We investigate how the voltage control of the exciton lateral dipole moment induces a transition from singly to doubly connected topology in type-II $\mathrm{In}\mathrm{As}$/${\mathrm{Ga}\mathrm{As}}_x{\mathrm{Sb}}_{1-x}$ quantum dots. The latter causes visible Aharonov-Bohm oscillations and a change of the exciton $g$ factor, which are modulated by the applied bias. The results are explained in the frame of realistic $\overrightarrow{k}\cdot \overrightarrow{p}$ and effective Hamiltonian models and could open a venue for new spin quantum memories beyond the $\mathrm{In}\mathrm{As}$/$\mathrm{Ga}\mathrm{As}$ realm.

Energy transport is of central importance in understanding a wide variety of transitions of physical states in nature. Recently, the coherence and noise have been identified for their existence and key roles in energy-transport processes, for instance, in a photosynthesis complex, DNA, and odor sensing etc., of which one may have to reveal the inner mechanics in the quantum regime. Here we present an analog of Newton’s cradle by manipulating a boundary-controlled chain on a photonic chip. Long-range interactions can be mediated by a long chain composed of $21$ strongly coupled sites, where single-photon excitations are transferred between two remote sites via simultaneous control of intersite weak and strong couplings. We observe a high retrieval efficiency in both uniform and defect-doped chain structures. Our results may offer a flexible approach to Hamiltonian engineering beyond geometric limitation, enabling the design and construction of quantum simulators on demand.

A spectrally uncorrelated biphoton state generated from the spontaneous nonlinear optical process is an important resource for quantum information. Currently, such a spectrally uncorrelated biphoton state can only be prepared from limited kinds of nonlinear media, thus limiting their wavelengths. In order to explore a wider wavelength range, here we theoretically study the generation of a spectrally uncorrelated biphoton state from 14 isomorphs of potassium dihydrogen phosphate (KDP) crystal. We find that 11 crystals from the “KDP family” still maintain similar nonlinear optical properties of KDP, such as KDP, DKDP, ADP, DADP, ADA, DADA, RDA, DRDA, RDP, DRDP, and KDA, which satisfy three kinds of the group-velocity matching conditions for spectrally uncorrelated biphoton-state generation from near-infrared to telecom wavelengths. Based on the uncorrelated biphoton state, we investigate the generation of a heralded pure-state single photon by detecting one member of the biphoton state to herald the output of the other. The purity of the heralded single photon is as high as 0.98 without using a narrow-band filter; the Hong-Ou-Mandel interference from independent sources can also achieve a visibility of 98%. This study may provide more and better single-photon sources for quantum-information processing at near-infrared and telecom wavelengths.

Previously, we developed a set of photon-free quantum random-number generators (QRNGs) using $\mathrm{In}\text{−}\mathrm{Ga}\text{−}\mathrm{As}$ single-photon avalanche diodes. We exploited the stochastic property of the quantum tunneling effect. Here, we utilize tunneling signals in $\mathrm{Si}$ diodes to generate random numbers. In our experiment, instead of applying periodic pulses to diodes, we apply a fixed voltage and detect the random time intervals between adjacent tunneling signals. This $\mathrm{Si}$ QRNG generates raw data with its ratio of min-entropy over ideal entropy reaching 0.98, which is higher than many other QRNG configurations. The final data rate is 6.98 MB/s and could reach 23 MB/s with a more efficient cooling system.

The twin-field quantum key distribution (TFQKD) protocol and its variants, e.g., phase-matching (PM) QKD and TFQKD based on sending or not sending, are highly attractive since they are able to overcome the well-known rate-loss limit for QKD protocols without a repeater: $R=O(\eta )$, with $\eta$ standing for the channel transmittance. However, all these protocols require active phase randomization and postselection that play an essential role together in their security proof. Counterintuitively, we find that in TFQKD, beating the rate-loss limit is still possible even if phase randomization and postselection in the coding mode are both removed, which means our final secure key rate $R=O(\sqrt{\eta })$. Furthermore, our protocol is more feasible in practice and more promising according to its higher final key rate in the valid distance. Our security proof counters a collective attack and can also counter a coherent attack in the asymptotical case.

