Reservoir-Free Decoherence in Flying Qubits.

An effective time-dependent Hamiltonian can be implemented by making a quantum system fly through an inhomogeneous potential, realizing, for example, a quantum gate on its internal degrees of freedom. However, flying systems have a spatial spread that will generically entangle the internal and spatial degrees of freedom, leading to decoherence in the internal state dynamics, even in the absence of any external reservoir. We provide formulas valid at all times for the dynamics, fidelity, and change of entropy for ballistic particles with small spatial spreads, quantified by Δx. This non-Markovian decoherence can be significant for ballistic flying qubits (scaling as Δx^{2}) but usually not for flying qubits carried by a moving potential well (scaling as Δx^{6}). We also discuss a method to completely counteract this decoherence for a ballistic qubit later measured.

Supergrowth and sub-wavelength object imaging.

We further develop the concept of supergrowth [Quantum Stud.: Math. Found.7, 285 (2020)10.1007/s40509-019-00214-5], a phenomenon complementary to superoscillation, defined as the local amplitude growth rate of a function higher than its largest wavenumber. We identify a canonical oscillatory function's superoscillating and supergrowing regions and find the maximum values of local growth rate and wavenumber. Next, we provide a quantitative comparison of lengths and relevant intensities between the superoscillating and the supergrowing regions of a canonical oscillatory function. Our analysis shows that the supergrowing regions contain intensities that are exponentially larger in terms of the highest local wavenumber compared to the superoscillating regions. Finally, we prescribe methods to reconstruct a sub-wavelength object from the imaging data using both superoscillatory and supergrowing point spread functions. Our investigation provides an experimentally preferable alternative to the superoscillation-based superresolution schemes and is relevant to cutting-edge research in far-field sub-wavelength imaging.

Super Interferometric Range Resolution.

We probe the fundamental underpinnings of range resolution in coherent remote sensing. We use a novel class of self-referential interference functions to show that we can greatly improve upon currently accepted bounds for range resolution. We consider the range resolution problem from the perspective of single-parameter estimation of amplitude versus the traditional temporally resolved paradigm. We define two figures of merit: (i) the minimum resolvable distance between two depths and (ii) for temporally subresolved peaks, the depth resolution between the objects. We experimentally demonstrate that our system can resolve two depths greater than 100× the inverse bandwidth and measure the distance between two objects to approximately 20 µm (35 000 times smaller than the Rayleigh-resolved limit) for temporally subresolved objects using frequencies less than 120 MHz radio waves.

Measurement-Based Quantum Thermal Machines with Feedback Control.

We investigated coupled-qubit-based thermal machines powered by quantum measurements and feedback. We considered two different versions of the machine: (1) a quantum Maxwell's demon, where the coupled-qubit system is connected to a detachable single shared bath, and (2) a measurement-assisted refrigerator, where the coupled-qubit system is in contact with a hot and cold bath. In the quantum Maxwell's demon case, we discuss both discrete and continuous measurements. We found that the power output from a single qubit-based device can be improved by coupling it to the second qubit. We further found that the simultaneous measurement of both qubits can produce higher net heat extraction compared to two setups operated in parallel where only single-qubit measurements are performed. In the refrigerator case, we used continuous measurement and unitary operations to power the coupled-qubit-based refrigerator. We found that the cooling power of a refrigerator operated with swap operations can be enhanced by performing suitable measurements.

Energetic Cost of Measurements Using Quantum, Coherent, and Thermal Light.

Quantum measurements are basic operations that play a critical role in the study and application of quantum information. We study how the use of quantum, coherent, and classical thermal states of light in a circuit quantum electrodynamics setup impacts the performance of quantum measurements, by comparing their respective measurement backaction and measurement signal to noise ratio per photon. In the strong dispersive limit, we find that thermal light is capable of performing quantum measurements with comparable efficiency to coherent light, both being outperformed by single-photon light. We then analyze the thermodynamic cost of each measurement scheme. We show that single-photon light shows an advantage in terms of energy cost per information gain, reaching the fundamental thermodynamic cost.

Efficiently fueling a quantum engine with incompatible measurements.

We propose a quantum harmonic oscillator measurement engine fueled by simultaneous quantum measurements of the noncommuting position and momentum quadratures of the quantum oscillator. The engine extracts work by moving the harmonic trap suddenly, conditioned on the measurement outcomes. We present two protocols for work extraction, respectively based on single-shot and time-continuous quantum measurements. In the single-shot limit, the oscillator is measured in a coherent state basis; the measurement adds an average of one quantum of energy to the oscillator, which is then extracted in the feedback step. In the time-continuous limit, continuous weak quantum measurements of both position and momentum of the quantum oscillator result in a coherent state, whose coordinates diffuse in time. We relate the extractable work to the noise added by quadrature measurements, and present exact results for the work distribution at arbitrary finite time. Both protocols can achieve unit work conversion efficiency in principle.

