In this paper, we present a device, designed to be used in a space payload, capable of correcting any arbitrary polarization state. The device consists of a set of three liquid-crystal variable-retarder (LCVR) cells, in which by selecting the appropriate values of the voltage to be applied in each of them, it can correct the undesirable effects. In particular, the degradation effects of the polarization states, such as ellipticity and rotation with respect to the reference system, are compensated. Among many other potential applications, this device has been conceived to be used in quantum-key-distribution (QKD) intersatellite link or satellite-ground polarization-coding protocols as a polarization-correction device (PCD). To demonstrate that these effects are corrected, we have carried out an experiment in which we have tested how the BB84 protocol states are compensated after being strongly perturbed, achieving polarization extinction ratio (PER) values around 39 dB. Our solution offers the advantage of being small and mechanism free, being the ideal solution to be implemented in small satellites. These satellites are the core to be used in future constellations required for a global quantum communications network.

A high-fidelity controlled-$Z$ (CZ) gate utilizing a double-transmon coupler (DTC) has recently been demonstrated as a building block for superconducting quantum processors. Like many other kinds of tunable couplers, however, the DTC requires a finite dc current for flux biasing the coupler at the idling point to turn off the coupling, necessitating extra care for wiring and heat-load management. To address this issue, we theoretically propose and experimentally realize a coupling scheme by introducing a shunt capacitance between the two transmons of the DTC at zero-flux bias, which demonstrates high-fidelity CZ-gate performance comparable to the previous DTC. Through a comprehensive error budget analysis using multiple randomized benchmarking methods, we also identify that the current fidelity is limited by the decoherence through the coupler. Moreover, we experimentally demonstrate the wide operational flux range of the capacitively shunted DTC, which solves the challenging issue of remnant flux existing even with careful magnetic shielding.

Amorphous-oxide layers in thin-film capacitors are linked to reduced transmon-qubit $T_1$ coherence times. $\mathrm{Ta}$-based capacitors outperform $\mathrm{Nb}$-based ones, suggesting that amorphous $\mathrm{Ta}$${}_2$$\mathrm{O}$${}_{5-x}$ is less lossy than $\mathrm{Nb}$${}_2$$\mathrm{O}$${}_{5-x}$. We investigate the microscopic features of these amorphous oxides using ab initio molecular dynamics and density functional theory, revealing the origins of the superior performance of $\mathrm{Ta}$${}_2$$\mathrm{O}$${}_{5-x}$. We establish that oxygen deficiency is less likely to occur in amorphous $\mathrm{Ta}$${}_2$$\mathrm{O}$${}_{5-x}$ than in $\mathrm{Nb}$${}_2$$\mathrm{O}$${}_{5-x}$ for $0\le x\le 0.25$ and that for a given oxygen deficiency $x$, metal $\mathrm{Ta}$—$\mathrm{Ta}$ bond formation is enhanced. Such bonds, which are accommodated by structural flaws in the amorphous network, capture electrons better than in amorphous $\mathrm{Nb}$${}_2$$\mathrm{O}$${}_{5-x}$. These thermochemical differences quench or highly suppress magnetic moments in amorphous $\mathrm{Ta}$${}_2$$\mathrm{O}$${}_{5-x}$ and eliminate a potential source of quasiparticles and magnetic flux noise. We also show that hyperfine couplings between $\mathrm{Nb}$ nuclei and local magnetic moments in $\mathrm{Nb}$${}_2$$\mathrm{O}$${}_{5-x}$ can form “two-level systems” (TLSs) or “two-level fluctuators” with energy splittings of 100–1000 MHz or higher. This reveals a TLS mechanism in amorphous $\mathrm{Nb}$${}_2$$\mathrm{O}$${}_{5-x}$ oxide layers that is likely inactive in $\mathrm{Ta}$${}_2$$\mathrm{O}$${}_{5-x}$. Our work provides a fundamental understanding of the materials chemistry and limitations imposed by native oxides of superconducting qubits that can be used to guide materials selection and processing.