We fabricate and investigate a stacked two-chip device, consisting of a lumped-element resonator on one chip, which is side coupled to a coplanar waveguide transmission line on a second chip. We present a full model to predict the behavior of the device dependent on the position of the lumped-element resonator with respect to the transmission line. We identify different regimes in which the device can be operated. One of them can be used to tune the coupling between the two subsystems. Another regime enables frequency tunability of the device, without leaving the overcoupled limit for internal quality factors of about ${10}^4$, while in the last regime the resonator properties are insensitive against small variations of the position. Finally, we measure the transmission characteristics of the resonator for different positions, demonstrating a good agreement with the model.

Use of the spatial degree of freedom vastly enhances the informational capacity of light at the cost of stringent requirements on the processing devices. Multimode quantum memories are a viable candidate for quantum and classical information processing; however, full use of the assets of high-dimensionality requires a flexible processing technique. We use a spatially varying ac Stark effect to perform arbitrary one-dimensional phase modulation of a coherent spin-wave state stored in a wavevector-multiplexed quantum memory. A far-field characterization and an interferometric near-field characterization of the introduced phase profiles are presented. Additionally, coherence between temporally separated partial readouts of a single coherent spin-wave state is demonstrated, offering possible applications in adaptive measurements via conditional spin-wave modulation.

Wavelengths in the telecommunication window (approximately 1.25–1.65 $\mu \mathrm{m}$) are ideal for quantum communication due to low transmission loss in fiber networks. To realize quantum networks operating at these wavelengths, long-lived quantum memories that couple to telecom-band photons with high efficiency need to be developed. We propose coupling neutral ytterbium atoms, which have a strong telecom-wavelength transition, to a silicon photonic crystal cavity. Specifically, we consider the ${}^3{P}_0{\leftrightarrow }^3{D}_1$ transition in neutral ${}^{171}\mathrm{Yb}$ to interface its long-lived nuclear spin in the metastable ${}^3{P}_0$ “clock” state with a telecom-band photon at $1.4\mu \mathrm{m}$. We show that $\mathrm{Yb}$ atoms can be trapped using a short-wavelength (approximately $470$ nm) tweezer at a distance of 350 nm from the silicon photonic crystal cavity. At this distance, due to the slowly decaying evanescent cavity field at a longer wavelength, we obtain a single-photon Rabi frequency of $g/2\pi \approx 100$ MHz and a cooperativity of $C\approx 47$ while maintaining a high photon collection efficiency into a single mode fiber. The combination of high system efficiency, telecom-band operation, and long coherence times makes this platform well suited for quantum optics on a silicon chip and long-distance quantum communication.

We theoretically investigate a near-quantum-limited parametric amplifier based on the nonlinear dynamics of quasiparticles flowing through a superconductor-insulator-superconductor junction. Photon-assisted tunneling, resulting from the combination of dc- and ac-voltage bias, gives rise to a strong parametric interaction for the electromagnetic modes reflected by the junction coupled to a transmission line. We show phase-sensitive and phase-preserving amplification, together with single- and two-mode squeezing. For an aluminum junction pumped at twice the center frequency, ${\omega }_0/2\pi =6\mathrm{GHz}$, we predict narrow-band phase-sensitive amplification of microwaves signals to more than 20 dB, and broadband phase-preserving amplification of 20 dB over a 3-dB bandwidth of 1.2 GHz. We also predict single- and two-mode squeezing reaching more than $-12\mathrm{dB}$ over a 3-dB bandwidth of 5.3-GHz. Moreover, with a simple impedance-matching circuit, we demonstrate a 3-dB bandwidth reaching 4.3 GHz for 20 dB of gain. A key feature of the device is that its performance can be controlled in situ with the applied dc- and ac-voltage biases.

For a frequency-tunable two-qubit system, a controlled-Z (cz) gate can be realized by adiabatic driving of the qubit system through an avoided level crossing between an auxiliary state and computational levels. Here we theoretically propose a fast cz gate using a shortcut to adiabaticity (STA). Experimentally, the STA cz gate is implemented with a 52-ns control pulse for two coupled superconducting Xmon qubits. The measured fidelity of the STA cz gate is higher than 96.0% in both quantum-process tomography and randomized benchmarking. The protocol allows a flexible design of the evolution time and control waveform. We suggest that this “fast adiabatic” cz gate can be directly applied to other multiqubit quantum systems.