Variational Principle for Optimal Quantum Controls in Quantum Metrology.

We develop a variational principle to determine the quantum controls and initial state that optimizes the quantum Fisher information, the quantity characterizing the precision in quantum metrology. When the set of available controls is limited, the exact optimal initial state and the optimal controls are, in general, dependent on the probe time, a feature missing in the unrestricted case. Yet, for time-independent Hamiltonians with restricted controls, the problem can be approximately reduced to the unconstrained case via Floquet engineering. In particular, we find for magnetometry with a time-independent spin chain containing three-body interactions, even when the controls are restricted to one- and two-body interaction, that the Heisenberg scaling can still be approximately achieved. Our results open the door to investigate quantum metrology under a limited set of available controls, of relevance to many-body quantum metrology in realistic scenarios.

Experimental demonstration of continuous quantum error correction.

The storage and processing of quantum information are susceptible to external noise, resulting in computational errors. A powerful method to suppress these effects is quantum error correction. Typically, quantum error correction is executed in discrete rounds, using entangling gates and projective measurement on ancillary qubits to complete each round of error correction. Here we use direct parity measurements to implement a continuous quantum bit-flip correction code in a resource-efficient manner, eliminating entangling gates, ancillary qubits, and their associated errors. An FPGA controller actively corrects errors as they are detected, achieving an average bit-flip detection efficiency of up to 91%. Furthermore, the protocol increases the relaxation time of the protected logical qubit by a factor of 2.7 over the relaxation times of the bare comprising qubits. Our results showcase resource-efficient stabilizer measurements in a multi-qubit architecture and demonstrate how continuous error correction codes can address challenges in realizing a fault-tolerant system.

Cross-cultural adaptation of the Spanish MINICHAL instrument into English for use in the United Kingdom.

BACKGROUND: Hypertension is a highly prevalent condition, with optimal treatment to BP targets conferring significant gains in terms of cardiovascular outcomes. Understanding why some patients do not achieve BP targets would be enhanced through greater understanding of their health-related quality of life (HRQoL). However, the only English language disease-specific instruments for measurement of HRQoL in hypertension have not been validated in accordance with accepted standards. It is proposed that the Spanish MINICHAL instrument for the assessment of HRQoL in hypertension could be translated, adapted and validated for use in the United Kingdom. The aim of the study was therefore to complete this process. METHODS: The MINICHAL authors were contacted and the original instrument obtained. This was then translated into English by two independent English-speakers, with these versions then reconciled, before back-translation and subsequent production of a 2nd reconciled version. Thereafter, a final version was produced after cognitive debriefing, for administration and psychometric analysis in the target population of patients living in the Exeter area (Southwest UK) aged 18-80 years with treatment-naïve grade II-III hypertension, before, during and after 18 weeks' intensive treatment. RESULTS: The English-language instrument was administered to 30 individuals (median age: 58.5 years, 53% male). Psychometric analysis demonstrated a floor effect, though no ceiling effect. Internal consistency for both state of mind (StM) and somatic manifestations (SM) dimensions of the instrument were acceptable (Cronbach's alpha = 0.81 and 0.75), as was test-retest reliability (ICC = 0.717 and 0.961) and construct validity, which was measured through co-administration with the EQ-5D-5L and Bulpitt-Fletcher instruments. No significant associations were found between scores and patient characteristics known to affect HRQoL. The EQ-5D-5L instrument found an improvement in HRQoL following treatment, with the StM and SM dimensions of the English language MINICHAL trending to support this (d = 0.32 and 0.02 respectively). CONCLUSIONS: The present study details the successful English translation and validation of the MINICHAL instrument for use in individuals with hypertension. The data reported also supports an improvement in HRQoL with rapid treatment of grade II-III hypertension, a strategy which has been recommended by contemporaneous European guidelines. Trial registration ISRCTN registry number: 57475376 (assigned 25/06/2015).

Enhanced on-chip frequency measurement using weak value amplification.

We present an integrated design to sensitively measure changes in optical frequency using weak value amplification with a multi-mode interferometer. The technique involves introducing a weak perturbation to the system and then post-selecting the data in such a way that the signal is amplified without amplifying the technical noise, as has previously been demonstrated in a free-space setup. We demonstrate the advantages of a Bragg grating with two band gaps for obtaining simultaneous, stable high transmission and high dispersion. The device is more robust and easily scalable than the free-space implementation, and provides amplified sensitivity compared to other methods of measuring changes in optical frequency on a chip, such as an integrated Mach-Zehnder interferometer.

