNQ-porphyrin-TAPD triad, following the predicted trend from calculated charge-recombination rates.

Conditional control of donor nuclear spins in silicon using stark shifts.

Electric fields can be used to tune donor spins in silicon using the Stark shift, whereby the donor electron wave function is displaced by an electric field, modifying the hyperfine coupling between the electron spin and the donor nuclear spin. We present a technique based on dynamic decoupling of the electron spin to accurately determine the Stark shift, and illustrate this using antimony donors in isotopically purified silicon-28. We then demonstrate two different methods to use a dc electric field combined with an applied resonant radio-frequency (rf) field to conditionally control donor nuclear spins. The first method combines an electric-field induced conditional phase gate with standard rf pulses, and the second one simply detunes the spins off resonance. Finally, we consider different strategies to reduce the effect of electric field inhomogeneities and obtain above 90% process fidelities.

Uncovering many-body correlations in nanoscale nuclear spin baths by central spin decoherence.

Central spin decoherence caused by nuclear spin baths is often a critical issue in various quantum computing schemes, and it has also been used for sensing single-nuclear spins. Recent theoretical studies suggest that central spin decoherence can act as a probe of many-body physics in spin baths; however, identification and detection of many-body correlations of nuclear spins in nanoscale systems are highly challenging. Here, taking a phosphorus donor electron spin in a (29)Si nuclear spin bath as our model system, we discover both theoretically and experimentally that many-body correlations in nanoscale nuclear spin baths produce identifiable signatures in decoherence of the central spin under multiple-pulse dynamical decoupling control. We demonstrate that under control by an odd or even number of pulses, the central spin decoherence is principally caused by second- or fourth-order nuclear spin correlations, respectively. This study marks an important step toward studying many-body physics using spin qubits.