This paper proposes an optimized quantum block-$ZXZ$ decomposition method that results in more optimal quantum circuits than the quantum Shannon decomposition, which was presented in 2005 by M. Möttönen, and J. J. Vartiainen [in Trends in quantum computing research, edited by S. Shannon (Nova Science Publishers, 2006) Chap. 7, p. 149, arXiv:quant-ph/0504100]. The decomposition is applied recursively to generic quantum gates, and can take advantage of existing and future small-circuit optimizations. Because our method uses only single-qubit gates and uniformly controlled rotation-Z gates, it can easily be adapted to use other types of multi-qubit gates. With the proposed decomposition, a general three-qubit gate can be decomposed using 19 cnot gates (rather than 20). For general $n$-qubit gates, the proposed decomposition generates circuits that have $\frac{22}{48}{4}^n-\frac{3}{2}{2}^n+\frac{5}{3}$ cnot gates, which is less than the best-known exact decomposition algorithm by $(4^{n-2}-1)/3$ cnot gates.

Hybrid magnonic systems have emerged as a promising direction for information propagation with preserved coherence. Because of the high tunability of magnons, their interactions with microwave photons can be engineered to probe novel phenomena based on strong photon-magnon coupling. Improving the photon-magnon coupling strength can be done by tuning the structure of microwave resonators to better interact with the magnon counterpart. Planar resonators have been explored due to their potential for on-chip integration, but only common modes from stripline-based resonators have been used. Here, we present a microwave spiral resonator supporting spoof localized surface plasmons (LSPs) and implement it in the investigation of photon-magnon coupling for hybrid magnonic applications. We showcase strong magnon-LSP photon coupling using a ferrimagnetic yttrium iron garnet sphere. We discuss the engineering capacity of the photon mode frequency and spatial field distributions of the spiral resonator via both experiment and simulation. As a result of the localized photon mode profiles, the resulting magnetic field concentrates near the surface dielectrics, giving rise to an enhanced magnetic filling factor. The strong coupling and large engineering space render the spoof LSPs an interesting contender in developing novel hybrid magnonic systems and functionalities.

Engineering high-fidelity two-qubit gates is an indispensable step toward practical quantum computing. For superconducting quantum platforms, one important setback is the stray interaction between qubits, which causes significant coherent errors. For transmon qubits, protocols for mitigating such errors usually involve fine-tuning the hardware parameters or introducing usually noisy flux-tunable couplers. In this work, we propose a simple scheme to cancel these stray interactions. The coupler used for such cancelation is a driven high-coherence resonator, where the amplitude and frequency of the drive serve as control knobs. Through the resonator-induced-phase interaction, the static $ZZ$ coupling can be entirely neutralized. We numerically show that such a scheme can enable short and high-fidelity entangling gates, including cross-resonance controlled-not (cnot) gates within 40 ns and adiabatic controlled-$Z$ gates within 140 ns. Our architecture is not only $ZZ$-free, but also contains no extra noisy components, such that it preserves the coherence times of fixed-frequency transmon qubits. With the state-of-the-art coherence times, the error of our cross-resonance cnot gate can be reduced to below ${10}^{-4}$.

Photonics has been a promising platform for implementing quantum technologies owing to its scalability and robustness. In this paper, we demonstrate the encoding of information in 32 time bins or dimensions of a single photon. A practical scheme for manipulating the single photon in high dimensions is experimentally realized to implement a compiled version of Shor’s algorithm on a single photon. Our work demonstrates the powerful information-processing capacity of a high-dimensional quantum system for complex quantum information tasks.

