Parasitic crosstalk in superconducting quantum devices is a leading limitation for quantum gates. We demonstrate the suppression of static ZZ crosstalk in a two-qubit, two-coupler superconducting circuit, where the frequency of a tunable coupler can be adjusted such that the ZZ interactions from each coupler destructively interfere. We verify the crosstalk elimination with simultaneous randomized benchmarking, and use a parametrically activated iswap interaction to achieve a Bell-state preparation fidelity of 98.5% and a $\sqrt{\mathrm{i}\mathrm{swap}}$-gate fidelity of 94.8% obtained via quantum-process tomography.

Generation and control of quantum states of light on an integrated platform have become essential tools for scalable quantum technologies. Chip-scale sources such as nonlinear optical microcavities have been demonstrated to efficiently generate entangled biphoton states. However, these systems have little control over the continuous-variable time-energy entanglement of the photons. We demonstrate such control by preparing biphoton states with asymmetric temporal wave functions. This is done by selectively modifying the density of states of the cavity modes participating in the interaction using Rayleigh-scattering-induced strong coupling of optical modes of a resonator. These states reveal exotic coherence properties and show a path forward for continuous-variable quantum-state engineering on a chip.

The ubiquitous presence of $1/f$ flux noise was a significant barrier to long coherence times in superconducting qubits until the development of qubits that could operate in static, flux-noise-insensitive configurations commonly referred to as “sweet spots.” Several proposals for entangling gates in superconducting qubits tune the flux bias away from these spots, thus reintroducing the dephasing problem to various degrees. Here we revisit one such proposal, where interactions are parametrically activated by rapidly modulating the flux bias of the qubits around these sweet spots, and study the effect of modulation on the sensitivity to flux noise. We explicitly calculate how dephasing rates depend on different components of the flux-noise spectrum, and show that, although the qubits are parked at flux-insensitive points, the modulation results in increased dephasing rate due to the multiplicative $1/f$ noise and white noise components. Remarkably, we find a sweet spot under flux modulation, which we dub the “ac sweet spot,” that is insensitive to $1/f$ flux noise. We show that simple filtering of the flux control signal additionally protects parametric entangling gates from white noise in the control electronics at this ac sweet spot, allowing interactions of quality that is limited only by higher-order effects and other sources of noise.

Information reconciliation is crucial for continuous-variable quantum key distribution (CV QKD) because its performance affects the secret key rate and maximal secure transmission distance. Fixed-rate error-correction codes limit the potential applications of the CV QKD because of the difficulty of optimizing such codes for different low SNRs. In this Paper, we propose a rateless reconciliation protocol combined multidimensional scheme with Raptor codes that not only maintains the rateless property but also achieves high efficiency in different SNRs using just one degree distribution. It significantly decreases the complexity of optimization and increases the robustness of the system. Using this protocol, the CV QKD system can operate with the optimal modulation variance, which maximizes the secret key rate. Simulation results show that the proposed protocol can achieve reconciliation efficiency of more than 95% within the range of SNR from $-20$ to 0 dB. It also shows that we can obtain a high secret key rate at arbitrary distances in a certain range and achieve a secret key rate of about $5\times {10}^{-4}$ bits/pulse at a maximum distance of 132 km (corresponding SNR is $-20$ dB) that is higher than previous works. The proposed protocol can maintain highly efficient key extraction under the wide range of SNRs and paves the way toward the practical application of CV QKD systems in flexible scenarios.

Quantum bits or qubits naturally decohere by becoming entangled with uncontrollable environments. Dynamical decoupling is, therefore, required to disentangle qubits from an environment by periodically reversing the qubit bases, but this causes rotation error to accumulate. Whereas a conventional qubit is rotated within the SU(2) two-level system, a geometric qubit defined in the degenerate subspace of a V-shaped SU(3) three-level system is geometrically rotated via the third ancillary level to acquire a geometric phase. We demonstrate that simply by introducing detuning, the dynamical decoupling of the geometric qubit on a spin triplet electron in a nitrogen-vacancy center in diamond can be made to spontaneously suppress error accumulation. The geometric dynamical decoupling extends the coherence time of the geometric qubit up to 1.9 ms, limited by the relaxation time, with 128 decoupling gates at room temperature. Our technique opens a route to holonomic quantum memory for use in various quantum applications requiring sequential operations.