Experimental demonstration of superresolution of partially coherent light sources using parity sorting: erratum.

The authors include references that appeared on arXiv during the preparation of their paper [Opt. Express29, 22034 (2021)10.1364/OE.427734].

Morphological and functional cardiac consequences of rapid hypertension treatment: a cohort study.

BACKGROUND: Left ventricular (LV) hypertrophy (LVH) in uncontrolled hypertension is an independent predictor of mortality, though its regression with treatment improves outcomes. Retrospective data suggest that early control of hypertension provides a prognostic advantage and this strategy is included in the 2018 European guidelines, which recommend treating grade II/III hypertension to target blood pressure (BP) within 3 months. The earliest LVH regression to date was demonstrated by echocardiography at 24 weeks. The effect of a rapid guideline-based treatment protocol on LV remodelling, with very early BP control by 18 weeks remains controversial and previously unreported. We aimed to determine whether such rapid hypertension treatment is associated with improvements in LV structure and function through paired cardiovascular magnetic resonance (CMR) scanning at baseline and 18 weeks, utilising CMR mass and feature tracking analysis. METHODS: We recruited participants with never-treated grade II/III hypertension, initiating a guideline-based treatment protocol which aimed to achieve BP control within 18 weeks. CMR and feature tracking were used to assess myocardial morphology and function immediately before and after treatment. RESULTS: We acquired complete pre- and 18-week post-treatment data for 41 participants. During the interval, LV mass index reduced significantly (43.5 ± 9.8 to 37.6 ± 8.3 g/m2, p < 0.001) following treatment, accompanied by reductions in LV ejection fraction (65.6 ± 6.8 to 63.4 ± 7.1%, p = 0.03), global radial strain (46.1 ± 9.7 to 39.1 ± 10.9, p < 0.001), mid-circumferential strain (- 20.8 ± 4.9 to - 19.1 ± 3.7, p = 0.02), apical circumferential strain (- 26.0 ± 5.3 to - 23.4 ± 4.2, p = 0.003) and apical rotation (9.8 ± 5.0 to 7.5 ± 4.5, p = 0.003). CONCLUSIONS: LVH regresses following just 18 weeks of intensive antihypertensive treatment in subjects with newly-diagnosed grade II/III hypertension. This is accompanied by potentially advantageous functional changes within the myocardium and supports the hypothesis that rapid treatment of hypertension could improve clinical outcomes. TRIAL REGISTRATION: ISRCTN registry number: 57475376 (assigned 25/06/2015).

Enhanced on-chip phase measurement by inverse weak value amplification.

Optical interferometry plays an essential role in precision metrology such as in gravitational wave detection, gyroscopes, and environmental sensing. Weak value amplification enables reaching the shot-noise-limit of sensitivity, which is difficult for most optical sensors, by amplifying the interferometric signal without amplifying certain technical noises. We implement a generalized form of weak value amplification on an integrated photonic platform with a multi-mode interferometer. Our results pave the way for a more sensitive, robust, and compact platform for measuring phase, which can be adapted to fields such as coherent communications and the quantum domain. In this work, we show a 7 dB signal enhancement in our weak value device over a standard Mach-Zehnder interferometer with equal detected optical power, as well as frequency measurements with 2 kHz sensitivity by adding a ring resonator.

Experimental demonstration of superresolution of partially coherent light sources using parity sorting.

Analyses based on quantum metrology have shown that the ability to localize the positions of two incoherent point sources can be significantly enhanced over direct imaging through the use of mode sorting. Here we theoretically and experimentally investigate the effect of partial coherence on the sub-diffraction limit localization of two sources based on parity sorting. With the prior information of a negative and real-valued degree of coherence, higher Fisher information is obtained than that for the incoherent case. Our results pave the way to clarifying the role of coherence in quantum-limited metrology.

Enhanced Weak-Value Amplification via Photon Recycling.

In a quantum-noise limited system, weak-value amplification using postselection normally does not produce more sensitive measurements than standard methods for ideal detectors: the increased weak value is compensated by the reduced power due to the small postselection probability. Here, we experimentally demonstrate recycled weak-value measurements using a pulsed light source and optical switch to enable nearly deterministic weak-value amplification of a mirror tilt. Using photon counting detectors, we demonstrate a signal improvement by a factor of 4.4±0.2 and a signal-to-noise ratio improvement of 2.10±0.06, compared to a single-pass weak-value experiment, and also compared to a conventional direct measurement of the tilt. The signal-to-noise ratio improvement could reach around six for the parameters of this experiment, assuming lower loss elements.

Confocal super-resolution microscopy based on a spatial mode sorter.