The quantum transduction between microwave and optical photons is essential for realizing scalable quantum computers with superconducting qubits. Due to the large frequency difference between microwave and optical ranges, the transduction needs to be done via intermediate bosonic modes or nonlinear processes. So far, the transduction efficiency $\eta$ via the magneto-optic Faraday effect (i.e., the light-magnon interaction) in the ferromagnet YIG has been demonstrated to be as small as $\eta \sim {10}^{-8}-{10}^{-15}$ due to the weak magneto-optic coupling. Here, we take advantage of the fact that three-dimensional topological insulator thin films exhibit a topological Faraday effect that is independent of the sample thickness in the terahertz regime. This leads to a large Faraday rotation angle and therefore enhanced light-magnon interaction in the thin-film limit. We show theoretically that the transduction efficiency between microwave and terahertz photons can be greatly improved to $\eta \sim {10}^{-4}$ by utilizing the heterostructures consisting of topological insulator thin films, such as ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and ferromagnetic insulator thin films, such as YIG.

Measuring the temperature of a quantum system is an essential task in almost all aspects of quantum technologies. Theoretically, an optimal strategy for thermometry often requires measuring energy, which demands full accessibility over the entire system as well as a complex entangled measurement basis. In this paper, we take a different approach and show that single-qubit sequential measurements in the computational basis not only allow for precise thermometry of a many-body system, but may also achieve precision beyond the thermometry capacity of the probe at equilibrium, given by the Cramér-Rao bound. Thus, using consecutive single-qubit measurements of the probe out of equilibrium is, in most cases, very beneficial, as it achieves lower-temperature uncertainties and avoids demanding energy measurements when compared with probes at thermal equilibrium. To obtain such precision, the time between the two subsequent measurements should be smaller than the thermalization time so that the probe never thermalizes. Therefore, the nonequilibrium dynamics of the system continuously imprint information about temperature in the state of the probe. To demonstrate the generality of our findings, we consider thermometry in both spin chains and the Jaynes-Cummings model.

We report on an experiment in which orbital-angular-momentum (OAM)-entangled photon pairs generated by the spontaneous parametric down-conversion process can be engineered to have particular symmetry properties. Our method is based on the use of a Dove-prism pair in conjunction with Hong-Ou-Mandel (HOM) interferometry resolved in transverse position and OAM. The latter allows us to engineer the postselected two-photon state to exhibit a specific type of symmetry. By selecting particular topological charge values for the pump and for the postselected two-photon state, we can transition from a symmetric two-photon state and a HOM dip to an antisymmetric state and a HOM peak. Spatial resolution allows us to obtain the HOM interferogram both at the single-pixel level and globally by summing over all sensor pixels. Furthermore, through spatially selective OAM projection of the detected photon pairs, we can define multiple transverse regions with different symmetry properties, as verified by our spatially resolved HOM apparatus. Although we used two transverse regions for this proof-of-concept demonstration, this method could in principle be scaled to a larger number of regions, leading to a new technique to be added to the existing toolbox for quantum technologies in the photonic domain.

Zero-noise extrapolation (ZNE), a technique to estimate quantum circuit expectation values through noise scaling and extrapolation, is well studied in the context of quantum computing. We examine the applicability of ZNE to the field of quantum sensing. Focusing on the problem of dc magnetometry using the Ramsey protocol, we show that the sensitivity (in the sense of the minimum detectable signal) does not improve upon using ZNE in the slope detection scheme. On the other hand, signals of sufficiently large magnitude can be estimated more accurately. Our results are robust across various noise models and design choices for the ZNE protocols, including both single-qubit and multiqubit entanglement-based sensing.

A nonadiabatic geometric quantum gate is realized by integrating nonadiabatic geometric phases with global geometric features into the unitary quantum control, thereby removing the limitation of a long evolution time in the adiabatic case. However, systematic errors are inevitable in practical quantum control; these lead to the deviation of the evolution from target conditions to inducing geometric phases, smearing the robustness of the induced geometric quantum gates. Here, we present a general theoretical framework with enhanced robustness for geometric quantum gates by preserving fundamental geometric conditions. We first analytically evaluate the influence of systematic errors on geometric gates and then propose an optimized approach to mitigate this influence. Numerical simulations indicate that, as the geometric conditions are still maintained in the presence of systematic errors in our scheme, the constructed geometric quantum gates exhibit strong robustness, far superior to that of conventional schemes. Furthermore, we propose implementing the scheme in superconducting quantum circuits, where geometric quantum gates can achieve high fidelity with current experimental parameters. Therefore, the enhanced gate performance highlights the promise of our scheme for large-scale quantum computations.

