Great challenges are encountered when solving problems based on continuously generated data streams due to the online data input fashion and limited memory space. Quantum properties are helpful in dealing with stream data. In this work, we design and experimentally implement a quantum streaming algorithm (QSA) with various lengths of input data in a single-defect spin system in silicon carbide at room temperature. We explore the schemes with trusted advice on the output results during the algorithm execution. Quite contrary to the classical case, outdated advice is still useful in the quantum case, as revealed by a proper performance function. Our work demonstrates the advantage of the QSA, which would be useful for investigating complex online problems. The room-temperature solid-state spin platform based on such a technological material opens the door for scalable quantum-information processing.

We propose a method to assist fault mitigation in quantum computation through the use of sensors co-located near physical qubits. Specifically, we consider using transition edge sensors co-located on silicon substrates hosting superconducting qubits to monitor for energy injection from ionizing radiation, which has been demonstrated to increase decoherence in transmon qubits. We generalize from these two physical device concepts and explore the potential advantages of co-located sensors to assist fault mitigation in quantum computation. In the simplest scheme, co-located sensors beneficially assist rejection of calculations potentially affected by environmental disturbances. Investigating the potential computational advantage further required development of an extension to the standard formulation of quantum error correction. In a specific case of the standard three-qubit, bit-flip quantum error-correction code, we show that given a 20% overall error probability per qubit, approximately 90% of repeated calculation attempts are correctable. However, when sensor-detectable errors account for 45% of overall error probability, the use of co-located sensors uniquely associated with independent qubits boosts the fraction of correct final-state calculations to 96%, at the cost of rejecting 7% of repeated calculation attempts.

We demonstrate a superconducting transmon qubit in which a Josephson junction has been engineered to act as its own parallel shunt capacitor. This merged-element transmon potentially offers a smaller footprint than conventional transmons. Because it concentrates the electromagnetic energy inside the junction, it reduces the relative electric field participation from other interfaces. By combining micrometer-scale $\mathrm{Al}$/$\mathrm{Al}\mathrm{O}$${}_x$/$\mathrm{Al}$ junctions with long oxidations, we produce functional devices with $E_J/E_C$ in the low-transmon regime ($E_J/E_C\lesssim 30$). Cryogenic $I$-$V$ measurements show a sharp $dI/dV$ structure with low subgap conduction. Qubit spectroscopy of tunable versions shows a small number of avoided level crossings, suggesting the presence of two-level systems. We observe mean $T_1$ times typically in the range of $10–90\mu \mathrm{s}$, with some annealed devices exhibiting $T_1>100\mu \mathrm{s}$ over several hours. The results suggest that energy relaxation in conventional small-junction transmons is not limited by junction loss.

Qubit coherence and gate fidelity are typically considered the two most useful metrics for characterizing a quantum processor. An equally useful metric is interqubit connectivity as it minimizes gate count and allows implementing algorithms efficiently with reduced error. However, interqubit connectivity in superconducting processors tends to be limited to nearest neighbor due to practical constraints in typical planar realizations. Here, we introduce a superconducting architecture that uses a ring resonator as a multiqubit coupling element to provide beyond nearest-neighbor connectivity without compromising on coupling uniformity or introducing fabrication complexities. We theoretically analyze the interqubit coupling as a function of frequency for a pair of qubits placed at different positions along the ring resonator and show that for carefully chosen operating frequency and angular spacing between the qubits, the variation of coupling can be minimized. For an operating frequency between the first two resonances of the ring resonator and a ${30}^{\circ }$ angular spacing for qubits, we compute interqubit coupling for a device capable of supporting up to 12 qubits with each qubit connected to nine other qubits. Using four qubits positioned strategically in the ring-resonator coupler, we experimentally verify the theoretical prediction for all possible angular spacings between the two qubits and demonstrate good agreement. Just like the standard bus resonator coupler, the coupling in the ring-resonator coupler is mediated via virtual photons since the operating frequency is far away from the resonant modes of the ring coupler. This ensures that any small internal loss in the ring resonator does not introduce decoherence during the coupling operation. We also compute extensions of this idea involving larger ring resonators and a multiring system and show the possibility of highly connected networks with larger number of qubits. Apart from being plug and play for existing superconducting architectures, our concept is scalable, adaptable to other platforms and has the potential to significantly accelerate progress in quantum computing, annealing, simulations, and error correction.