Correlated noise across multiqubit architectures is known to be highly detrimental to the operation of error-correcting codes and the long-term feasibility of quantum processors. The recent discovery of spatially dependent correlated noise in multiqubit architectures of superconducting qubits arising from the impact of cosmic radiation and high-energy particles giving rise to quasiparticle poisoning within the substrate has led to intense investigations of mitigation strategies to address this. Correlated noise in semiconductor spin qubits is also now receiving attention, as this platform begins to scale. Here we report the magnitude, frequency, and spatial dependence of noise correlations between four silicon quantum dot pairs as a function of interdot distance at frequencies from 0.3 to 1 mHz. We find the magnitude of charge-noise correlations, quantified by the magnitude square coherence $C_{xy}$, are significantly suppressed from $>0.5$ to $<0.1$ as the interdot distance increases from 75 to 300 nm. Using an analytical model we confirm that, in contrast to superconducting qubits, the dominant source of correlated noise arises from low-frequency charge noise from the presence of two level fluctuators (TLFs) at the native silicon-silicon dioxide surface. Knowing this, we conclude with a timely discussion of charge-noise mitigation strategies.

Building a scalable, universal, high-performance quantum processor is a formidable challenge. In particular, the problem of realizing fast, high-performance two-qubit gates of high fidelity has yet to be addressed. Here, we propose a building block for a scalable quantum processor consisting of two transmons and a tunable three-mode coupler allowing for $ZZ$ interaction control. We experimentally demonstrate a native cz gate with a pulse duration of 60 ns, achieving a two-qubit gate fidelity above $98\mathrm{\%}$, limited mostly by the qubit coherence time. Numerical simulations show that by optimizing the gate duration, the fidelity can be pushed over $99.97\mathrm{\%}$.

Lithium niobate (LN) is a promising material for building acoustic resonators used in quantum applications, but its performance is limited by poorly understood material defects called two-level systems (TLSs). In this work, we fabricate high-performance acoustic resonators from LN with quality factors up to $6\times {10}^7$ and use them to separate bulk and surface contributions to TLS loss. By comparing these bulk acoustic wave (BAW) resonators with previous surface-acoustic-wave and phononic-crystal studies, we show that devices with high surface-participation ratios are limited by surface TLSs, while our BAW devices reveal an intrinsic bulk TLS limit. Through systematic surface treatments and microscopy, we demonstrate that BAW resonator performance remains unchanged despite surface modifications, confirming operation in a bulk-limited regime. Our work establishes quantitative bounds on both surface and bulk TLS losses in LN within the context of the material growth and fabrication approaches we have pursued, and it provides guidance for future device engineering and materials development.

Traveling-wave parametric amplifiers (TWPAs) have become an essential tool for the readout of quantum circuits and the search for dark matter. We report on the implementation of an rf-superconducting quantum interference device (SQUID)-based Josephson TWPA with an average saturation power of −84 dBm while providing an average power gain of 20 dB from 3.5 to 8.5 GHz. This wide bandwidth is enabled by reducing the curvature of the dispersion in the signal band while suppressing detrimental mixing processes. With 2393 rf-SQUIDs, our device has a comparable number of nonlinear elements but achieves ten times higher saturation power than previously reported devices. This shows that using rf-SQUIDs results in a more favorable trade-off between device length and saturation power. Harnessing wideband characterization techniques, we determine the TWPA’s excess noise above the quantum limit to be 0.8 to 1.5 photons. In addition, we validate the signal-to-noise ratio improvement as a practical measure for tuning the TWPA to minimize the total system noise and demonstrate that its optimum does not coincide with the highest gain.

