Multimode Storage of Quantum Microwave Fields in Electron Spins over 100 ms.

We report long coherence times (up to 300 ms) for near-surface bismuth donor electron spins in silicon coupled to a superconducting microresonator, biased at a clock transition. This enables us to demonstrate the partial absorption of a train of weak microwave fields in the spin ensemble, their storage for 100 ms, and their retrieval, using a Hahn-echo-like protocol. Phase coherence and quantum statistics are preserved in the storage.

Fast Gate-Based Readout of Silicon Quantum Dots Using Josephson Parametric Amplification.

Spins in silicon quantum devices are promising candidates for large-scale quantum computing. Gate-based sensing of spin qubits offers a compact and scalable readout with high fidelity, however, further improvements in sensitivity are required to meet the fidelity thresholds and measurement timescales needed for the implementation of fast feedback in error correction protocols. Here, we combine radio-frequency gate-based sensing at 622 MHz with a Josephson parametric amplifier, that operates in the 500-800 MHz band, to reduce the integration time required to read the state of a silicon double quantum dot formed in a nanowire transistor. Based on our achieved signal-to-noise ratio, we estimate that singlet-triplet single-shot readout with an average fidelity of 99.7% could be performed in 1 µs, well below the requirements for fault-tolerant readout and 30 times faster than without the Josephson parametric amplifier. Additionally, the Josephson parametric amplifier allows operation at a lower radio-frequency power while maintaining identical signal-to-noise ratio. We determine a noise temperature of 200 mK with a contribution from the Josephson parametric amplifier (25%), cryogenic amplifier (25%) and the resonator (50%), showing routes to further increase the readout speed.

Pulsed electron spin resonance spectroscopy in the Purcell regime.

In EPR, spin relaxation is typically governed by interactions with the lattice or other spins. However, it has recently been shown that given a sufficiently strong spin-resonator coupling and high resonator quality factor, the spontaneous emission of microwave photons from the spins into the resonator can become the main relaxation mechanism, as predicted by Purcell. With increasing attention on the use of microresonators for EPR to achieve high spin-number sensitivity it is important to understand how this novel regime influences measured EPR signals, for example the amplitude and temporal shape of the spin-echo. We study this regime theoretically and experimentally, using donor spins in silicon, under different conditions of spin-linewidth and coupling homogeneity. When the spin-resonator coupling is distributed inhomogeneously, we find that the effective spin-echo relaxation time measured in a saturation recovery sequence strongly depends on the parameters for the detection echo. When the spin linewidth is larger than the resonator bandwidth, the different Fourier components of the spin echo relax with different characteristic times - due to the role of the resonator in driving relaxation - which results in the temporal shape of the echo becoming dependent on the repetition time of the experiment.

Linear Hyperfine Tuning of Donor Spins in Silicon Using Hydrostatic Strain.

We experimentally study the coupling of group V donor spins in silicon to mechanical strain, and measure strain-induced frequency shifts that are linear in strain, in contrast to the quadratic dependence predicted by the valley repopulation model (VRM), and therefore orders of magnitude greater than that predicted by the VRM for small strains |ϵ|<10^{-5}. Through both tight-binding and first principles calculations we find that these shifts arise from a linear tuning of the donor hyperfine interaction term by the hydrostatic component of strain and achieve semiquantitative agreement with the experimental values. Our results provide a framework for making quantitative predictions of donor spins in silicon nanostructures, such as those being used to develop silicon-based quantum processors and memories. The strong spin-strain coupling we measure (up to 150 GHz per strain, for Bi donors in Si) offers a method for donor spin tuning-shifting Bi donor electron spins by over a linewidth with a hydrostatic strain of order 10^{-6}-as well as opportunities for coupling to mechanical resonators.

Controlling spin relaxation with a cavity.

Spontaneous emission of radiation is one of the fundamental mechanisms by which an excited quantum system returns to equilibrium. For spins, however, spontaneous emission is generally negligible compared to other non-radiative relaxation processes because of the weak coupling between the magnetic dipole and the electromagnetic field. In 1946, Purcell realized that the rate of spontaneous emission can be greatly enhanced by placing the quantum system in a resonant cavity. This effect has since been used extensively to control the lifetime of atoms and semiconducting heterostructures coupled to microwave or optical cavities, and is essential for the realization of high-efficiency single-photon sources. Here we report the application of this idea to spins in solids. By coupling donor spins in silicon to a superconducting microwave cavity with a high quality factor and a small mode volume, we reach the regime in which spontaneous emission constitutes the dominant mechanism of spin relaxation. The relaxation rate is increased by three orders of magnitude as the spins are tuned to the cavity resonance, demonstrating that energy relaxation can be controlled on demand. Our results provide a general way to initialize spin systems into their ground state and therefore have applications in magnetic resonance and quantum information processing. They also demonstrate that the coupling between the magnetic dipole of a spin and the electromagnetic field can be enhanced up to the point at which quantum fluctuations have a marked effect on the spin dynamics; as such, they represent an important step towards the coherent magnetic coupling of individual spins to microwave photons.