Tunable couplers in superconducting quantum computers have enabled fast and accurate two-qubit gates, with reported high fidelities over 99% in various architectures and gate-implementation schemes. However, there are few tunable couplers whose performance in multiqubit systems is clarified, except for the most widely used one: single-transmon coupler (STC). Achieving similar accuracy to isolated two-qubit systems remains challenging due to various undesirable couplings but is necessary for scalability. In this work, we numerically analyze a system of three fixed-frequency qubits coupled via two double-transmon couplers (DTCs) where nearest-neighbor qubits are highly detuned and also next-nearest-neighbor ones are nearly resonant. The DTC is a recently proposed tunable coupler, which consists of two fixed-frequency transmons coupled through a common loop with an additional Josephson junction. We find that the DTC cannot only reduce undesired residual couplings sufficiently, as well as in isolated two-qubit systems, but also enables implementations of 30-ns $\mathrm{CZ}$ gates and individual and simultaneous 10-ns $\pi /2$ pulses with fidelities over 99.99%. For comparison, we also investigate the system where the DTCs are replaced by the STCs. The results show that the DTC outperforms the STC in terms of both residual coupling suppression and gate accuracy in the above systems. From these results, we expect that the DTC architecture is promising for realizing high-performance, scalable superconducting quantum computers.

Quantum key distribution (QKD) could help to share a secure key between two distant peers. In recent years, twin-field (TF) QKD has been widely investigated because of its long transmission distance. One of the popular variants of TF QKD is sending-or-not-sending (SNS) QKD, which has been experimentally verified to realize 1000-km level fiber key distribution. In this article, we introduce phase postselection into the SNS protocol. With this modification, the probability of selecting “sending” can be substantially improved. The numerical simulation shows that the transmission distance can be improved both with and without the actively odd-parity pairing method. The case of discrete phase randomization is also analyzed, a similar improvement on distance can be seen.

In recent years, notable progress has been made in the study of hole qubits in planar germanium, and circuit quantum electrodynamics (circuit QED) has emerged as a promising approach for achieving long-range coupling and scaling up of qubits. Here, we demonstrate the coupling between holes in a planar germanium double quantum dot (DQD) and photons in a microwave cavity. Specifically, a real-time calibrated virtual gate method is developed to characterize this hybrid system, which in turn allows us to determine the typical parameters sequentially through a single-parameter fitting instead of conventional multiparameter fitting with additional uncertainty, giving a hole-photon coupling rate of $g_0$/2π = 21.7 MHz. This work is a step toward further research on hole-photon interactions and long-range qubit coupling in planar germanium. The experimental method developed in this work contributes to a more accurate and efficient characterization of hybrid cavity-QED systems.

A crucial requirement for quantum computing—in particular, for scalable quantum computing and error correction—is fast and high-fidelity qubit readout. For semiconductor-based qubits, one limiting factor for local low-power signal amplification is the output swing of the charge sensor. We demonstrate $\mathrm{Ga}\mathrm{As}$ and Si asymmetric sensing dots (ASDs) specifically designed to provide a significantly larger response compared to conventional charge-sensing dots. Our ASD design features a drain reservoir strongly decoupled from the sensor dot, which mitigates negative feedback effects found in conventional sensors. This results in a boosted output swing of $3\mathrm{mV}$, which exceeds the response in the conventional regime of our device by more than a factor of 10. The enhanced output signal paves the way for employing very low-power readout amplifiers in close proximity to the qubit.