In the noisy intermediate-scale quantum era, one of the most crucial bottlenecks for the realization of universal quantum computation is error correction. Stabilizer code is the most recognizable type of quantum error-correction code. A scalable efficient decoder is most desired for the application of the quantum error-correction codes. In this work, we propose a self-attention U-Net quantum decoder (SU-NetQD) for toric code, which outperforms the minimum-weight perfect-matching decoder, especially in the circuit-level noise environments. Specifically, with our SU-NetQD, we achieve lower logical error rates compared with MWPM and discover an increased trend of code threshold as the increase of noise bias. We obtain a high threshold of 0.231 for the extremely biased noise environment. The combination of low-level decoder and high-level decoder is the key innovation for the high accuracy of our decoder. With transfer learning mechanics, our decoder is scalable for cases with different code distances. Our decoder provides a practical tool for quantum noise analysis and promotes the practicality of quantum error-correction codes and quantum computing.

Quantum process tomography (QPT) is the gold standard for fully characterizing quantum processes, yet it is resource intensive. In this work, we shift our focus from global to local quantum processes, which are beneficial for practical scenarios, such as distributed quantum computing and quantum networks. We term these local processes as reduced quantum processes and present a comprehensive theory to describe them. To experimentally characterize all $k$-reduced quantum processes of an $n$-qubit quantum process, we introduce our primary method, quantum process overlapping tomography (QPOT), and compare it with two alternative approaches: ancilla QPT and parallel QPT. Suppose the target quantum process involves $T$ local interactions, each acting on $l_i$ qubits, with the total number of affected qubits satisfying $l:=\sum _{i=1}^T{l}_i\le n$. The experimental configurations required for each method are as follows: QPOT requires $2^l\times e^{\mathcal{O}(k)}\times \log (n)$ experimental configurations. Ancilla QPT requires $e^{\mathcal{O}(k)}\times \log (2n)$ experimental configurations but additionally demands $n$ ancilla qubits and the preparation of maximally entangled states. Parallel QPT requires approximately $\sum _{i=1}^{n^k}{2}^{m_i}\times {12}^k$ experimental configurations, where $m_i$ denotes the number of qubits interacting with the $i\mathrm{th}$ reduced quantum process. Among these methods, QPOT exhibits superior efficiency and broader applicability. The efficacy of our methods is confirmed through experimental demonstrations conducted on IBM hardware, aligning well with our theory.

Room-temperature photonic crystals (PCs) in the optical frequency range have been widely studied and applied, while those in the microwave frequency range have received relatively less attention, as quantum interferences of microwaves is strongly suppressed at room temperature. In this work, we study the quantum scattering of microwave photons by periodically arranged emitters. We experimentally demonstrated two kinds of microwave PCs generated by periodically arranged microwave cavities and Josephson two-level atoms. We investigate how the bandwidth and central frequency of the generated band gap are influenced by the scattering defects, the dissipation of the scatterers, the photon-scatter coupling strengths, and the separated distances between the nearest-neighbor scatterers. In particular, we show that the central frequency of the band gap generated by the current biased Josephson-junction (CBJJ) array can be dynamically controlled by adjusting the biased dc current. The microwave PCs with controllable band gaps demonstrated here can be applied for microwave quantum information processing.

Magnons, the quanta of collective spin excitations in magnetic materials, may enable functionalities, such as nonreciprocity and transduction in hybrid quantum devices. To assess the potential of such applications, it is necessary to understand magnon dynamics beyond the simple harmonic oscillator regime, where theory predicts effects like population-dependent damping and quantum fluctuations in the form of magnon shot noise. Probing these phenomena requires sensors with high sensitivity and the ability to resolve magnon properties across different excitation regimes. Here, we demonstrate accurate and sensitive detection of magnon population and decay over a wide range of occupation numbers. We use a superconducting qubit to probe magnons in a ferrimagnet over approximately 2000 excitations. Using qubit control and parametrically induced qubit-magnon interactions, we demonstrate few-excitation sensitive detection of magnons with a dynamic range of approximately 30 dB, and are able to accurately resolve their decay with few-ns sensitivity. These capabilities offer a powerful and practical technique for probing magnon dynamics in or beyond the linear regime over a wide range of excitations.

