We introduce a microwave-assisted spectroscopy technique to determine the relative ratio of fluorescence emitted by nitrogen-vacancy (N-$V$) centers in diamond that are negatively charged ($\mathrm{N}\text{−}{V}^{-}$) and neutrally charged ($\mathrm{N}\text{−}{V}^0$) and present its application to studying spin-dependent ionization in N-$V$ ensembles and enhancing N-$V$-magnetometer sensitivity. Our technique is based on selectively modulating the $\mathrm{N}\text{−}{V}^{-}$ fluorescence with a spin-state-resonant microwave drive to isolate, in situ, the spectral shape of the $\mathrm{N}\text{−}{V}^{-}$ and $\mathrm{N}\text{−}{V}^0$ contributions to an N-$V$-ensemble sample’s fluorescence. As well as serving as a reliable means to characterize the charge state, the method can be used as a tool to study spin-dependent ionization in N-$V$ ensembles. As an example, we apply the microwave technique to a high-N-$V$-density diamond sample and find evidence for an additional spin-dependent ionization pathway, which we present here alongside a rate-equation model of the data. We further show that our method can be used to enhance the contrast of optically detected magnetic resonance (ODMR) on N-$V$ ensembles and may lead to significant sensitivity gains in N-$V$ magnetometers dominated by technical noise sources, especially where the $\mathrm{N}\text{−}{V}^0$ population is large. With the high-N-$V$-density diamond sample investigated here, we demonstrate an up to 4.8-fold enhancement in the ODMR contrast. We also propose a second postprocessing method of increasing the ODMR contrast in shot-noise-limited applications. The techniques presented here may also be applied to other solid-state defects, as long as their fluorescence can be selectively modulated by means of a microwave drive. We demonstrate this utility by applying our method to isolate room-temperature spectral signatures of the V2-type silicon vacancy from an ensemble of V1 and V2 silicon vacancies in $4H$ silicon carbide.

Electric field tuning of single $\mathrm{In}\mathrm{As}$ quantum dots (QDs) provides an important tool for physics experiments and for the development of scalable and in situ–tunable devices. Control of a single QD with either a growth direction or an in-plane electric field is well established, but a device that can apply the two-dimensional (2D) vector electric fields necessary to simultaneously and independently tune more than one parameter has not yet been demonstrated. We use comsol multiphysics simulations to illustrate the device-design challenges in applying a uniform 2D electric field in a $\mathrm{Ga}\mathrm{As}$ solid-state system and to systematically explore the effects of electrode coverage, mesa size, and doping on the device performance. We develop and present a design that can apply 2D electric fields to a single QD while remaining compatible with optical experiments. We then fabricate and characterize a proof-of-concept device that validates the design presented here. We discuss the applications for this device and the potential for full 2D-electric-field control of a single QD or a quantum-dot molecule.

Single molecules in solids have been considered as an attractive class of solid-state single quantum systems because they can be chemically synthesized to have stable transitions at desired wavelengths. Here, we report and demonstrate single dibenzoterrylene molecules in crystalline anthracene nanosheets as a solid-state platform for delicate photophysics studies and as building blocks of single-photon devices. The nanosheet sample enables studies of single-molecule photophysics at room temperature, including in situ observation of single-molecule insertion-site jump, quantitative measurements of the associated changes of dipole-moment orientation and magnitude, and determination of the excitation-power-dependent intersystem crossing rate and the triplet lifetime. Moreover, we demonstrate the flexible assembly of the nanosheets into a planar Yagi-Uda antenna device to achieve a Gaussian-like angular emission pattern from single dibenzoterrylene molecules. The low thickness (approximately 100 nm), good photostability, and mechanical stability make the dibenzoterrylene-in-anthracene-nanosheet system an excellent candidate for static quantum nodes in integrated photonic circuits.

The nitrogen-vacancy center in diamond has been explored extensively as a light-matter interface for quantum information applications, however, it is limited by low coherent photon emission and spectral instability. Here, we present a promising interface based on an alternative defect with superior optical properties (the germanium-vacancy) coupled to a finesse-11 000 fiber cavity, resulting in a ${31}_{-15}^{+11}$-fold increase in the spectral density of zero-phonon-line emission. This work sets the stage for cryogenic experiments, where we predict a measurable increase in the spontaneous emission rate.

