We demonstrate the generation of multimode entangled states of propagating microwaves. The entangled states are generated by our parametrically pumping a multimode superconducting cavity. By combining different pump frequencies, applied simultaneously to the device, we can produce different entanglement structures in a programable fashion. The Gaussian output states are fully characterized by our measuring the full covariance matrices of the modes. The covariance matrices are absolutely calibrated by our using an in situ microwave calibration source, a shot-noise tunnel junction. Applying a variety of entanglement measures, we demonstrate both full inseparability and genuine tripartite entanglement of the states. Our method is easily extensible to more modes.

Preserving coherence long enough to perform meaningful calculations is one of the major challenges on the pathway to large-scale quantum-computer implementations. Noise coupled in from the environment is the main factor contributing to decoherence but can be mitigated via engineering design and control solutions. However, this is possible only after acquisition of a thorough understanding of the dominant noise sources and their spectrum. In the work reported here, we use a silicon quantum dot spin qubit as a metrological device to study the noise environment experienced by the qubit. We compare the sensitivity of this qubit to electrical noise with that of an implanted silicon-donor qubit in the same environment and measurement setup. Our results show that, as expected, a quantum dot spin qubit is more sensitive to electrical noise than a donor spin qubit due to the larger Stark shift, and the noise-spectroscopy data show pronounced charge-noise contributions at intermediate frequencies (2–20 kHz).

We report a single-stage bidirectional interface capable of linking ${\mathrm{Sr}}^{+}$ trapped-ion qubits in a long-distance quantum network. Our interface converts photons between the ${\mathrm{Sr}}^{+}$ emission wavelength at 422 nm and the telecom $C$ band to enable low-loss transmission over optical fiber. We have achieved both up- and down-conversion at the single-photon level with efficiencies of 9.4% and 1.1%, respectively. Furthermore, we demonstrate noise levels that are low enough to allow for genuine quantum operation in the future.

Self-assembled quantum dots (QDs) provide an ideal platform for realization of on-demand maximally entangled photon pairs, which is essential for quantum-information technologies. When the two intermediate exciton states are indistinguishable in energy, the biexciton cascade process naturally yields the maximally entangled photon pairs. However, because of the low symmetry of QDs, the two bright states should belong to different irreducible representations in realistic experiments, and their energy difference, called the “fine-structure splitting” (FSS), is much larger than their homogeneous broadening, giving rise to only classically correlated photon pairs by erasing the “which-way” information. In this work, we show that since these two states belong to different representations, their lifetimes should also be slightly different, which is termed “exciton lifetime asymmetry.” In contrast to the extensively studied FSS, investigation of this lifetime asymmetry is missing in the literature. Here we conduct an investigation of the exciton lifetime asymmetry in self-assembled QDs and present a theory to deduce this asymmetry indirectly from measurable qualities of QDs. We further reveal that the intrinsic lifetimes and their asymmetries are more fundamental quantities of QDs, which determine the upper bound of the extrinsic lifetime asymmetries. Exact relations between lifetime asymmetries, FSS, and the polarization angle are also derived. These relations may be measured in experiments by studying the degree of polarization, as well its evolution under external stress. Our findings provide a complete description of the symmetry of the optical properties of QDs, which can give an important basis to deepen our understanding of QDs.

We describe and implement a family of entangling gates activated by radio-frequency flux modulation applied to a tunable transmon that is statically coupled to a neighboring transmon. The effect of this modulation is the resonant exchange of photons directly between levels of the two-transmon system, obviating the need for mediating qubits or resonator modes and allowing for the full utilization of all qubits in a scalable architecture. The resonance condition is selective in both the frequency and amplitude of modulation and thus alleviates frequency crowding. We demonstrate the use of three such resonances to produce entangling gates that enable universal quantum computation: one $i\mathrm{SWAP}$ gate and two distinct controlled-Z gates. We report interleaved randomized benchmarking results indicating gate error rates of 6% for the $i\mathrm{SWAP}$ (duration 135 ns) and 9% for the controlled-Z gates (durations 175 and 270 ns), limited largely by qubit coherence.

