Sympathetic cooling is a technique often employed to mitigate motional heating in trapped-ion quantum computers. However, choosing system parameters such as number of coolants and cooling duty cycle for optimal gate performance requires evaluating trade-offs between motional errors and other slower errors such as qubit dephasing. The optimal parameters depend on cooling power, heating rate, and ion spacing in a particular system. In this study, we aim to analyze best practices for sympathetic cooling of long chains of trapped ions using analytical and computational methods. We use a case study to show that optimal cooling performance is achieved when coolants are placed at the center of the chain and provide a perturbative upper bound on the cooling limit of a mode given a particular set of cooling parameters. In addition, using computational tools, we analyze the trade-off between the number of coolant ions in a chain and the center-of-mass mode heating rate. We also show that cooling as often as possible when running a circuit is optimal when the qubit coherence time is otherwise long. These results provide a roadmap for how to choose sympathetic cooling parameters to maximize circuit performance in trapped-ion quantum computers using long chains of ions.

In the field of quantum information, hybrid quantum systems comprising magnons and solid-state spins are gaining significant interest. Despite their potential, the scalability of magnon-based quantum devices faces challenges, primarily due to the necessity for close proximity in direct magnon coupling. In this paper, we introduce an approach that enables remote coupling of magnon arrays through microwave photons within a superconducting coplanar waveguide resonator. By applying periodic time-varying magnetic fields, we demonstrate the ability to finely control magnon frequencies, thereby facilitating the adjustment of magnon-magnon hopping dynamics. With appropriate magnetic modulation, the resulting coupled magnon chain emulates a topological reservoir, exemplified by the Su-Schrieffer-Heeger model. When interacting with solid-state spins, such as nitrogen-vacancy centers in diamond, our configuration enables the magnon chain to act as a mediator for tunable and chiral spin-spin interactions. This leads to the generation of significant entanglement between pairs of spins. Our work proposes a scalable quantum device architecture based on magnons and solid-state spins, offering broad possibilities for advances in quantum information processing.

High-rate, high-fidelity entanglement distribution is essential to the creation of a quantum internet, but recent achievements in fiber (248 km at a 9-${\mathrm{s}}^{-1}$ rate) and satellite-based (1200 km at a 1.1-${\mathrm{s}}^{-1}$ rate) entanglement distribution fall far short of what is needed. Chen et al. [Phys. Rev. Appl. 19, 054209 (2023)] proposed a means for dramatically increasing entanglement-distribution rates via a scheme they called zero-added-loss multiplexing (ZALM). ZALM’s quantum transmitter employs a pair of Sagnac-configured spontaneous parametric down-converters (SPDCs), channelization via dense wavelength-division multiplexing (DWDM) filtering, and partial Bell-state measurements (BSMs) to realize a heralded source of frequency-multiplexed polarization-entangled biphotons. Each biphoton is transmitted to Alice and Bob along with a classical message identifying its frequency channel and whether a ${\psi }^{-}$ singlet or a ${\psi }^{+}$ triplet was heralded. Alice’s and Bob’s quantum receivers then use DWDM filtering and temporal-mode conversion to interface their received biphotons to intracavity color-center quantum memories. This paper delves deeply into ZALM’s SPDCs, partial BSMs, and Duan-Kimble loading of Alice’s and Bob’s quantum memories. Its principal results—the density operators for the SPDC sources and the quantum memories—allow heralding probability, heralding efficiency, and fidelity to be evaluated for both the polarization-entangled biphotons and the loaded quantum memories, thus enabling exploration of the parameter space for optimizing ZALM’s performance. Even without a comprehensive optimization analysis, the paper’s examples already demonstrate two critical features of the ZALM architecture: (1) the necessity of achieving a near-separable channelized biphoton wave function to ensure that the biphoton sent to Alice and Bob is of high purity; and (2) the premium placed on Alice’s and Bob’s temporal-mode converters enabling narrowband push-pull memory loading to ensure that the arriving biphoton’s state is faithfully transferred to the intracavity color centers.