There are two main factors limiting the performance of quantum key distribution—channel transmission loss and noise. Previously, a linear bound was believed to put an upper limit on the rate-transmittance performance. Remarkably, the recently proposed twin-field and phase-matching quantum key distribution schemes have been proven to overcome the linear bound. In practice, due to the intractable phase fluctuation of optical signals in transmission, these schemes suffer from large error rates, which renders the experimental realization extremely challenging. Here, we close this gap by proving the security based on a different principle—encoding symmetry. With the symmetry-based security proof technique, we can decouple the privacy from the channel disturbance, and eventually remove the limitation of secure key distribution on bit error rates. As a direct application, we show that the phase-matching scheme can yield positive key rates even with high bit error rates up to 50%. In the simulation, with typical experimental parameters, the key rate is able to break the linear bound with an error rate of 13%. Meanwhile, we provide a simple finite-data size analysis for the phase-matching scheme under this symmetry-based analysis, which can break the bound with a reasonable data size of ${10}^{12}$. Encouraged by high loss and error tolerance, we expect the approach based on symmetry-protected privacy will provide a different insight into the security of quantum key distribution.

Fast, robust two-qubit gate operation with low susceptibility to crosstalk are the key to scalable quantum-information processing. A parametrically driven gate is inherently insensitive to crosstalk, while superadiabatic control can speed up the gate without losing accuracy. We propose and experimentally implement superadiabatic two-qubit gates using parametric modulation on superconducting quantum circuits. Our results demonstrate the preservation of adiabaticity at a gate speed close to the quantum limit, in addition to robustness against control instability. We demonstrate a controlled-$Z$ gate with an error rate of $5.8\mathrm{\%}$, mainly limited by qubit decoherence, promising future improvement and scalable implementation.

A semiclassical model is presented for characterizing the linear response of elementary quantum-optical systems involving cavities, optical fibers, and atoms. Formulating the transmission and reflection spectra using a scattering-wave (transfer-matrix) approach, the calculations become easily scalable. To demonstrate how useful this method is, we consider the example of a simple quantum network, i.e., two cavity-QED systems connected via an optical fiber. Differences between our quasiexact transfer-matrix approach and a single-mode, linearized quantum-optical model are demonstrated for parameters relevant to recent experiments with coupled nanofiber-cavity-QED systems.

The dominant source of decoherence in contemporary frequency-tunable superconducting qubits is $1/f$ flux noise. To understand its origin and find ways to minimize its impact, we systematically study flux noise amplitudes in more than 50 flux qubits with varied superconducting quantum interference device (SQUID) geometry parameters and compare our results to a microscopic model of magnetic spin defects located at the interfaces surrounding the SQUID loops. Our data are in agreement with an extension of the previously proposed model, based on numerical simulations of the current distribution in the investigated SQUIDs. Our results and detailed model provide a guide for minimizing the flux noise susceptibility in future circuits.

We make use of a custom-designed cryo-CMOS multiplexer to enable multiple quantum devices to be characterized in a single cooldown of a dilution refrigerator. Combined with a packaging approach that integrates cryo-CMOS chips and a hot-swappable parallel-device test platform, we describe how this setup takes the standard wiring configuration of a dilution refrigerator as input and expands the capability for batch characterization of quantum devices at milli-Kelvin temperatures and high magnetic fields. The architecture of the cryo-CMOS multiplexer is discussed, and its performance is benchmarked using few-electron quantum dots and Hall mobility mapping measurements.

Using a high quality factor 3 GHz bulk acoustic wave resonator device, we demonstrate the acoustically driven single quantum spin transition ($|m_s=0⟩\leftrightarrow |\pm 1⟩$) for diamond nitrogen-vacancy (N-$V$) centers and characterize the corresponding stress susceptibility. A key challenge is to disentangle the unintentional magnetic driving field generated by the device current from the intentional stress driving within the device. We quantify these driving fields independently using Rabi spectroscopy before studying the more complicated case in which both are resonant with the single quantum spin transition. By building an equivalent circuit model to describe the device’s current and mechanical dynamics, we quantitatively model the experiment to establish their relative contributions and compare with our results. We find that the stress susceptibility of the N-$V$ center spin single quantum transition is around $\sqrt{2}(0.5\pm 0.2)$ times that for double quantum transition ($|+1⟩\leftrightarrow |-1⟩$). Although acoustic driving in the double quantum basis is valuable for quantum-enhanced sensing applications, double quantum driving lacks the ability to manipulate N-$V$ center spins out of the $|m_s=0⟩$ initialization state. Our results demonstrate that efficient all-acoustic quantum control over N-$V$ centers is possible, and is especially promising for sensing applications that benefit from the compact footprint and location selectivity of acoustic devices.