The duration and fidelity of qubit readout are critical factors for applications in quantum-information processing as they limit the fidelity of algorithms which reuse qubits after measurement or apply feedback based on the measurement result. Here we present fast multiplexed readout of five qubits using a single 1.2-GHz-wide readout channel. Using a readout pulse length of 80 ns and populating readout resonators for less than 250 ns, we find an average probability of correct assignment for the five measured qubits to be $97\mathrm{\%}$. The differences between the individual readout errors and those found when measuring the qubits simultaneously are within $1\mathrm{\%}$. We employ individual Purcell filters for each readout resonator to suppress off-resonant driving, which we characterize by measuring the dephasing imposed on unintentionally measured qubits. We expect the readout scheme presented here to become particularly useful for the selective readout of individual qubits in multiqubit quantum processors.

Quantum digital signatures (QDSs) offer the information-theoretic security verified by quantum mechanics to send a message that is robust against forging and repudiation. We report a proof-of-principle demonstration of a passive decoy-state QDS system that can avoid leaking the modulation information of decoy states. To realize this, four local detection events are generated from the idler light of a parametric down-conversion process, and are then used as decoy states to estimate channel parameters more accurately. As a result, we successfully sign a more-than-one-bit message within 7 s at 100 km. Besides, we achieve a transmission distance of 200 km, which represents a further step toward the practical application of QDSs.

Time-frequency duality, which enables the control of optical waveforms by manipulating the amplitudes and phases of electromagnetic fields, plays a pivotal role in a wide range of modern optics. The conventional one-dimensional (1D) time-frequency duality has been successfully applied to characterize the behavior of classical light, such as ultrafast optical pulses from a laser. However, the 1D treatment is not enough to characterize quantum-mechanical correlations in the time-frequency behavior of multiple photons, such as the biphotons from parametric down-conversion. Two-dimensional (2D) treatment is essentially required, but has not been fully demonstrated yet due to technical problems. Here, we study the 2D time-frequency duality of biphotons, by measuring two-photon distributions in both the frequency and the time domain. It is discovered that the generated biphotons satisfy the Fourier-transform-limited conditions in a two-photon spectral distribution, but do not satisfy those conditions in the single-photon marginal distribution. Our study may pave the way for the quantum-optical synthesizer, the next-generation technology beyond the traditional optical synthesis in the time-frequency domain.

Scalable quantum computing is based on realizable accurate quantum gates. For neutral atoms, it is an outstanding challenge to design a high-fidelity two-qubit entangling gate without resorting to difficult techniques such as shaping laser pulses or cooling atoms to motional ground states. By using spin echo to suppress the blockade error, we propose an easily realizable controlled-phase Rydberg quantum gate of high intrinsic fidelity. In the context of spin echo, we show that the fundamental blockade error of the traditional Rydberg gate, of the order of $ϵ\sim {10}^{-3}$, actually results from two “clockwise” rotations of Rabi frequencies ${\overline{\mathrm{\Omega }}}_{\pm }=V\pm \sqrt{V^2+{\mathrm{\Omega }}^2}$. In our “echo” sequence, such an error can be suppressed to the order of ${ϵ}^2$ by adding two “anticlockwise” rotations with frequencies $-{\overline{\mathrm{\Omega }}}_{\pm }$. With the blockade error effectively removed, the error caused by Rydberg-state decay becomes the final fundamental limit to the gate accuracy, which, in principle, can be reduced beyond the level of ${10}^{-5}$. Furthermore, due to the small population involved in the “echo” process, the spin-echo gate is robust against the variation of Rydberg blockade caused by the drift of the qubits, so that it can still be much more accurate than that of a traditional Rydberg gate even for qubits cooled only to the submillikelvin regime.

We study a circuit, the Josephson sampler, that embeds a real vector into an entangled state of $n$ qubits, and optionally samples from it. We estimate two fidelity measures and a variety of entropies and entanglement measures for this circuit, versus depth, on the 16-qubit ibmqx5 chip. This provides a quantitative assessment of the actual performance of the circuit on a first-generation quantum computer. To assess its expressiveness, we also measure its ability to generate quantum chaos as measured by out-of-time-order correlators (OTOCs). We conjecture that OTOCs can be used to validate a source of pseudorandom unitaries with only a single instance, which has application beyond the Josephson sampler. The circuit requires nearest-neighbor controlled-Z gates on a chain and is especially well suited for first-generation superconducting architectures.