In this work, we study the current-phase relation ($\mathrm{C}\mathrm{\Phi }\mathrm{R}$) of lithographically fabricated molybdenum germanium (${\mathrm{Mo}}_{79}{\mathrm{Ge}}_{21}$) nanobridges that is intimately linked with the nanobridge’s kinetic inductance. We do this by imbedding the nanobridges in a superconducting quantum interference device (SQUID). We observe that, for temperatures far below $T_c$, the $\mathrm{C}\mathrm{\Phi }\mathrm{R}$ is linear, as long as the condensate is not weakened by the presence of a supercurrent. We demonstrate lithographic control over the nanobridge kinetic inductance, which scales with the nanobridge aspect ratio. This allows the $I_c(B)$ characteristic of the SQUID to be tuned. The SQUID properties that can be controlled in this way include the SQUID’s sensitivity and the positions of the critical-current maxima. These observations can be of use for the design and operation of future superconducting devices, such as magnetic memories or flux qubits.

Quantum secure direct communication (QSDC) realizes the transmission of secret messages directly in a quantum channel. A continuous-variable- (CV) based quantum-communication system allows high-speed, large-capacity information transmission in optical telecommunication systems. In this work, we propose a QSDC protocol based on Gaussian mapping. As for Gaussian modulation, the designed Gaussian-mapping scheme effectively solves the problem, which results from the nonuniformity of the secret-message bitstream, and is applicable to different modulation variances. The system performance analysis shows that the proposed scheme can realize inerrant Gaussian modulation of secret messages, and secure transmission of messages under information theory. Moreover, our work hopes to stimulate discussion on experimental realization and practical advance of CV-QSDC protocol.

Rydberg electromagnetically induced transparency (EIT) enables the realization of atom-based Systeme-International-traceable microwave sensing, imaging, and communication devices by exploiting the strong microwave electric dipole coupling of highly excited Rydberg states. Essential to the development of robust devices is a careful characterization of the sensor performance and systematic uncertainties. In this work, we present a comparison of microwave-induced EIT splitting in a cesium atomic vapor for four possible Rydberg couplings, $65S_{1/2}\to 65P_{1/2}$, $66S_{1/2}\to 66P_{3/2}$, $79D_{5/2}\to 81P_{3/2}$, and $62D_{5/2}\to 60F_{7/2}$, at microwave transition frequencies around 13 GHz. Our work highlights the impact of multiphoton couplings on neighboring Rydberg states in breaking both the symmetry and linearity of the observed splitting, with excellent agreement between experimental observations and a theoretical model accounting for multiphoton couplings. We identify an optimal angular-state choice for robust microwave measurements, as well as demonstrating an alternative regime in which microwave polarization can be measured.

Control of the dynamics of mechanical resonators is central to quantum science and metrology applications. Optomechanical control of diamond resonators is attractive owing to the excellent physical properties of diamond and its ability to host electronic spins that can be coherently coupled to mechanical motion. Using a confocal microscope, we demonstrate tunable amplification and damping of the motion of a diamond nanomechanical resonator. Observation of both normal-mode cooling from room temperature to 80 K and amplification into self-oscillations with $60\mu \mathrm{W}$ of optical power is observed via waveguide optomechanical readout. This system is promising for quantum spin optomechanics, as it is predicted to enable optical control of stress-spin coupling with rates of approximately $1$ MHz (100 THz) to ground (excited) states of diamond nitrogen-vacancy centers.

Radio-frequency (rf) reflectometry offers a fast and sensitive method for charge sensing and spin readout in gated quantum dots. We focus in this work on the implementation of rf readout in accumulation-mode gate-defined quantum dots, where the large parasitic capacitance poses a challenge. We describe and test two methods for mitigating the effect of the parasitic capacitance, one by on-chip modifications and a second by off-chip changes. We demonstrate that on-chip modifications enable high-performance charge readout in $\mathrm{Si}$/${\mathrm{Si}}_x$${\mathrm{Ge}}_{1-x}$ quantum dots, achieving a fidelity of 99.9% for a measurement time of $1\mu \mathrm{s}$.