In the Gaussian-modulated coherent state quantum key distribution (QKD) protocol, the sender first generates Gaussian-distributed random numbers and then encodes them on weak laser pulses actively by performing amplitude and phase modulations. Recently, an equivalent passive QKD scheme has been proposed by exploring the intrinsic field fluctuations of a thermal source [B. Qi, P. G. Evans, and W. P. Grice, Phys. Rev. A 97, 012317 (2018)]. This passive QKD scheme is especially appealing for chip-scale implementation since no active modulation is required. In this paper, we conduct an experimental study of the passively encoded QKD scheme using an off-the-shelf amplified spontaneous emission source operated in continuous-wave mode. Our results show that the excess noise introduced by the passive state preparation scheme can be effectively suppressed by applying optical attenuation and a secure key can be generated over metro-area distances.

A cavity quantum electrodynamical (QED) system beyond the strong-coupling regime is expected to exhibit intriguing quantum phenomena. Here we report a direct measurement of the photon-dressed qubit transition frequencies up to four photons by harnessing the same type of state transitions in an ultrastrongly coupled circuit QED system realized by inductively coupling a superconducting flux qubit to a coplanar-waveguide resonator. This demonstrates a convincing observation of the photon-dressed Bloch-Siegert shift in the ultrastrongly coupled quantum system. Moreover, our results show that the photon-dressed Bloch-Siegert shift becomes more pronounced as the photon number increases, which is a characteristic of the quantum Rabi model.

The application in cryptography of quantum algorithms for prime factorization fostered the interest in quantum computing. However, quantum computers, and particularly quantum annealers, can also be helpful to construct secure cryptographic keys. Indeed, finding robust Boolean functions for cryptography is an important problem in sequence ciphers, block ciphers, and hash functions, among others. Due to the superexponential size $\mathcal{O}(2^{2^n})$ of the associated space, finding $n$-variable Boolean functions with global cryptographic constraints is computationally hard. This problem has already been addressed employing generic low-connected incoherent D-Wave quantum annealers. However, the limited connectivity of the Chimera graph, together with the exponential growth in the complexity of the Boolean-function design problem, limit the problem scalability. Here, we propose a special-purpose coherent quantum-annealing architecture with three couplers per qubit, designed to optimally encode the bent-function design problem. A coherent quantum annealer with this tree-type architecture has the potential to solve the eight-variable bent-function design problem, which is classically unsolved, with only $127$ physical qubits and $126$ couplers. This paves the way to reach useful quantum supremacy within the framework of quantum annealing for cryptographic purposes.

Most quantum processors requires pulse sequences for controlling quantum states. Here we present an alternative algorithm for computing an optimal pulse sequence so as to perform a specific task, being an implementation of a quantum gate or a quantum-state preparation. In our method, we reduce greatly the number of parameters to be fitted by using a limited number of functions as the modulations for the amplitude and phase of the radio-frequency pulses. Unlike other algorithms, ours uses some approximations to make it fast and scalable, and uses some efficient methods to obtain pulses that are robust to common types of experimental errors. We demonstrate the success of the proposed algorithm by performing several real experiments for 4-, 7-, and 12-quantum-bit systems using NMR. In addition, we show the efficiency of the algorithm, finding pulses for controlling with good fidelity the quantum states of spins in fictional square two-dimensional lattices containing 16, 36, and 100 qubits.

We design superinductors made of strongly disordered superconductors for implementation in “hybrid” superconducting quantum circuits. The superinductors are fabricated as meandered nanowires made from granular aluminum films. Optimization of the device geometry enables the realization of superinductors with an inductance of approximately 1 $\mu \mathrm{H}$ and a self-resonance frequency of over 3 GHz. These compact superinductors are attractive for a wide range of applications, from superconducting circuits for quantum computing to microwave elements of cryogenic parametric amplifiers and kinetic inductance photon detectors.

A single-photon source is an essential tool for the emerging field of quantum technologies. Ideally, it should be spectrally compatible with other photonic devices while providing a high flux of narrow-band photons. The single organic dye molecule dibenzanthanthrene under cryogenic conditions possesses the given characteristics and therefore constitutes a prominent single-photon source. Nevertheless, the implementation of such a single-photon source requires a complex experimental setup involving a cryostat with a confocal microscope for the effective collection of the molecular emission. In the approach presented here we use a single emitter coupled directly to the end facet of an optical fiber. This has the potential to transfer a single-photon source based on a quantum emitter from a proof-of-principle type of setup to a scalable “plug-and-play” device. We present successful coupling of a single organic molecule to an optical fiber and record the excitation spectrum, measure the saturation curve, and analyze the contributions of Raman background fluorescence. The single-photon nature is proven by an antibunched autocorrelation recording, which reveals coherent Rabi oscillations.