Reaching the quantum limit of sensitivity in electron spin resonance.

The detection and characterization of paramagnetic species by electron spin resonance (ESR) spectroscopy is widely used throughout chemistry, biology and materials science, from in vivo imaging to distance measurements in spin-labelled proteins. ESR relies on the inductive detection of microwave signals emitted by the spins into a coupled microwave resonator during their Larmor precession. However, such signals can be very small, prohibiting the application of ESR at the nanoscale (for example, at the single-cell level or on individual nanoparticles). Here, using a Josephson parametric microwave amplifier combined with high-quality-factor superconducting microresonators cooled at millikelvin temperatures, we improve the state-of-the-art sensitivity of inductive ESR detection by nearly four orders of magnitude. We demonstrate the detection of 1,700 bismuth donor spins in silicon within a single Hahn echo with unit signal-to-noise ratio, reduced to 150 spins by averaging a single Carr-Purcell-Meiboom-Gill sequence. This unprecedented sensitivity reaches the limit set by quantum fluctuations of the electromagnetic field instead of thermal or technical noise, which constitutes a novel regime for magnetic resonance. The detection volume of our resonator is ∼ 0.02 nl, and our approach can be readily scaled down further to improve sensitivity, providing a new versatile toolbox for ESR at the nanoscale.

XactMice: humanizing mouse bone marrow enables microenvironment reconstitution in a patient-derived xenograft model of head and neck cancer.

The limitations of cancer cell lines have led to the development of direct patient-derived xenograft models. However, the interplay between the implanted human cancer cells and recruited mouse stromal and immune cells alters the tumor microenvironment and limits the value of these models. To overcome these constraints, we have developed a technique to expand human hematopoietic stem and progenitor cells (HSPCs) and use them to reconstitute the radiation-depleted bone marrow of a NOD/SCID/IL2rg(-/-) (NSG) mouse on which a patient's tumor is then transplanted (XactMice). The human HSPCs produce immune cells that home into the tumor and help replicate its natural microenvironment. Despite previous passage on nude mice, the expression of epithelial, stromal and immune genes in XactMice tumors aligns more closely to that of the patient tumor than to those grown in non-humanized mice-an effect partially facilitated by human cytokines expressed by both the HSPC progeny and the tumor cells. The human immune and stromal cells produced in the XactMice can help recapitulate the microenvironment of an implanted xenograft, reverse the initial genetic drift seen after passage on non-humanized mice and provide a more accurate tumor model to guide patient treatment.

Hybrid optical-electrical detection of donor electron spins with bound excitons in silicon.

Electrical detection of spins is an essential tool for understanding the dynamics of spins, with applications ranging from optoelectronics and spintronics, to quantum information processing. For electron spins bound to donors in silicon, bulk electrically detected magnetic resonance has relied on coupling to spin readout partners such as paramagnetic defects or conduction electrons, which fundamentally limits spin coherence times. Here we demonstrate electrical detection of donor electron spin resonance in an ensemble by transport through a silicon device, using optically driven donor-bound exciton transitions. We measure electron spin Rabi oscillations, and obtain long electron spin coherence times, limited only by the donor concentration. We also experimentally address critical issues such as non-resonant excitation, strain, and electric fields, laying the foundations for realizing a single-spin readout method with relaxed magnetic field and temperature requirements compared with spin-dependent tunnelling, enabling donor-based technologies such as quantum sensing.

Quantum information storage for over 180 s using donor spins in a 28Si "semiconductor vacuum".

A quantum computer requires systems that are isolated from their environment, but can be integrated into devices, and whose states can be measured with high accuracy. Nuclear spins in solids promise long coherence lifetimes, but they are difficult to initialize into known states and to detect with high sensitivity. We show how the distinctive optical properties of enriched (28)Si enable the use of hyperfine-resolved optical transitions, as previously applied to great effect for isolated atoms and ions in vacuum. Together with efficient Auger photoionization, these resolved hyperfine transitions permit rapid nuclear hyperpolarization and electrical spin-readout. We combine these techniques to detect nuclear magnetic resonance from dilute (31)P in the purest available sample of (28)Si, at concentrations inaccessible to conventional measurements, measuring a solid-state coherence time of over 180 seconds.