Heralding techniques are useful in quantum communication to circumvent losses without resorting to error-correction schemes or quantum repeaters. Such techniques are realized, for example, by monitoring for photon loss at the receiving end of the quantum channel while not disturbing the transmitted quantum state. We describe and experimentally benchmark a scheme that incorporates error detection in a direct quantum channel connecting two transmon qubits using traveling microwave photons. This is achieved by encoding the quantum information as a time-bin superposition of a single photon, which simultaneously realizes high communication rates and high fidelities. The presented scheme is straightforward to implement in circuit quantum electrodynamics and is fully microwave controlled, making it an interesting candidate for future modular quantum-computing architectures.

We propose a theoretical scheme to achieve strong photon blockade via a single atom in a cavity. By utilizing the optical Stark shift, the dressed-state splitting between the higher and lower branches is enhanced, which results in a significant increase in the lower (higher) branch and a decrease in the higher (lower) branch for a negative (positive) Stark shift, and dominates the time evolution of the photon-number oscillations as well. Furthermore, two-photon excitation is suppressed via quantum interference for the optimal phase and Rabi frequency of an external microwave field. It is shown that the interplay between quantum interference and the enhanced vacuum Rabi splitting gives rise to a strong photon blockade that can be used to realize a high-quality single-photon source beyond the regime of strong atom-cavity coupling. In particular, by tuning the optical Stark shift, the second-order correlation function in our scheme can be made three orders of magnitude smaller than in the Jaynes-Cummings model, and, correspondingly, a large cavity photon number appears as well. Our proposal may imply exciting opportunities for potential applications in quantum networks and quantum information processing.

Quantum processors rely on classical electronic controllers to manipulate and read out the state of quantum bits (qubits). As the performance of the quantum processor improves, nonidealities in the classical controller can become the performance bottleneck for the whole quantum computer. To prevent such limitation, this paper presents a systematic study of the impact of the classical electrical signals on the qubit fidelity. All operations, i.e., single-qubit rotations, two-qubit gates, and readout, are considered, in the presence of errors in the control electronics, such as static, dynamic, systematic, and random errors. Although the presented study could be extended to any qubit technology, it currently focuses on single-electron spin qubits, because of several advantages, such as purely electrical control and long coherence times, and for their potential for large-scale integration. As a result of this study, detailed electrical specifications for the classical control electronics for a given qubit fidelity can be derived. We also discuss how qubit fidelity is affected by the limited performance of the general-purpose room-temperature equipment typically employed to control the few qubits available today. Ultimately, we show that tailor-made electronic controllers can achieve significantly lower power, cost, and size, as required to support the scaling up of quantum computers.

We present a numerical study of a microelectromechanical-system-based design of a fiber cavity integrated with an ion-trap system. Each fiber mirror is supported by a microactuator that controls the mirror’s position in three dimensions. The mechanical stability is investigated by a feasibility analysis, which shows that the actuator offers stable support of the fiber. The actuators move the fibers’ positions continuously with a stroke of more than $10\mu \mathrm{m}$, with mechanical resonance frequencies on the order of kilohertz. A calculation of the trapping potential shows that a separation between the ion and the fiber consistent with strong ion-cavity coupling is feasible. Our miniaturized ion-photon interface constitutes a viable approach to integrated hardware for quantum information.