In the hardware implementation of quantum computation, the control pulses for quantum states and gate operations must be calibrated to correct errors induced by unknown drifts of system parameters. The calibration can be iteratively done by checking the control precision, but the required randomized benchmarking or quantum tomography is experimentally expensive. In this paper, we propose that the control calibration and the quantum tomography can be taken as two separate but collaborative learning processes. Combining the gradient-descent pulse engineering and the adaptive tomography, the resulting c-GRAPE algorithm can substantially reduce the high cost of measurements without sacrificing the control accuracy. The effectiveness is demonstrated by numerical simulation examples.

Magnetic flux quantization in superconductors allows the implementation of fast and energy-efficient digital superconducting circuits. However, information representation in magnetic flux severely limits the functional density and is a long-standing problem. Here, we introduce the concept of superconducting digital circuits that do not utilize magnetic flux and have no inductors. We argue that neither the use of geometric nor kinetic inductance is promising for the scaling down of superconducting circuits. The key idea of our approach is the utilization of bistable Josephson junctions, allowing the representation of information through the Josephson energy. Since the proposed circuits are composed only of Josephson junctions, they can be called all-Josephson junction (all-JJ) circuits. We present a methodology for the design of circuits consisting of conventional and bistable junctions. We analyze the principles of the circuit’s functioning, ranging from simple logic cells to an 8-bit parallel adder. The utilization of bistable junctions in the all-JJ circuits is promising for the simplification of schematics and a decrease of the JJ count, leading to space efficiency.

In recent years, unshielded atomic systems have been attracting researchers’ attention, in which decoherence is one of the major problems, especially for high-precision measurements. The nonlinear Zeeman effect and magnetic field gradient are the main decoherence sources of atomic electron spin in the Earth-field range. Here, we propose a method to cancel out the two dominant broadening effects simultaneously by an all-optical dynamic decoupling approach based on Raman scattering in the ${}^{87}\mathrm{Rb}$ Zeeman sublevels. By adjusting the parameters of the Raman lasers, we realize spin control along an arbitrary direction. We analyze the state evolution of atomic spin under the Raman light-control sequence in detail. The results show that both the nonlinear Zeeman effect and magnetic field gradient can be significantly suppressed.

Quantum phase estimation (QPE) is the key subroutine of several quantum computing algorithms as well as a central ingredient in quantum computational chemistry and quantum simulation. While QPE strategies have focused on the estimation of a single phase, applications to the simultaneous estimation of several phases may bring substantial advantages; for instance, in the presence of spatial or temporal constraints. In this work, we study a Bayesian algorithm for the parallel (simultaneous) estimation of multiple arbitrary phases. The protocol gives access to correlations in the Bayesian multiphase distribution resulting in covariance matrix elements scaling as $O(N_T^{-2})$, with respect to the total number of quantum resources $N_T$. The parallel estimation allows to surpass the sensitivity of sequential single-phase estimation strategies for optimal linear combinations of phases. Furthermore, the algorithm proves a certain noise resilience and can be implemented using single photons and standard optical elements in currently accessible experiments.

We use a semiclassical formalism to optimize a microwave single-photon detector based on switching events of a current-biased Josephson junction coupled to a resonator. To detect very rare events, the average time between dark counts ${\tau }_{\mathrm{dark}}$ should be maximized taking into account that the switching time ${\tau }_{\mathrm{sw}}$ should be sufficiently small. We demonstrate that these times can be tuned in a wide range by changing the junction parameters, and ${\tau }_{\mathrm{dark}}/{\tau }_{\mathrm{sw}}\sim {10}^9$ can be achieved. Therefore, a junction-resonator arrangement can be used to detect extremely low photon fluxes (e.g., for searching for galactic axions).

We discuss the realization of a universal set of ultrafast single- and two-qubit operations with superconducting quantum circuits and investigate the most relevant physical and technical limitations that arise when pushing for faster and faster gates. With the help of numerical optimization techniques, we establish a fundamental bound on the minimal gate time, which is determined independently of the qubit design solely by its nonlinearity. In addition, important practical restrictions arise from the finite qubit transition frequency and the limited bandwidth of the control pulses. We show that, for highly anharmonic flux qubits and commercially available control electronics, elementary single- and two-qubit operations can be implemented in about 100 ps with residual gate errors below ${10}^{-4}$. Under the same conditions, we simulate the complete execution of a compressed version of Shor’s algorithm for factoring the number 15 in about 1 ns. These results demonstrate that, compared to state-of-the-art implementations with transmon qubits, a hundredfold increase in the speed of gate operations with superconducting circuits is still feasible.