Electrically detected magnetic resonance of neutral donors interacting with a two-dimensional electron gas.

We have measured the electrically detected magnetic resonance of donor-doped silicon field-effect transistors in resonant X- (9.7 GHz) and W-band (94 GHz) microwave cavities. The two-dimensional electron gas resonance signal increases by 2 orders of magnitude from X to W band, while the donor resonance signals are enhanced by over 1 order of magnitude. Bolometric effects and spin-dependent scattering are inconsistent with the observations. We propose that polarization transfer from the donor to the two-dimensional electron gas is the main mechanism giving rise to the spin resonance signals.

Electrically detected magnetic resonance in a W-band microwave cavity.

We describe a low-temperature sample probe for the electrical detection of magnetic resonance in a resonant W-band (94 GHz) microwave cavity. The advantages of this approach are demonstrated by experiments on silicon field-effect transistors. A comparison with conventional low-frequency measurements at X-band (9.7 GHz) on the same devices reveals an up to 100-fold enhancement of the signal intensity. In addition, resonance lines that are unresolved at X-band are clearly separated in the W-band measurements. Electrically detected magnetic resonance at high magnetic fields and high microwave frequencies is therefore a very sensitive technique for studying electron spins with an enhanced spectral resolution and sensitivity.

Entangling remote nuclear spins linked by a chromophore.

Molecular nanostructures may constitute the fabric of future quantum technologies, if their degrees of freedom can be fully harnessed. Ideally one might use nuclear spins as low-decoherence qubits and optical excitations for fast controllable interactions. Here, we present a method for entangling two nuclear spins through their mutual coupling to a transient optically excited electron spin, and investigate its feasibility through density-functional theory and experiments on a test molecule. From our calculations we identify the specific molecular properties that permit high entangling power gates under simple optical and microwave pulses; synthesis of such molecules is possible with established techniques.

High-cooperativity coupling of electron-spin ensembles to superconducting cavities.

Electron spins in solids are promising candidates for quantum memories for superconducting qubits because they can have long coherence times, large collective couplings, and many qubits could be encoded into spin waves of a single ensemble. We demonstrate the coupling of electron-spin ensembles to a superconducting transmission-line cavity at strengths greatly exceeding the cavity decay rates and comparable to the spin linewidths. We also perform broadband spectroscopy of ruby (Al2O3:Cr(3+)) at millikelvin temperatures and low powers, using an on-chip feedline. In addition, we observe hyperfine structure in diamond P1 centers.

Quantum computing with an electron spin ensemble.

We propose to encode a register of quantum bits in different collective electron spin wave excitations in a solid medium. Coupling to spins is enabled by locating them in the vicinity of a superconducting transmission line cavity, and making use of their strong collective coupling to the quantized radiation field. The transformation between different spin waves is achieved by applying gradient magnetic fields across the sample, while a Cooper pair box, resonant with the cavity field, may be used to carry out one- and two-qubit gate operations.

NT-proBNP does not rise on acute ascent to high altitude.

The response of brain natriuretic peptide (BNP) to acute ascent to altitude is of interest as a surrogate for ventricular function and because BNP is involved in the normal homeostasis of the pulmonary vasculature. The structurally related hormone atrial natriuretic pressure (ANP) has been demonstrated to be elevated at altitude and implicated in natriuresis. We measured plasma concentrations of ANP and NT-proBNP (a more stable BNP precursor) in 10 healthy non-HAPE-susceptible lowlanders during acute exposure to 5200 m on the Apex 2 expedition to Bolivia. Systolic pulmonary artery pressure (PASP) was measured using tricuspid regurgitant jet estimation by echocardiography. Despite a significant rise in the PASP, NT-proBNP did not rise. A small decrease in NT-pro BNP occurred after 7 days at high altitude. There was no significant change in ANP levels. The lack of any increase in NT-proBNP in healthy resting subjects supports the view that ventricular function is well preserved and suggests that BNP is not playing a significant role in altered pulmonary artery pressure.

Value of BNP to estimate cardiac risk in patients on cardioactive treatment in primary care.