We develop a concept for a traveling-wave parametric amplifier driven by a magnetic flux wave. The circuit consists of a serial array of symmetric dc superconducting quantum-interference devices coupled inductively to a separate superconducting $LC$ transmission line carrying the pump wave. The adjusted phase velocity of the pump flux wave of frequency ${\omega }_p$ ensures amplification of the signal (${\omega }_s$) and idler (${\omega }_i$) waves, with frequencies obeying the relation ${\omega }_s+{\omega }_i={\omega }_p$. The advantage of the proposed flux-driven linear circuit includes a large gain over a wide frequency range and overcoming of the pump depletion problem. Unlike conventional traveling-wave amplifiers, the signal and pump in the proposed circuit are applied to different ports, what can greatly simplify the whole measurement setup. The experimental parameters and characteristics of this amplifier are evaluated and show promise for applications in quantum-information single-photon circuits.

A superconducting nanowire single-photon detector (SNSPD) has played a significant role in numerous applications for visible and near-infrared photon detection. SNSPDs with high system detection efficiency (SDE greater than 90%) would enable remarkable experiments in quantum information processing. Currently, niobium nitride- ($\mathrm{Nb}\mathrm{N}$) based SNSPDs are widely used in practical applications since they can operate in an inexpensive compact closed-cycle cryostat. However, it is a challenge to realize a $\mathrm{Nb}\mathrm{N}$ SNSPD with saturated intrinsic detection efficiency (IDE) while maintaining its high SDE at near-infrared wavelengths. We develop a postprocessing method to enhance the IDE of $\mathrm{Nb}\mathrm{N}$ SNSPDs to saturation without sacrificing SDE through defect engineering by a 20-keV helium ion irradiation. The enhancement of IDE is achieved because of the irradiation-induced reduction of the superconducting energy gap and the electron density of states at the Fermi level as determined by the electrical transport measurements. The change in the optical absorptance of the irradiated SNSPD is insignificant at the resonant wavelength as confirmed by the measured optical reflectance and SDE. By taking advantage of the IDE enhancement, the SDE of an irradiated device is significantly increased from 49% to 92% at 2.2 K for a 1550-nm wavelength.

Quantum plasmonics experiments have on multiple occasions resulted in the observation of quantum coherence of discrete plasmons, which exhibit remarkable preservation of quantum interference visibility, a seemingly surprising feature for systems mixing light and matter with high Ohmic losses during propagation. However, most experiments to date used essentially weakly confined plasmons, which experience limited light-matter hybridization, thus limiting the potential for decoherence. In this paper, we investigate experimentally the robustness of coherence preservation in a plasmonic system: our setup is based on a hole-array chip supporting plasmons near the surface plasma frequency, where plasmonic dispersion and confinement are much stronger than in previous experiments, making the plasmons much more susceptible for decoherence processes. We, however, report preservation of quantum coherence even in these extreme conditions. We generate polarization-entangled pairs of photons using type-I spontaneous parametric down-conversion and transmit one of the photons through a plasmonic hole array that is numerically designed to convert incident single photons into highly dispersive single surface-plasmon polaritons. Our results show that the quality of photon entanglement after the plasmonic channel is unperturbed by the introduction of a highly dispersive plasmonic element. Our findings provide a lower bound of 100 fs for the pure dephasing time for dispersive plasmons in gold, and show that even in a highly dispersive regime surface plasmons preserve quantum mechanical correlations, making possible harnessing of the power of extreme light confinement for integrated quantum photonics.

Among various multipartite entangled states, Dicke states stand out because their entanglement is maximally persistent and robust under particle losses. Although they have attracted much attention for their potential applications in quantum information processing and foundational studies, the characterization of Dicke states remains as a challenging task in experiments. Here we propose efficient and practical protocols for verifying arbitrary $n$-qubit Dicke states in both adaptive and nonadaptive ways. Our protocols require only two distinct settings based on Pauli measurements besides permutations of the qubits. To achieve infidelity $ϵ$ and confidence level $1-\delta$, the total number of tests required is only $O(n{ϵ}^{-1}\ln {\delta }^{-1})$. This performance is exponentially more efficient than all previous protocols based on local measurements, including quantum state tomography and direct fidelity estimation, and is comparable to the best global strategy. Our protocols are readily applicable with current experimental techniques and are able to verify Dicke states of hundreds of qubits.