We reexamine the well-known Duan-Kimble entanglement scheme, wherein the state of a single-photon qubit is entangled with a quantum memory consisting of a single-atom qubit in a strongly coupled optical cavity, providing the capability to load the photon’s state into the memory. We correct a common error appearing in some subsequent papers regarding the validity of the single-photon reflectivity function that characterizes the essential phase shift at the heart of the protocol. Using the validated analytical solution, we introduce an improved scheme—the push-pull configuration—where the photon and cavity are tuned at the midpoint between atomic resonances and show that it can outperform the original on-off configuration in which the photon and cavity are tuned exactly to one of the atomic resonances. The performance metric used is the final memory-state fidelity versus the heralding probability, which determines the memory-loading rate. The results should play a role in optimizing future quantum repeater schemes based on the Duan-Kimble protocol.

We propose and demonstrate transmission-based dispersive readout of a superconducting qubit using an all-pass resonator, which preferentially emits readout photons toward the output. This is in contrast to typical readout schemes, which intentionally mismatch the feedline at one end so that the readout signal preferentially decays toward the output. We show that this intentional mismatch creates scaling challenges, including larger spread of effective resonator linewidths due to nonideal impedance environments and added infrastructure for impedance matching. A future architecture using multiplexed all-pass readout resonators would avoid the need for intentional mismatch and potentially improve the scaling prospects of quantum computers. As a proof-of-concept demonstration of “all-pass readout,” we design and fabricate an all-pass readout resonator that demonstrates insertion loss below 1.17 dB at the readout frequency and a maximum insertion loss of 1.53 dB across its full bandwidth for the lowest three states of a transmon qubit. We demonstrate qubit readout with an average single-shot fidelity of 98.1% in 600 ns; to assess the effect of larger dispersive shift, we implement a shelving protocol and achieve a fidelity of 99.0% in 300 ns.

Discrete-modulated continuous-variable quantum key distribution offers a pragmatic solution, greatly simplifying experimental procedures, while retaining robust integration with classical optical communication. Theoretical analyses have progressively validated the comprehensive security of this protocol, paving the way for practical experimentation. However, imperfect sources in practical implementations introduce noise. The traditional approach is to assume that eavesdroppers can control all of the source noise, which overestimates the ability of eavesdroppers and underestimates the secret-key rate. Some parts of source noise are intrinsic and cannot be manipulated by the eavesdropper, so they can be seen as trusted noise. We tailor a trusted-noise model specifically for the discrete-modulated protocol and upgrade the security analysis accordingly. Simulation results demonstrate that this approach successfully mitigates the negative impact of an imperfect source on system performance, while maintaining security of the protocol. Furthermore, our method can be used in conjunction with a trusted-detector-noise model, effectively reducing the influence of both source noise and detector noise in the experimental setup. This is a meaningful contribution to the practical deployment of discrete-modulated-continuous-variable-quantum-key-distribution systems.

Characterizing temporally correlated noise and “non-Markovian” qubit dynamics is a key prerequisite for achieving noise-tailored error mitigation and optimal device performance. Quantum noise spectroscopy can provide quantitative estimation of the noise spectral features; however, in its current form it is highly vulnerable to implementation nonidealities, notably, state preparation and measurement (SPAM) errors. Further to that, existing protocols have been mostly developed for dephasing-dominated noise processes, with competing dephasing and relaxation effects being largely unaccounted for. We introduce quantum noise spectroscopy protocols inspired by spin-locking techniques that enable the characterization of arbitrary temporally correlated multiaxis noise on a qubit with fixed energy splitting, while remaining resilient to realistic static SPAM errors. By validating the performance of our protocol in both numerical simulation and on cloud-based IBM quantum processors, we demonstrate the successful separation and estimation of native noise spectrum components as well as SPAM error rates. We find that SPAM errors can significantly alter or mask important noise features, with spectra overestimated by up to 26.4% in a classical noise regime. Clear signatures of nonclassical noise are manifest in the reconstructed IBM-qubit dephasing spectra, once SPAM-error effects are compensated for. Our work provides a timely tool for benchmarking realistic sources of noise in qubit devices.