Solid-state single-photon emitters are attractive for the realization of integrated quantum systems due to their experimental convenience and scalability. Unfortunately, however, their complex mesoscopic environments cause photons from different emitters to be spectrally distinguishable. Here we demonstrate spectral alignment of two solid-state single-photon emitters by utilizing the strain gradient. Multiple germanium-vacancy (GeV) color centers in diamond are created in fabricated diamond microcantilevers using focused ion-beam implantation. The strain response of the electronic energy levels of the GeV is measured by inducing an electrically controlled strain in the device. Leveraging the large strain gradient, we tune two GeVs in the spot to emit at the same optical wavelength. Simultaneous resonant excitation of two spectrally aligned GeVs is demonstrated using a narrow linewidth laser. This strain gradient tuning can be used to realize spectrally identical quantum emitters for on-chip integrated quantum systems.

We propose a realizable circuit QED architecture for engineering states of a superconducting resonator off-resonantly coupled to an ancillary superconducting qubit. The qubit-resonator dispersive interaction together with a microwave drive applied to the qubit gives rise to a Kerr resonator with two-photon driving that enables us to efficiently engineer the quantum state of the resonator, such as generation of the Schrödinger cat states for resonator-based universal quantum computation. Moreover, the presented architecture is easily scalable for solving an optimization problem mapped into the Ising spin-glass model and thus serves as a platform for quantum annealing. Although various scalable architectures with superconducting qubits have been proposed for realizing a quantum annealer, the existing annealers are currently limited to the coherent time of the qubits. Here, based on the protocol for realizing a two-photon driven Kerr resonator in three-dimensional circuit QED (3D cQED), we propose a flexible and scalable hardware for implementing a quantum annealer that combines the advantage of the long coherence times attainable in 3D cQED and the recently proposed resonator-based Lechner-Hauke-Zoller (LHZ) scheme. In the proposed resonator-based LHZ annealer, each spin is encoded in the subspace formed by two coherent states of the 3D microwave superconducting resonator with opposite phase and thus the fully connected Ising model is mapped onto the network of the resonator with local tunable three-resonator interaction. This hardware architecture provides a promising physical platform for realizing a quantum annealer with improved coherence.

We investigate a hybrid quantum system where an ensemble of nitrogen-vacancy (NV) centers in diamond is interfaced with a piezomagnetic superlattice that supports surface-phonon polaritons (SPHPs). We show that the strong magnetic coupling between the collective spin waves in the NV spin ensemble and the quantized SPHPs can be realized, due to the subwavelength nature of the SPHPs and relatively long spin-coherence times. The magnon-polariton coupling allows different modes of the SPHPs to be mapped and orthogonally stored in different spatial modes of excitation in the solid medium. Because of its easy implementation and high tunability, the proposed hybrid structure with NV spins and piezoactive superlattices could be used for quantum memory and quantum computation.

Electrically active defects have a significant impact on the performance of electronic devices based on wide-band-gap materials. This issue is ubiquitous in diamond science and technology, since the presence of charge traps in the active regions of different classes of diamond-based devices (detectors, power diodes, transistors) can significantly affect their performance, due to the formation of space charge, memory effects, and the degradation of the electronic response associated with radiation-induced damage. Among the most common defects in diamond, the nitrogen-vacancy (N-V) center possesses unique spin properties that enable high-sensitivity field sensing at the nanoscale. Here, we demonstrate that N-V ensembles can be successfully exploited to perform direct local mapping of the internal electric-field distribution of a graphite-diamond-graphite junction exhibiting electrical properties dominated by trap- and space-charge-related conduction mechanisms. By means of optically detected magnetic resonance measurements, we performed both point-by-point readout and spatial mapping of the electric field in the active region at different bias voltages. In this novel “self-diagnostic” approach, defect complexes represent not only the source of detrimental space-charge effects but also a unique tool for their direct investigation, by providing an insight on the conduction mechanisms that could not be inferred in previous studies on the basis of conventional electrical and optical characterization techniques.

Developing fast, accurate, and scalable techniques for quantum-state readout is an active area in semiconductor-based quantum computing. Here, we present results on dispersive sensing of silicon corner state quantum dots coupled to lumped-element electrical resonators via the gate. The gate capacitance of the quantum device is placed in parallel with a superconducting spiral inductor resulting in resonators with loaded $Q$ factors in the 400–800 range. We utilize resonators operating at 330 and 616 MHz, and achieve charge sensitivities of 7.7 and $1.3\mu e/\sqrt{\mathrm{Hz}}$, respectively. We perform a parametric study of the resonator to reveal its optimal operation points and perform a circuit analysis to determine the best resonator design. The results place gate-based sensing on a par with the best reported radio-frequency single-electron transistor sensitivities while providing a fast and compact method for quantum-state readout.