Spatial resolution is one of the most important specifications of an imaging system. Recent results in the quantum parameter estimation theory reveal that an arbitrarily small distance between two incoherent point sources can always be efficiently determined through the use of a spatial mode sorter. However, extending this procedure to a general object consisting of many incoherent point sources remains challenging, due to the intrinsic complexity of multi-parameter estimation problems. Here, we generalize the Richardson-Lucy (RL) deconvolution algorithm to address this challenge. We simulate its application to an incoherent confocal microscope, with a Zernike spatial mode sorter replacing the pinhole used in a conventional confocal microscope. We test different spatially incoherent objects of arbitrary geometry, and we find that the resolution enhancement of sorter-based microscopy is on average over 30% higher than that of a conventional confocal microscope using the standard RL deconvolution algorithm. Our method could potentially be used in diverse applications such as fluorescence microscopy and astronomical imaging.

Two-Qubit Engine Fueled by Entanglement and Local Measurements.

We introduce a two-qubit engine that is powered by entanglement and local measurements. Energy is extracted from the detuned qubits coherently exchanging a single excitation. Generalizing to an N-qubit chain, we show that the low energy of the first qubit can be up-converted to an arbitrarily high energy at the last qubit by successive neighbor swap operations and local measurements. We finally model the local measurement as the entanglement of a qubit with a meter, and we identify the fuel as the energetic cost to erase the correlations between the qubits. Our findings extend measurement-powered engines to composite working substances and provide a microscopic interpretation of the fueling mechanism.

Quantum measurement arrow of time and fluctuation relations for measuring spin of ultracold atoms.

The origin of macroscopic irreversibility from microscopically time-reversible dynamical laws-often called the arrow-of-time problem-is of fundamental interest in both science and philosophy. Experimentally probing such questions in quantum theory requires systems with near-perfect isolation from the environment and long coherence times. Ultracold atoms are uniquely suited to this task. We experimentally demonstrate a striking parallel between the statistical irreversibility of wavefunction collapse and the arrow of time problem in the weak measurement of the quantum spin of an atomic cloud. Our experiments include statistically rare events where the arrow of time is inferred backward; nevertheless we provide evidence for absolute irreversibility and a strictly positive average arrow of time for the measurement process, captured by a fluctuation theorem. We further demonstrate absolute irreversibility for measurements performed on a quantum many-body entangled wavefunction-a unique opportunity afforded by our platform-with implications for studying quantum many-body dynamics and quantum thermodynamics.

Conditional teleportation of quantum-dot spin states.

Among the different platforms for quantum information processing, individual electron spins in semiconductor quantum dots stand out for their long coherence times and potential for scalable fabrication. The past years have witnessed substantial progress in the capabilities of spin qubits. However, coupling between distant electron spins, which is required for quantum error correction, presents a challenge, and this goal remains the focus of intense research. Quantum teleportation is a canonical method to transmit qubit states, but it has not been implemented in quantum-dot spin qubits. Here, we present evidence for quantum teleportation of electron spin qubits in semiconductor quantum dots. Although we have not performed quantum state tomography to definitively assess the teleportation fidelity, our data are consistent with conditional teleportation of spin eigenstates, entanglement swapping, and gate teleportation. Such evidence for all-matter spin-state teleportation underscores the capabilities of exchange-coupled spin qubits for quantum-information transfer.

Rapid treatment of moderate to severe hypertension using a novel protocol in a single-centre, before and after interventional study.

Rapid treatment to target in hypertension may have beneficial effects on long-term outcomes. This has led to a new recommendation in the 2018 European hypertension guidelines for patients with grade II/III hypertension to be treated to target within three months. However, whether it is feasible and safe to quickly manage treatment-naïve grade II/III hypertension to target was unclear. We examined this using a single-centre before and after interventional study, treating newly diagnosed, never-treated, grade II/III hypertensive patients with a daytime average systolic ABP ≥ 150 mmHg to target within 18 weeks. The proportion at office target BP at 18 weeks was determined, together with office and ambulatory BP change from baseline to after the intervention. The protocol was designed to maximise medication adherence, including a low threshold for treatment adaptation. Safety was evaluated through close monitoring of adverse events and protocol discontinuation. Fifty-five participants were enrolled with 54 completing the protocol. 69 ± 12.3% were at office target BP at their final visit, despite a high average starting BP of 175/103 mmHg, as a consequence of significant reductions in both office and ambulatory BP. Of those at office target BP, 51% were above target on ambulatory measurement. Adherence testing demonstrated that 92% of participants were adherent to treatment at their final visit. Therefore we conclude that the accelerated management of treatment-naïve grade II/III hypertension is feasible and safe to implement in routine practice and there is no evidence to suggest it causes harm. Further large-scale randomised studies of rapid, adaptive treatment, including a cost-effectiveness analysis, are required.