The noise-biased Kerr-cat qubit is an attractive candidate for fault-tolerant quantum computation; however, its initialization faces challenges due to the squeezing pump-induced frequency shift. Here we propose and demonstrate a dynamic compensation method to mitigate the effect of pump-induced frequency shift during the Kerr-cat qubit initialization. Using a nonlinearity-engineered multiloop superconducting quantum interference device, we realize a stabilized Kerr-cat qubit and validate the advantages of the dynamic compensation method by increasing the initialization fidelity from 57% to 78%, with a projected fidelity of 91% after exclusion of state preparation and measurement errors. Our results not only advance the practical implementation of Kerr-cat qubits, but also provide valuable insights into the fundamental adiabatic dynamics of these systems. This work is valuable for scalable quantum processors that leverage the noise-biased properties of Kerr-cat qubits.

Integrating two-dimensional (2D) materials into circuit quantum electrodynamics (cQED) devices is an emerging field in recent years. This integration not only facilitates the exploration of potential applications in quantum information processing but also enables the study of material’s fundamental properties using microwave techniques. While most studies employ 2D coplanar architectures with scalability potential, three-dimensional (3D) cavity-based cQED devices, due to their simpler design, offer the advantage of a quicker turnaround to probe the composite Josephson junctions (JJs). Here, we construct the first flux-tunable, 3D cavity-compatible superconducting quantum circuit based on 2D materials, featuring a graphene superconducting quantum interference device (SQUID) shunted by a capacitor that is accessible by both dc and microwave probes. We have shown how flux-modulated cavity frequency can be linked to the SQUID critical current under the influence of Fraunhofer’s pattern. In addition, we extracted the symmetry information of the SQUIDs based on dc analysis, and correlated this with the flux-modulated cavity frequency observed in microwave measurements. Our platform can extend to topological materials, holding the prospect of establishing valid topological JJs with a dc probe while allowing a fast microwave probe to avoid quasiparticle poisoning.

Quantum secret sharing (QSS) plays a pivotal role in multiparty quantum communication, ensuring the secure distribution of private information among multiple parties. However, the security of QSS schemes can be compromised by attacks exploiting imperfections in measurement devices. Here, we propose a reconfigurable approach to implement QSS based on measurement-device-independent (MDI) principles, utilizing linear two-photon Bell-state analysis. By employing single-qubit conjugate operations for encoding private classical information, our approach offers reconfigurability, allowing for the inclusion of additional parties without sacrificing efficiency. Furthermore, we demonstrate the robust security of our MDI QSS scheme against intereavesdropping by dishonest participants and establish lower bounds for secure communication among three legitimate parties. This work presents a flexible configuration for implementing multiparty secure quantum communication with imperfect measurement devices and represents a significant advancement in the development of secure quantum communication technologies.

The design and optimization of quantum codes is generally hard because of the prohibitive computational cost. The recently introduced quantum lego framework provides a powerful method for generating complex quantum error-correcting codes (QECCs) out of simple ones. We gamify this process and unlock a new avenue for code design and discovery using reinforcement learning (RL). Already for simple code measures and modest qubit numbers, we produce codes that are both optimal and novel. Moreover, we argue that this framework is both scalable, when combined with efficient tensor contraction methods, and flexible, since we can specify arbitrary properties of the code to be optimized. We train on two such properties, maximizing the code distance, and minimizing the probability of logical error under biased Pauli noise. For the first, we show that the trained agent identifies ways to increase code distance beyond naive concatenation, saturating the linear programming bound for CSS (Calderbank, Shor, Steane) codes on 13 qubits. With a learning objective to minimize the logical error probability under biased Pauli noise, we find the best-known CSS code at this task for $\lesssim 20$ qubits. Compared to other (locally deformed) CSS codes, including Surface, XZZX, and two-dimensional Color codes, our $[[17,1,3]]$ code construction actually has lower adversarial distance, yet better protects the logical information, highlighting the importance of QECC desiderata. Lastly, we comment on how this RL framework can be used in conjunction with physical quantum devices to tailor a code without explicit characterization of the noise model.

Qudits provide a resource-efficient alternative to qubits for quantum information processing. The multilevel nature of the transmon, with its individually resolvable transition frequencies, makes it an attractive platform for superconducting circuit-based qudits. In this work, we systematically analyze the trade-offs associated with encoding high-dimensional quantum information in fixed-frequency transmons. Designing high $E_J/E_C$ ratios of up to 325, we observe up to 12 levels ($d=12$) on a single transmon. Despite the decreased anharmonicity, we demonstrate process infidelities $e_f<3\times {10}^{-3}$ for qubitlike operations in each adjacent-level qubit subspace in the lowest ten levels. Furthermore, we achieve a ten-state readout assignment fidelity of 93.8% with the assistance of deep neural network classification of a multitone dispersive measurement. We find that the Hahn echo time $T_{2E}$ for the higher levels is close to the limit of $T_1$ decay, primarily limited by bosonic enhancement. We verify the recently observed Josephson harmonic model, finding that it yields better predictions for the transition frequencies and charge dispersion. Finally, we show strong $ZZ$-like coupling between the higher energy levels in a two-transmon system. Our high-fidelity control and readout methods, in combination with our comprehensive characterization of the transmon model, suggest that the high-$E_J/E_C$ transmon is a powerful tool for exploring excited states in circuit quantum electrodynamics.