High-quality, distributed quantum entanglement is the distinctive resource for quantum communication and forms the foundation for the unequaled level of security that can be assured in quantum key distribution. While the entanglement provider does not need to be trusted, the secure key rate drops to zero if the entanglement used is too noisy. In this paper, we show experimentally that QPA is able to increase the secure key rate achievable with QKD by improving the quality of distributed entanglement, thus increasing the quantum advantage in QKD. Beyond that, we show that QPA enables key generation at noise levels that previously prevented key generation. These remarkable results were only made possible by the efficient implementation exploiting hyperentanglement in the polarization and energy-time degrees of freedom. We provide a detailed characterization of the gain in secure key rate achieved in our proof-of-principle experiment at different noise levels. The results are paramount for the implementation of a global quantum network linking quantum processors and ensuring future-proof data security.

Environmental fluctuations degrade the performance of solid-state qubits but can in principle be mitigated by real-time Hamiltonian estimation down to timescales set by the estimation efficiency. We implement a physics-informed and an adaptive Bayesian estimation strategy and apply them in real time to a semiconductor spin qubit. The physics-informed strategy propagates a probability distribution inside the quantum controller according to the Fokker-Planck equation, appropriate for describing the effects of nuclear spin diffusion in gallium arsenide. Evaluating and narrowing the anticipated distribution by a predetermined qubit probe sequence enables improved dynamical tracking of the uncontrolled magnetic field gradient within the singlet-triplet qubit. The adaptive strategy replaces the probe sequence by a small number of qubit probe cycles, with each probe time conditioned on the previous measurement outcomes, thereby further increasing the estimation efficiency. The combined real-time estimation strategy efficiently tracks low-frequency nuclear spin fluctuations in solid-state qubits, and can be applied to other qubit platforms by tailoring the appropriate update equation to capture their distinct noise sources.

We demonstrate parametric all-optical modulation in a periodically poled lithium niobate microring resonator on a chip. It uses quantum Zeno blockade between two distinct waves, a signal and a pump, through their sum-frequency generation at a large per-photon efficiency of $8.2$ MHz. With nanosecond pump pulses with $6$-mW peak power, $85.7\mathrm{\%}$ modulation extinction is observed, achieving an efficiency increase of more than 30 times compared with previous implementations. With only $2$-mW pump peak power, $43.0\mathrm{\%}$ modulation extinction is observed for a twice-as-strong signal of $4$ mW. This demonstrates that optical transistors with cascadability and fan-out are possible with just parametric nonlinear optics. These results, together with inherent advantages in such photonic integrated circuits, open the door to scalable technology for all-optical and quantum information processing.

Hybrid quantum photonic systems connect classical photonics to the quantum world and promise to deliver efficient light-matter quantum interfaces while leveraging the advantages of both, the classical and the quantum, subsystems. However, combining efficient, scalable photonics and solid-state quantum systems with desirable optical and spin properties remains a formidable challenge. In particular, the access to individual spin states and coherent mapping to photons remains unsolved for hybrid systems. In this paper, we demonstrate all-optical initialization and readout of the electron spin of a negatively charged silicon-vacancy center in a nanodiamond coupled to a silicon nitride photonic crystal cavity. We characterize relevant parameters of the coupled emitter-cavity system and determine the silicon-vacancy center’s spin-relaxation and spin-decoherence rate. Our results mark a key step towards the realization of a hybrid spin-photon interface based on silicon nitride photonics and the silicon-vacancy center’s electron spin in nanodiamonds with potential use for quantum networks, quantum communication, and distributed quantum computation.

Future quantum technologies such as quantum communication, quantum sensing, and distributed quantum computation, will rely on networks of shared entanglement between spatially separated nodes. In this work, we provide improved protocols and policies for entanglement distribution along a linear chain of nodes, both homogeneous and inhomogeneous, that take practical limitations into account. For a wide range of parameters, our policies improve upon previously known policies, such as the “swap-as-soon-as-possible” policy, with respect to both the waiting time and the fidelity of the end-to-end entanglement. This improvement is greatest for the most practically relevant cases, namely, for short coherence times, high link losses, and highly asymmetric links. To obtain our results, we model entanglement distribution using a Markov decision process, and then we use the $Q$-learning reinforcement-learning (RL) algorithm to discover alternative policies. These policies are characterized by dynamic, state-dependent memory cutoffs, and collaboration between the nodes. In particular, we quantify this collaboration between the nodes. Our quantifiers tell us how much “global” knowledge of the network every node has, specifically, how much knowledge two distant nodes have of each other’s states. In addition to the usual figures of merit, these quantifiers add an extra dimension to the performance analysis and practical implementation of quantum repeaters. Finally, our understanding of the performance of large quantum networks is currently limited by the computational inefficiency of simulating them using RL or other optimization methods. The other main contribution of our work is to address this limitation. We present a method for nesting policies in order to obtain policies for large repeater chains. By nesting our RL-based policies for small repeater chains, we obtain policies for large repeater chains that improve upon the swap-as-soon-as-possible policy, and thus we pave the way for a scalable method for obtaining policies for long-distance entanglement distribution under practical constraints.