We study theoretically the dynamics of a driven-dissipative qubit-resonator system. Specifically, a transmon qubit is coupled to a transmission-line resonator; this system is considered to be probed via a resonator, by means of either continuous or pulsed measurements. Analytic results obtained in the semiclassical approximation are compared with calculations in the semiquantum theory as well as with the previous experiments. We demonstrate that the temperature dependence of the resonator frequency shift can be used for the system thermometry and that the dynamics, which display a pinched-hysteretic curve, can be useful for realization of the memory devices termed quantum memcapacitors.

Decoherence remains a major challenge in quantum computing hardware, and a variety of physical-layer controls provide opportunities to mitigate the impact of this phenomenon through feedback and feed-forward control. In this work, we compare a variety of machine-learning algorithms derived from diverse fields for the task of state estimation (retrodiction) and forward prediction of future qubit-state evolution for a single qubit subject to classical, non-Markovian dephasing. Our approaches involve the construction of a dynamical model capturing qubit dynamics via autoregressive or Fourier-type protocols using only a historical record of projective measurements. A detailed comparison of achievable prediction horizons, model robustness, and measurement-noise-filtering capabilities for Kalman filters (KFs) and Gaussian process regression (GPR) algorithms is provided. We demonstrate superior performance from the autoregressive KF relative to Fourier-based KF approaches and focus on the role of filter optimization in achieving suitable performance. Finally, we examine several realizations of GPR using different kernels and discover that these approaches are generally not suitable for forward prediction. We highlight the linkages between predictive performance and kernel structure, and we identify ways in which forward predictions are susceptible to numerical artifacts.

We report on the conversion to telecommunication wavelengths of single photons emitted by a nitrogen-vacancy (NV) defect in diamond. By means of difference frequency generation, we convert spin-selective photons at 637 nm, associated with the coherent NV zero-phonon line, to the target wavelength of 1588 nm in the $L$-telecommunication band. The successful conversion is evidenced by time-resolved detection revealing a telecommunication-photon lifetime identical to that of the original 637-nm photon. Furthermore, we show by second-order correlation measurements that the single-photon statistics are preserved. The overall efficiency of this one-step conversion reaches 17% in our current setup, along with a signal-to-noise ratio of approximately 7 despite the low probability $(<{10}^{-3})$ of an incident 637-nm photon. This result shows the potential for efficient telecommunication-photon–NV-center interfaces and marks an important step towards future long-range entanglement-based quantum networks.

We propose a quantum algorithm for simulating spin models based on the periodic modulation of transmon qubits. Using the Floquet theory, we derive an effective time-averaged Hamiltonian, which is of the general $XYZ$ class, different from the isotropic $XY$ Hamiltonian typically realized by the physical setup. As an example, we provide a simple recipe to construct a transverse Ising Hamiltonian in the Floquet basis. For a 1D system, we demonstrate numerically the dynamical simulation of the transverse Ising Hamiltonian and quantum annealing to its ground state. We benchmark the Floquet approach with a digital simulation procedure and demonstrate that it is advantageous for limited resources and finite anharmonicity of the transmons. The described protocol represents a hardware-efficient quantum software and can serve as a simple yet reliable path towards configurable quantum simulators with currently existing superconducting chips.

Most quantum error-correcting codes assume that the decoherence of each physical qubit is independent of the decoherence of any other physical qubit. We can test the validity of this assumption in an experimental setup where a microwave feedline couples to multiple qubits by examining correlations between the qubits. We investigate the correlations between fluxonium qubits located in a single waveguide. Despite being in a wide-bandwidth electromagnetic environment, the qubits have measured relaxation times in excess of $100\mu \mathrm{s}$. We use cascaded Josephson parametric amplifiers to measure the quantum jumps of two fluxonium qubits simultaneously. No correlations are observed between the relaxation times of the two fluxonium qubits, which indicates that the sources of relaxation are local to each qubit in our setup. Our correlation analysis can be generalized to different types of qubits and our architecture can easily be scaled to monitor larger numbers of qubits.