Quantum secure direct communications (QSDC) can directly transmit secret messages through a quantum channel without keys. The imperfect photon source is a major obstacle for QSDC’s practical implementation. The unwanted vacuum state and multiphoton components emitted from imperfect photon source largely reduce QSDC’s secrecy message capacity and even threaten its security. In the paper, we propose a high-efficient passive decoy-state QSDC protocol with the heralded single-photon source (HSPS). We adopt a spontaneous parametric down-conversion source to emit entangled photon pairs in two spatial modes. By detecting the photons in one of the two correlated spatial modes, we can infer the photon-number distribution of the other spatial mode. Meanwhile, our protocol allows a simple passive preparation of the signal states and decoy state. The HSPS can effectively reduce the probability of vacuum state and increase QSDC’s secrecy message capacity. Meanwhile, the passive decoy-state method can simplify the experimental operations and enhance QSDC’s robustness against the third-party side-channel attacks. Under the communication distance of 10 km, the secrecy message capacity of our QSDC protocol can achieve 81.85 times with average photon number of 0.1 and 12.79 times with average photon number of 0.01 of that in the original single-photon-based QSDC protocol without the HSPS. Our QSDC protocol has longer maximal communication distance about 17.975 km with average photon number of 0.01. Our work serves as a major step toward the further development of practical passive decoy-state QSDC systems.

We identify a major source of quantum decoherence in three-dimensional superconducting radio-frequency (SRF) resonators and two-dimensional transmon qubits composed of oxidized niobium: oxygen vacancies in the niobium pentoxide, which drive two-level system (TLS) losses. By probing the effect of sequential in situ vacuum-baking treatments on the rf performance of bulk $\mathrm{Nb}$ SRF resonators and on the oxide structure of a representative $\mathrm{Nb}$ sample using TOF SIMS, we find a nonmonotonic evolution of cavity quality factor $Q_0$, which correlates with the interplay of ${\mathrm{Nb}}_2{\mathrm{O}}_5$ vacancy generation and oxide-thickness reduction. We localize this effect to the oxide itself and present the insignificant role of diffused interstitial oxygen in the underlying $\mathrm{Nb}$ by regrowing the oxide via wet oxidation, which reveals a mitigation of aggravated TLS losses. We hypothesize that such vacancies in the pentoxide serve as magnetic impurities and are a source of TLS-driven rf loss.

Continuous-variable quantum key distribution (CV-QKD) with plug-and-play design offers a promising route in simplifying the system implementation and shows intriguing prospects for quantum access network applications. However, such a scheme makes it possible for the eavesdropper (Eve) to completely control the source, helping her to gain more information since the laser travels through the unsecured channel before being modulated, which will severely compromise the performance of the system and limit its potential application. To fight against the security loophole, we propose a passive source monitoring scheme based on a combination of beam splitter and homodyne detector, as well as source noise suppression. The corresponding entanglement-based model is established to estimate the secret key rate for the proposed scheme. We show that the performance of the plug-and-play CV-QKD system can be significantly improved by using the source monitoring scheme compared with the untrusted source model. With typical parameters, the maximum transmission distance can be promoted by more than 50%, and the secret key rate can be increased by more than 25% when the transmission distance is longer than 50 km. This study provides a feasible approach for improving the security and performance of the plug-and-play CV-QKD and holds positive potential for practical applications.

Ion-ion coupling over long distances represents a highly useful resource for quantum technologies, for example, to sympathetically cool or interconnect qubits in ion-based quantum computing architectures. In this respect, the recently demonstrated wire-mediated ion-ion coupling stands due to the simplification of its trap layout and its prospects for deterministic entanglement. However, the strength of such coherent ion-wire-ion coupling is typically weak, hindering its practical utilization. Here, we propose a wire-mediated scheme for coherent ion-electron coupling. The scheme not only enables the sympathetic cooling of electrons via advanced ion-cooling techniques, but also allows promotion of the effective ion-ion coupling strength by orders of magnitudes via electron mediation. Our work thus paves a way toward quantum information processing in ion-electron hybrid quantum systems.