A continuous-variable quantum key distribution (CV QKD) using a true local (located at the receiver) oscillator (LO) has been proposed to remove any possibility of side-channel attacks associated with transmission of the LO as well as reduce the cross-pulse contamination. Here we report an implementation of true LO-CV QKD using “off-the-shelf” components and conduct QKD experiments using the fiber optical network at Oak Ridge National Laboratory. A phase reference and quantum signal are time multiplexed and then wavelength division multiplexed with the classical communications that “coexist” with each other on a single optical network fiber. This is the first demonstration of CV QKD with a receiver-based true LO over a deployed fiber network, a crucial step for its application in real-world situations.

Superconducting parametric amplifiers play a crucial role in the preparation and readout of quantum states at microwave frequencies, enabling high-fidelity measurements of superconducting qubits. Most existing implementations of these amplifiers rely on the nonlinearity from Josephson junctions, superconducting quantum interference devices, or disordered superconductors. Additionally, frequency tunability arises typically from either flux or current biasing. In contrast, semiconductor-based parametric amplifiers are tunable by local electric fields, which impose a smaller thermal load on the cryogenic setup than current and flux biasing and lead to vanishing crosstalk to other on-chip quantum systems. In this work, we present a gate-tunable parametric amplifier that operates without Josephson junctions, using a proximitized semiconducting nanowire. This design achieves near-quantum-limited performance, featuring more than 20-dB gain and a 30-MHz gain-bandwidth product. The absence of Josephson junctions results in advantages, including substantial saturation powers of $-120$ dBm, magnetic field compatibility up to $500\mathrm{mT}$, and frequency tunability over a range of 15 MHz. Our realization of a parametric amplifier supplements efforts towards gate-controlled superconducting electronics, further advancing the abilities for high-performing quantum measurements of semiconductor-based and superconducting quantum devices.

Atomic engineering in a solid-state material has the potential to functionalize the host with novel phenomena. STM-based lithographic techniques have enabled the placement of individual phosphorus atoms at selective lattice sites of silicon with atomic precision. Here we show that by placing four phosphorus donors spaced 10–15 nm apart from their neighbors in a linear chain, one can realize coherent spin coupling between the end dopants of the chain, analogous to the superexchange interaction in magnetic materials. Since phosphorus atoms are a promising building block of a silicon quantum computer, this enables spin coupling between their bound electrons beyond nearest neighbors, allowing the qubits to be separated by 30–45 nm. The added flexibility in architecture brought about by this long-range coupling not only reduces gate densities but can also reduce correlated noise between qubits from local noise sources that are detrimental to error-correction codes. We base our calculations on a full-configuration-interaction technique in the atomistic tight-binding basis, solving the four-electron problem exactly, over a domain of a million silicon atoms. Our calculations show that superexchange can be tuned electrically through gate voltages where it is less sensitive to charge noise and donor-placement errors.

Microwave-to-optical transducers are integral to the future of superconducting quantum computing, as they would enable scaling and long-distance communication of superconducting quantum processors through optical-fiber links. However, optically induced microwave noise poses a significant challenge in achieving quantum transduction between microwave and optical frequencies. In this work, we study light-induced microwave noise in an integrated electro-optical transducer harnessing the Pockels effect of thin-film lithium niobate. We reveal three sources of added noise with distinctive time constants ranging from sub-$100\mathrm{ns}$ to milliseconds. Our results provide insights into the mechanisms and corresponding mitigation strategies for light-induced microwave noise in superconducting microwave-optical transducers and pave the way toward realizing the ultimate goal of quantum transduction.