A Sagnac-loop structure can help overcome the major difficulty in the practical implementation of a twin-field quantum key distribution (TFQKD) network, namely, the need to stabilize the phase of a quantum state over many kilometers of fiber. Unfortunately, Rayleigh backscattering noise limits the signal-to-noise ratio for Sagnac systems containing long fibers and lossy photonic devices. Here, we solve this problem by sending optical pulses in long on-off bursts and using time postselection on measurements taken with free-run single-photon avalanche detectors. We also investigate the impact of the residual phase noise uncompensated by the Sagnac structure and find that the variance of the phase noise scales as loop length to the third power, verifying an existing calculation in the literature. We measure the interference visibility in Sagnac loops of varying length without active phase stabilization and achieve $>97\mathrm{\%}$ visibility in 200-km ultralow-loss fiber, which is, to our knowledge, the longest fiber Sagnac interferometer demonstrated. Our results suggest that a Sagnac system is feasible for long-distance TFQKD networks, a key step towards the practical implementation of metropolitan quantum networks.

High-dimensional quantum entanglement is an essential resource in quantum technology since it provides benefits for increasing the information capacity and processing speed. Thus, the controlled harnessing of high-dimensional entanglement has long been hailed as a necessary prerequisite towards practical quantum applications. By using a deterministic quantum state filter implemented through quantum interference, we present a generalized formulation for the complete high-dimensional symmetric and antisymmetric Bell basis and experimentally prepare four-dimensional orbital angular momentum Bell states that provide the well-behaved symmetric or antisymmetric properties. Additionally, we use a concise yet efficient scan of temporal delay to directly observe high-dimensional two-photon interference effects in spatial modes. These results provide an alternative way to harness high-dimensional entanglement and may facilitate the use of quantum interference for more complex quantum information processing tasks beyond qubits.

Achieving high-fidelity two-qubit gates is a key requirement for the realization of large-scale quantum computation and simulation. In this paper, we propose a microwave-control protocol for the implementation of a two-qubit gate employing two transmon qubits coupled via a fixed-frequency transmon coupler. This protocol entails applying two microwave pulses exclusively to the coupler, thereby inducing interaction between fixed-frequency transmon qubits. This interaction facilitates the realization of $\sqrt{i\mathrm{SWAP}}$ gates. Additionally, we explore the implementation of the gate scheme in two distinct qubit architectures (ABA and ABC). Without considering decoherence, the ABA architecture $\sqrt{i\mathrm{SWAP}}$ gate has a fidelity of 94.60% in 346 ns, while the ABC architecture achieves 99.63% in 378 ns. When considering the implementation of the $\sqrt{i\mathrm{SWAP}}$-like gate, we show that gate fidelity can exceed 99.9% in both architectures.

Motivated by recent advancements highlighting $\mathrm{Ta}$ as a promising material in low-loss superconducting circuits and showing long coherence times in superconducting qubits, we have explored the effect of cryogenic temperatures on the growth of $\mathrm{Ta}$ and its integration in superconducting circuits. Cryogenic growth of $\mathrm{Ta}$ using a low-temperature molecular beam epitaxy (MBE) system is found to stabilize single-phase $\alpha$-$\mathrm{Ta}$ on several different substrates, which include ${\mathrm{Al}}_2{\mathrm{O}}_3(0001)$, $\mathrm{Si}$(001), $\mathrm{Si}$(111), ${\mathrm{Si}\mathrm{N}}_x$, and $\mathrm{Ga}\mathrm{As}$(001). The substrates are actively cooled down to cryogenic temperatures and remain <20 K during the $\mathrm{Ta}$ deposition. X-ray $\theta$-$2\theta$ diffraction (XRD) after warming to room temperature indicates the formation of polycrystalline $\alpha$-$\mathrm{Ta}$. The 50-nm $\alpha$-$\mathrm{Ta}$ films grown on ${\mathrm{Al}}_2{\mathrm{O}}_3(0001)$ at a substrate manipulator temperature of 7 K have a room temperature resistivity (${\rho }_{300\mathrm{K}}$) of $13.4\text{μ}\mathrm{\Omega }\mathrm{cm}$, a residual resistivity ratio (RRR) of 17.3 and a superconducting transition temperature ($T_C$) of 4.14 K, which are comparable to bulk values. In addition, atomic force microscopy (AFM) indicates that the film grown at 7 K with an rms roughness of 0.45 nm was significantly smoother than the one grown at room temperature. Similar properties are found for films grown on other substrates. Results for films grown at higher substrate manipulator temperatures show higher ${\rho }_{300\mathrm{K}}$, lower RRR and $\mathrm{Tc}$, and increased $\beta$-$\mathrm{Ta}$ content. Coplanar waveguide resonators with a gap width of $3\text{μ}\mathrm{m}$ fabricated from cryogenically grown $\mathrm{Ta}$ on $\mathrm{Si}$(111) and ${\mathrm{Al}}_2{\mathrm{O}}_3(0001)$ show low-power $Q_i$ of 1.9 million and 0.7 million, respectively, indicating polycrystalline $\alpha$-$\mathrm{Ta}$ films may be promising for superconducting qubit applications even though they are not fully epitaxial.