Waveguide-based spin-photon interfaces on the $\mathrm{Ga}\mathrm{As}$ platform have emerged as a promising system for a variety of quantum information applications directly integrated into planar photonic circuits. The coherent control of spin states in a quantum dot can be achieved by applying circularly polarized laser pulses that may be coupled into the planar waveguide vertically through radiation modes. However, proper control of the laser polarization is challenging since the polarization is modified through the transformation from the far field to the exact position of the quantum dot in the nanostructure. Here, we demonstrate polarization-controlled excitation of a quantum-dot electron spin and use that to perform coherent control in a Ramsey interferometry experiment. The Ramsey interference reveals an inhomogeneous dephasing time of $2.2\pm 0.1$ ns, which is comparable to the values so far only obtained in bulk media. We analyze the experimental limitations in spin initialization fidelity and Ramsey contrast and identify the underlying mechanisms.

Directional transmission or amplification of microwave signals is indispensable in various applications involving sensitive measurements. In this work we show experimentally how to use a generic cavity optomechanical setup to nonreciprocally amplify microwave signals above 3 GHz in one direction by 9 dB and simultaneously attenuate the transmission in the opposite direction by 21 dB. We use a device including two on-chip superconducting resonators and two metallic drumhead mechanical oscillators. Application of four microwave pump-tone frequencies allows the design of constructive or destructive interference for a signal tone depending on the propagation direction. The device can also be configured as an isolator with lossless nonreciprocal transmission and 18 dB of isolation.

Quantum reservoir computing provides a framework for exploiting the natural dynamics of quantum systems as a computational resource. It can implement real-time signal processing and solve temporal machine-learning problems in general, which requires memory and nonlinear mapping of the recent input stream using the quantum dynamics in the computational supremacy region, where the classical simulation of the system is intractable. A nuclear-magnetic-resonance spin-ensemble system is one of the realistic candidates for such physical implementations, which is currently available in laboratories. In this paper, considering these realistic experimental constraints for implementing the framework, we introduce a scheme, which we call a spatial multiplexing technique, to effectively boost the computational power of the platform. This technique exploits disjoint dynamics, which originate from multiple different quantum systems driven by common input streams in parallel. Accordingly, unlike designing a single large quantum system to increase the number of qubits for computational nodes, it is possible to prepare a huge number of qubits from multiple but small quantum systems, which are operationally easy to handle in laboratory experiments. We numerically demonstrate the effectiveness of the technique using several benchmark tasks and quantitatively investigate its specifications, range of validity, and limitations in detail.

We report on the implementation and detailed modeling of a Josephson parametric amplifier (JPA) made from an array of eighty superconducting quantum interference devices (SQUIDs), forming a nonlinear quarter-wave resonator. This device is fabricated using a very simple single-step fabrication process. It shows a large bandwidth (45 MHz), an operating frequency tunable between 5.9 and 6.8 GHz, and a large input saturation power ($-117\mathrm{dBm}$) when biased to obtain 20 dB of gain. Despite the length of the SQUID array being comparable to the wavelength, we present a model based on an effective nonlinear $LC$ series resonator that quantitatively describes these figures of merit without fitting parameters. Our work illustrates the advantage of using array-based JPA since a single-SQUID device showing the same bandwidth and resonant frequency would display a saturation power 15 dB lower.

We propose a fast spin-initialization method for a single electron trapped in an electrostatic quantum dot. The dot is created in a nanodevice composed of a catalytically grown indium antimonide ($\mathrm{In}\mathrm{Sb}$) nanowire and nearby gates to which control voltages are applied. Initially, we insert a single electron of arbitrary spin into the wire. Operations on spin are performed using the Rashba spin-orbit interaction induced by an electric field. First, a single pulse of voltages applied to lateral gates is used to split the electron wave packet into two parts with opposite spin orientations. Next, another voltage pulse applied to the remaining gates rotates the spins of both parts in opposite directions by $\pi /2$. In this way, initially opposite-spin parts eventually point in the same direction, along the axis of the quantum wire. We thus set spin in a predefined direction regardless of its initial orientation. This is achieved in time less than $60\mathrm{ps}$, without the use of microwaves, photons, or external magnetic fields.

We generate correlated photon pairs in an all-pass silicon microring resonator using continuous-wave pumped spontaneous four-wave mixing and we characterize in detail the optical bistability in the nonclassical single-photon regime. Special attention is given to the resonance-locked method by balancing the pump-induced heating and the active off-chip cooling, leading to stable operation of the photon-pair source. The maximal coincidence rate of $20.53\pm 0.34\mathrm{Hz}$, maximal coincidence-to-accidental ratio of $654\pm 125$, and minimal zero-delayed heralded second-order correlation of $0.14\pm 0.09$ are achieved. The wavelengths of photon pairs remain unchanged at different pump power, indicating that our source is more compatible with the common communication systems than those using conventional on-resonance strategies, that tunes the pump wavelength. This work not only experimentally validates the nonclassical bistability behaviors, but also puts forward a simple method of pump-resonance matching for high-quality performance.