Superconducting cavities have emerged as a key tool for measuring the spin states of quantum dots. So far, however, few experiments have explored longitudinal couplings between quantum dots and cavities, and no solid-state qubit experiments have explicitly probed the “adiabatic” regime, where the Purcell decay is strongly suppressed. Here we report measurements of a double-quantum-dot charge qubit coupled to a high-impedance resonator via a “flip-chip” design geometry. By applying an adiabatic ac drive to the qubit through two different channels, and studying the effects of qubit energy detuning, interdot tunneling, and driving strength, we are able to unequivocally confirm the presence of a longitudinal coupling between the qubit and the cavity, while the qubit remains in its ground state. Since this coupling is proportional to the driving amplitude, and is therefore switchable, it has the potential to become a powerful tool in qubit experiments.

Quantum key distribution (QKD) protocols have been proven to be secure on the basis of fundamental physical laws; however, the proofs consider a well-defined setting and encoding of the sent quantum signals only. Side channels, where the encoded quantum state is correlated with properties of other degrees of freedom of the quantum channel, allow an eavesdropper to obtain information unnoticeably, as demonstrated in a number of hacking attacks on the quantum channel. However, also classical radiation emitted by the devices may be correlated, leaking information on the potential key, especially when combined with novel data-analysis methods. We demonstrate here a side-channel attack using a deep convolutional neural network to analyze the recorded classical, radio-frequency electromagnetic emissions. Even at a distance of a few centimeters from the electronics of a QKD sender containing frequently used electronic components, we are able to recover virtually all information about the secret key. However, as shown here, countermeasures can enable a significant reduction of both the emissions and the amount of secret-key information leaked to the attacker. Our analysis methods are independent of the actual device and thus provide a starting point for assessing the presence of classical side channels in QKD devices.

Superconducting qubits typically use a dispersive readout scheme, where a resonator is coupled to a qubit such that its frequency is qubit-state dependent. Measurement is performed by driving the resonator, where the transmitted resonator field yields information about the resonator frequency and thus the qubit state. Ideally, we could use arbitrarily strong resonator drives to achieve a target signal-to-noise ratio in the shortest possible time. However, experiments have shown that when the average resonator photon number exceeds a certain threshold, the qubit is excited out of its computational subspace in a process we refer to as a measurement-induced state transition (MIST). These transitions degrade readout fidelity, and constitute leakage, which precludes further operation of the qubit in, for example, error correction. Here we study these transitions experimentally with a transmon qubit by measuring their dependence on qubit frequency, average resonator photon number, and qubit state, in the regime where the resonator frequency is lower than the qubit frequency. We observe signatures of resonant transitions between levels in the coupled qubit-resonator system that exhibit noisy behavior when measured repeatedly in time. We provide a semiclassical model of these transitions based on the rotating-wave approximation and use it to predict the onset of state transitions in our experiments. Our results suggest the transmon is excited to levels near the top of its cosine potential following a state transition, where the charge dispersion of higher transmon levels explains the observed noisy behavior of state transitions. Moreover, we show that occupation in these higher energy levels poses a major challenge for fast qubit reset.

${}^{171}{\mathrm{Yb}}^{3+}$ in ${\mathrm{YVO}}_4$ is a promising candidate for building quantum networks with good optical addressability, excellent spin properties and a secondary nuclear-spin quantum register. However, the associated long optical lifetime necessitates coupling to optical resonators for faster emission of single photons and to facilitate control of single ${}^{171}\mathrm{Yb}$ ions. Previously, single ${}^{171}\mathrm{Yb}$ ions were addressed by coupling them to monolithic photonic crystal cavities fabricated via lengthy focused ion beam milling. Here, we design and fabricate a hybrid platform based on ions coupled to the evanescently decaying field of a GaAs photonic crystal cavity. For the most strongly coupled ion close to the GaAs-YVO interface, we find a 64-fold reduction in lifetime corresponding to a Purcell enhancement of 179. For an ion with a Purcell enhancement of 21, we experimentally detect and demonstrate coherent optical control. The results show a promising route toward a quantum network with ${}^{171}\mathrm{Yb}$-${\mathrm{YVO}}_4$ using a highly scalable platform that can readily be applied to other quantum emitters in the near-infrared.