Treatment of laboratory-grown diamond by electron irradiation and annealing has enabled quantum sensors based on negatively charged nitrogen-vacancy ($\mathrm{N}$-$V^{-}$) centers to demonstrate record sensitivities. Here we investigate the irradiation and annealing process applied to 28 diamond samples using an ambient-temperature, all-optical approach. As the presence of the neutrally charged nitrogen-vacancy ($\mathrm{N}$-$V^0$) center is deleterious to sensor performance, this photoluminescence decomposition analysis is first used to determine the concentration ratio of $\mathrm{N}$-$V^{-}$ to $\mathrm{N}$-$V^0$ in diamond samples from the measured photoluminescence spectrum. The analysis hinges on (i) isolating each $\mathrm{N}$-$V$ charge state’s emission spectrum and (ii) measuring the $\mathrm{N}$-$V^{-}$ to $\mathrm{N}$-$V^0$ emission ratio, which is found to be $2.5\pm 0.5$ under low-intensity 532-nm illumination. Using the photoluminescence-decomposition-analysis method, we measure the effects of irradiation and annealing on conversion of substitutional nitrogen to $\mathrm{N}$-$V$ centers. Combining these measurements with a phenomenological model for diamond irradiation and annealing, we extract an estimated monovacancy creation rate of $0.52\pm 0.26{\mathrm{cm}}^{-1}$ for 1-MeV electron irradiation and an estimated monovacancy diffusion coefficient of 1.8 ${\mathrm{nm}}^2/$s at $850{}^{\circ }\mathrm{C}$. Finally, we find that irradiation doses of ${10}^{18}{e}^{-}$/${\mathrm{cm}}^2$ or more deteriorate the $\mathrm{N}$-$V^{-}$ decoherence time $T_2$, whereas $T_1$ is unaffected up to the the maximum investigated dose of $5\times {10}^{18}{e}^{-}$/${\mathrm{cm}}^2$.

The growing field of quantum computing is based on the concept of a q-bit, which is a delicate superposition of 0 and 1, requiring cryogenic temperatures for its physical realization along with challenging coherent coupling techniques for entangling them. By contrast, a probabilistic bit or a $p$-bit is a robust classical entity that fluctuates between 0 and 1 and can be implemented at room temperature using present-day technology. Here, we show that a probabilistic coprocessor built out of room-temperature $p$-bits can be used to accelerate simulations of a special class of quantum many-body systems that are sign-problem–free or “stoquastic,” leveraging the well-known Suzuki-Trotter decomposition that maps a $d$-dimensional quantum many-body Hamiltonian to a $d+1$-dimensional classical Hamiltonian. This mapping allows an efficient emulation of a quantum system by classical computers and is commonly used in software to perform quantum Monte Carlo (QMC) algorithms. By contrast, we show that a compact, embedded magnetic tunnel junction (MTJ)-based coprocessor can serve as a highly efficient hardware accelerator for such QMC algorithms, providing an improvement in speed of several orders of magnitude compared to optimized CPU implementations. Using realistic device-level spice simulations, we demonstrate that the correct quantum correlations can be obtained using a classical $p$-circuit built with existing technology and operating at room temperature. The proposed coprocessor can serve as a tool to study stoquastic quantum many-body systems, overcoming challenges associated with physical quantum annealers.