Silicon carbide is a promising material platform for hosting various color centers that are suitable for quantum information processing. Here, we report the design and demonstration of an integrated electro-optic modulator that can directly interface silicon-vacancy centers in the 4H-silicon-carbide-on-insulator platform. Despite a relatively low electro-optic coefficient ($0.22$ pm/V), the optimized silicon carbide modulator is able to work in the 920-nm range with a propagation loss of less than $0.5$ dB/cm, featuring a 3-dB bandwidth of around 500 MHz, an extinction ratio of 8–12 dB for an operating peak-to-peak voltage of 10 V, and a footprint of less than $0.1{\mathrm{mm}}^2$. With such modulation performance, this approach described provides a cost-effective solution for the chip-scale implementation of the electro-optic modulation technology required for interfacing with color centers in the silicon carbide platform.

Quantum discord represents a correlation beyond classicality that could be useful for many quantum information tasks and therefore is viewed as a valuable quantum resource. Theoretically, whether or not a bipartite quantum state has a nonzero amount of quantum discord can be determined once its density matrix is given. Experimentally, however, it is not easy to perform a reliable tomography on the state, especially when considering the imperfection of devices and the high dimensionality of the system. Here, inspired by the so-called dimension witness, we propose a new approach using uncharacterized measurements to witness quantum discord of an unknown bipartite state within an arbitrary-dimension system. Furthermore, $2\times 2$ measurements are sufficient for any bipartite state of arbitrarily large dimension. For any two-qubit state, we show that the maximum of the witness happens when one performs mutually orthogonal basis measurements on each qubit. Moreover, the witness value decreases linearly in response to the errors, which indicates its nonsensitivity to device imperfections and experimental feasibility.

Optically active solid-state spin systems have been extensively studied in quantum technologies. We introduce a new readout scheme, termed “time-to-space” (T2S) encoding, which decouples spin manipulation from optical readout both temporally and spatially. This is achieved by simultaneously controlling the spin state within a region of interest, followed by rapid scanning of the optical readout position using an acousto-optic modulator. Time tracking allows the optical readout position to be encoded as a function of time. Using nitrogen-vacancy center ensembles in diamond, we demonstrate that the T2S scheme enables correlated experiments for efficient common-mode noise cancellation in various nano- and microscale sensing scenarios. Additionally, we show scalable multipixel imaging that does not require a camera and has the potential to accelerate data acquisition by several hundred times compared to conventional scanning methods. We anticipate widespread adoption of this technique, as it requires no additional components beyond those commonly used in experiments with optically adressable spin systems.

Despite progress towards achieving low error rates with superconducting qubits, error-prone two-qubit gates remain a bottleneck for realizing large-scale quantum computers. Therefore, a systematic framework to design high-fidelity gates becomes imperative. One type of two-qubit gate in superconducting qubits is the controlled-phase (cphase) gate, which utilizes a conditional interaction between higher-energy levels of the qubits controlled by a baseband flux pulse on one of the qubits or a tunable coupler. In this work, we study an adiabatic implementation of cphase gates and formulate the design of the control trajectory for the gate as a pulse-design problem. We show in simulation that the Chebyshev-based trajectory can, in certain cases, enable gates with gate infidelity lower by an average of $23.3\mathrm{\%}$ when compared to the widely used Slepian-based trajectory.

The accurate modeling of mode hybridization and calculation of radiative relaxation rates have been crucial to the design and optimization of superconducting quantum devices. In this work, we introduce a spectral theory for the electrohydrodynamics of superconductors that enables the extraction of the relaxation rates of excitations in a general three-dimensional distribution of superconducting bodies. Our approach addresses the long-standing problem of formulating a modal description of open systems that is both efficient and allows second quantization of the radiative hybridized fields. This is achieved through the implementation of finite but transparent boundaries through which radiation can propagate into and out of the computational domain. The resulting spectral problem is defined within a coarse-grained formulation of the electrohydrodynamical equations that is suitable for the analysis of the nonequilibrium dynamics of multiscale superconducting quantum systems.