UNLABELLED: Cardiac dysfunction may be suspected in patients with cardiovascular disease but identifying those with the highest risk is problematic. B-type natriuretic peptide (BNP) is a strong marker of heart failure in un-treated patients. This study evaluates a combined BNP and clinical algorithm for detecting cardiac dysfunction and the risk of death, in patients receiving cardioactive medication. METHODS: 459 stable general practice patients, who were taking typical heart failure drugs for any indication, were included. Echocardiography, ECG, and assay of NT-proANP and BNP (two methods) were performed. Regression models were used to identify items in a Risk Score to detect cardiac dysfunction. RESULTS: A Risk Score based on history of myocardial infarction (1 point), abnormal ECG (2 points), atrial fibrillation (1 point) and raised BNP (1-2 points) detected cardiac dysfunction with an AUC of ROC of 0.85. A Risk Score > or = 2 had a sensitivity of 90%, specificity of 58%, and positive and negative predictive values of 37% and 96%. Risk Score and LVEF<0.36 also predicted mortality. Abnormal BNP defined as either >100 pg/ml (Shionogi), or as age and sex related values, had similar predictive value. CONCLUSION: In patients on cardioactive medication, a structured Risk Score based on raised BNP, history of MI, atrial fibrillation and abnormal ECG was useful for identifying patients for immediate further examination and those who could be evaluated later.

Elevated troponin levels are associated with sympathoadrenal activation in acute ischaemic stroke.

BACKGROUND: It has been hypothesised that elevated serum troponin levels in acute stroke are due to myocardial damage caused by sympathoadrenal activation, which, in turn, may be due particularly to insular damage. We aimed to determine the factors associated with troponin elevation in ischaemic stroke and the prognostic value of this finding. METHODS: We studied 222 consecutive acute ischaemic stroke admissions. Serum troponin I and catecholamines were measured. Ischaemic damage on brain computed tomography (CT) scan was graded using the Alberta Stroke Program Early CT Score (ASPECTS). Electrocardiograms were classified using the Minnesota Code and the European Society of Cardiology/American College of Cardiology criteria for acute myocardial infarction. The Rankin scale was recorded at 30 days. RESULTS: Forty-five patients (20%) had troponin I >0.2 microg/l. These troponin-positive patients had higher epinephrine levels (median 0.27 vs. 0.17 nmol/l; p = 0.0002) and were more likely to have electrocardiograms coded as definite or possible acute myocardial infarction (odds ratio 3.35; 95% CI 1.26-8.93), compared with those with troponin < or = 0.2 microg/l, in univariate analysis. There were no significant associations between troponin I score and ASPECTS or insular damage on brain CT. In logistic regression analyses, elevated troponin was significantly associated with age, elevated serum creatinine and epinephrine; however, increased troponin was not an independent predictor of death or dependency (Rankin >2) at 30 days. CONCLUSIONS: Raised troponin I is associated with elevation of circulating epinephrine in acute ischaemic stroke. Activation of the sympathoadrenal system may be an important contributor to myocardial damage in these patients. Increased troponin is not associated with insular damage and does not independently predict poor outcome.

NT-proBNP can be used to detect right ventricular systolic dysfunction in pulmonary hypertension.

Right ventricular systolic dysfunction (RVSD) at baseline (pre-treatment) predicts early death in patients with pulmonary hypertension (PH). However, RVSD can only be detected reliably by prohibitively invasive or expensive techniques. N-terminal B-type natriuretic peptide concentration ([NT-proBNP]) correlates with RV function in PH; however, an [NT-proBNP] threshold that indicates RVSD in individual patients has not previously been determined. Twenty-five patients with PH (pulmonary arterial hypertension (n = 19) or chronic thromboembolic PH (n = 6)) underwent cardiovascular magnetic resonance (CMR) imaging and NT-proBNP measurement at baseline. [NT-proBNP] was correlated against RV dimensions and ejection fraction (RVEF) measured directly by CMR imaging. The ability of NT-proBNP to detect RVSD (defined as a CMR-derived RVEF >2 SDS below control values) was tested and predictors of [NT-proBNP] identified. [NT-proBNP] correlated negatively with RVEF. RVSD was present in nine out of 25 patients. An [NT-proBNP] threshold of 1,685 pg.mL(-1) was sensitive (100%) and specific (94%) in detecting RVSD. RVEF and RV mass index independently predicted [NT-proBNP]. In pulmonary hypertension, a baseline N-terminal B-type natriuretic peptide concentration of >1,685 ng.L(-1) suggests right ventricular systolic dysfunction, and thus an increased risk of early death. N-terminal B-type natriuretic peptide could prove useful as an objective, noninvasive means of identifying patients with pulmonary hypertension who have right ventricular systolic dysfunction at presentation.