We report on the isolation of single negatively-charged-silicon-vacancy ($\mathrm{Si}$-$V^{-}$) centers in nanodiamonds. We observe the fine structure of single $\mathrm{Si}$-$V^{-}$ centers with reduced inhomogeneous ensemble linewidth below the excited-state splitting, stable optical transitions, good polarization contrast, and excellent spectral stability under resonant excitation. On the basis of our experimental results, we develop an analytical strain model where we extract the ratio between strain coefficients of excited and ground states as well the intrinsic zero-strain spin-orbit splittings. The observed strain values are as low as the best values in low-strain bulk diamond. We achieve our results by means of $\mathrm{H}$-plasma treatment of the diamond surface and in combination with resonant and off-resonant excitation. Our work paves the way for indistinguishable, single-photon emission. Furthermore, we demonstrate controlled nanomanipulation by an atomic-force-microscope cantilever of one- and two-dimensional alignments with an accuracy of about 10 nm, as well as new tools including dipole rotation and cluster decomposition. Combined, our results show the potential to utilize $\mathrm{Si}$-$V^{-}$ centers in nanodiamonds for controlled interfacing via optical coupling of individually-well-isolated atoms for bottom-up assemblies of complex quantum systems.

A quantum photonic integrated circuit (QPIC) is a promising tool for constructing integrated devices for quantum-technology applications. In the optical regime, silicon photonics empowered by complementary-metal-oxide-semiconductor (CMOS) technology provides optical components useful for realizing large-scale QPICs. Optical nonlinearity at the single-photon level is required for QPICs to facilitate photon-photon interaction. However, to date, realization of optical elements with deterministic (i.e., not probabilistic) single-photon nonlinearity by use of silicon-based components is challenging despite the enhancement of the functionality of QPICs based on silicon photonics. In this study, we realize a strongly coupled $\mathrm{In}\mathrm{As}$/$\mathrm{Ga}\mathrm{As}$ quantum-dot–cavity-quantum-electrodynamics system on a CMOS-processed silicon photonic chip. The heterogeneous integration of the $\mathrm{Ga}\mathrm{As}$ cavity on the silicon chip is performed by transfer printing. The cavity-quantum-electrodynamics system on the CMOS photonic chip realized in this work is a promising candidate for an on-chip single-photon nonlinear element, which constitutes the fundamental component for future applications based on QPICs, such as coherent manipulation and nondestructive measurement of qubit states via a cavity, and an efficient single-photon filter and router.

Quantum key distribution (QKD) guarantees the secure communication between legitimate parties with quantum mechanics. High-dimensional QKD (HDQKD) not only increases the secret key rate but also tolerates higher quantum bit error rate (QBER). Many HDQKD experiments have been realized by utilizing orbital-angular-momentum (OAM) photons as the degree of freedom (DOF) of OAM of the photon is a prospective resource for HD quantum information. In this work, we propose that a high-quality HDQKD based on polarization-OAM hybrid states can be realized by utilizing state mapping between different DOFs and we characterize this HDQKD. Both the preparation and measurement procedures of the proof-of-principle verification experiment are simple and stable. Our experiment verifies that a $0.60\mathrm{\%}\pm 0.06\mathrm{\%}$ QBER and a $1.849\pm 0.008$-bit secret key rate per sifted signal can be achieved for a four-dimensional QKD with the weak coherent light source and decoy state method.

At low temperatures, microwave cavities are often preferred for the readout and control of a variety of systems. In this paper, we present design and measurements on an optomechanical device based on a three-dimensional rectangular waveguide cavity. We show that by suitably modifying the electromagnetic field corresponding to the fundamental mode of the cavity, the equivalent circuit capacitance can be reduced to 29 fF. By coupling a mechanical resonator to the modified electromagnetic mode of the cavity, we achieve a capacitance participation ratio of $43\mathrm{\%}$. We demonstrate an optomechanical cooperativity, $C\sim$ 40, characterized by performing measurements in the optomechanically induced absorption limit. In addition, due to a low-impedance environment between the two halves of the cavity, our design has the flexibility of incorporating a dc bias across the mechanical resonator, often a desired feature in tunable optomechanical devices.