Indistinguishable single photons are key ingredients for a plethora of quantum-information-processing applications, ranging from quantum communications to photonic quantum computing. A mainstream platform to produce indistinguishable single photons over a wide spectral range is based on biphoton generation through spontaneous parametric down-conversion in nonlinear crystals. The purity of the biphotons produced is, however, limited by their spectral correlations. Here we present a design recipe, based on a machine-learning framework, for the engineering of biphoton joint spectral amplitudes over a wide spectral range. By customizing the poling profile of the ${\mathrm{K}\mathrm{Ti}\mathrm{O}\mathrm{P}\mathrm{O}}_4$ crystal, we show, numerically, that spectral purities of 99.22%, 99.99%, and 99.82%, respectively, can be achieved in the 1310-, 1550-, and 1600-nm bands after applying a moderate 8-nm filter. The machine-learning framework thus enables the generation of near-indistinguishable single photons over the entire telecommunication band without resorting to the ${\mathrm{K}\mathrm{Ti}\mathrm{O}\mathrm{P}\mathrm{O}}_4$ crystal’s group-velocity-matching wavelength window near 1582 nm.

We study theoretically how loss impacts the amplification and squeezing performance of a generic quantum traveling-wave parametric amplifier. Unlike previous studies, we analyze how having different levels of loss at signal and idler frequencies can dramatically alter properties compared to the case of frequency-independent loss. We find that loss asymmetries increase the amplifier’s added noise in comparison to the symmetric loss case. More surprisingly, even small levels of loss asymmetry can completely destroy any quantum squeezing of symmetric collective-output quadratures, while nonetheless leaving the output state strongly entangled.

We experimentally demonstrate a photon-pair source, based on continuous-wave pumped spontaneous four-wave mixing in a silicon microring resonator, which efficiently generates photon pairs with high coincidence counts, high brightness, and high coincidence-to-accidental ratio at multiple resonance wavelengths matching a standard International Telecommunication Union (ITU) frequency grid. The signal-idler joint spectral intensity is measured with a very high resolution of $2\mathrm{pm}$ over multiple wavelengths by an optical spectrum analyzer through probe-swept stimulated four-wave mixing, allowing an accurate quantification of the spectral correlation. Strong spectral correlation and uniform degree of quantum correlation are observed from the source, and the measured amount of spectral modes agrees well with that obtained by unheralded second-order correlation measurements.

A transducer capable of converting quantum information stored as microwaves into telecom-wavelength signals is a critical piece of future quantum technology as it promises to enable the networking of quantum processors. Cavity optomechanical devices that are simultaneously coupled to microwave fields and optical resonances are being pursued in this regard. Yet even in the classical regime, developing optical modulators based on cavity optomechanics could provide alternatives to current technology. Here we demonstrate a magnetically mediated wavelength-conversion technique based on mixing high-frequency tones with an optomechanical torsional resonator. This process can act either as an optical phase modulator or an amplitude modulator depending on the experimental configuration, and the carrier modulation is always coherent with the input tone. Such coherence allows classical information transduction and transmission via the technique of phase-shift keying. We demonstrate that we can encode up to eight bins of information, corresponding to three bits, simultaneously and demonstrate the transmission of a 52 500-pixel image over 6 km of optical fiber with just 0.67% error. Furthermore, we show that magneto-optomechanical transduction can be described in a fully quantum manner, implying that this is a possible approach to signal transduction in the quantum regime.

The reference-frame-independent quantum key distribution (RFI QKD) protocol can reduce the requirements on the alignment of reference frames in practical systems. However, compared with the Bennett-Brassard (BB84) QKD protocol, the main drawback of RFI QKD is that Alice needs to prepare six encoding states in three mutually unbiased bases ($X$, $Y$, and $Z$), and Bob also needs to measure the quantum state in three such bases. Here, we show that the RFI QKD protocol can be secured in a case where Alice sends fewer states. In particular, we find that transmitting three states (two eigenstates of the $Z$ basis and one of the eigenstates of the $X$ basis) is sufficient to obtain comparable secret-key rates and distances covered, even when security against coherent attacks with statistical fluctuations of the finite-key size is considered. Finally, a proof-of-principle experiment based on time-bin encoding is demonstrated to show the feasibility of our scheme, and with the three-state protocol, the experimental complexity is effectively alleviated; in particular, a high-speed phase modulator is no longer required by Alice.