Entanglement-based quantum key distribution (QKD) protocols require robust and stable photon-pair sources in terms of high heralding efficiencies or photon-pair generation rates even under harsh environmental conditions, e.g., when operated in the field. Here we report on a type-0 spontaneous parametric down-conversion photon source in periodically poled crystals based on simultaneous operation of second harmonic generation and spontaneous parametric down-conversion (SPDC) in a double-pass configuration within the same nonlinear crystal, further simplifying the hardware requirements of SPDC photon sources. Our source is flexible, tunable, alignment-free, relatively broadband, and completely fiber coupled. Its rack-compatible and modular setup allows a straightforward plug-and-play integration of coding modules, e.g., interferometers to enable various QKD protocols such as phase or phase-time coding. We compare the double-pass configuration with our single-pass type-II and type-0 SPDC source configurations. To evaluate the conversion efficiencies of our modules, we use data postprocessing to remove artifacts from detector after-pulsing and dead times of the detectors.

In this study, we employed a quantum computer to solve a low-energy effective Hamiltonian for spin defects in diamond (so-called nitrogen-vacancy centres) and wurtzite-type aluminium nitride, which are anticipated to be qubits. The probabilistic imaginary-time evolution (PITE) method, designed for use in a fault-tolerant quantum computer era, was employed to calculate the ground and excited states of the spin singlet state, as represented by the effective Hamiltonian. It is difficult to compute the spin singlet state correctly using density functional theory, which should be described by multiple Slater determinants. To mitigate the effects of quantum errors inherent in current quantum computers, we implemented a $\text{⟦}n+2,n,2\text{⟧}$ quantum error detection (QED) code called the Iceberg code. Despite the inevitable destruction of the encoded state resulting from the measurement of the ancilla qubit at each PITE step, we were able to successfully re-encode and recover the logical success state. In the implementation of the PITE, it was observed that the effective Hamiltonian comprises large diagonal components and relatively small nondiagonal components, which is frequently the case with quantum chemistry calculations. An efficient implementation of Hamiltonian simulations, in which the diagonal components dominate, was developed on a quantum computer based on the second-order Trotter-Suzuki decomposition. This is the first instance of an encoded PITE circuit being executed on a trapped-ion quantum computer. Our results demonstrate that QED effectively reduces quantum errors and that we successfully obtained both the ground and excited states of the spin singlet state. Our demonstration clearly indicates that ${\mathrm{Zr}}_{\mathrm{Al}}{\mathrm{V}}_{\mathrm{N}}$, ${\mathrm{Ti}}_{\mathrm{Al}}{\mathrm{V}}_{\mathrm{N}}$, and ${\mathrm{Hf}}_{\mathrm{Al}}{\mathrm{V}}_{\mathrm{N}}$ defects have a high potential as spin qubits for quantum sensors.

Characterization of quantum devices generates insights into their sources of disturbances. State-of-the-art characterization protocols often focus on incoherent noise and eliminate coherent errors when using Pauli or Clifford twirling techniques. This approach biases the structure of the effective noise and adds a circuit and sampling overhead. We motivate the extension of an incoherent local Pauli noise model to coherent errors and present a practical characterization protocol for an arbitrary gate layer. Notably, the coherent noise estimation is robust to Pauli noise. We demonstrate our protocol on a superconducting hardware platform and identify the leading coherent errors. To verify the characterized noise structure, we mitigate its coherent and incoherent components using a gate-level coherent noise mitigation scheme in conjunction with probabilistic error cancelation. The proposed characterization procedure opens up possibilities for device calibration, hardware development, and improvement of error mitigation and correction techniques.

Quantized surface acoustic waves (SAWs) in piezoelectric media have been studied recently, and they can be used to control the electric dipoles of quantum systems via the electric field produced through the piezoelectric effect. However, it is not easy nor convenient to manipulate magnetic moments directly by an electric field. Here we study a quantum theory of SAWs in a piezomagnetic medium. We show that the intrinsic properties of the piezomagnetic medium enable the SAW in the piezomagnetic medium to interact directly with magnetic moments of quantum systems via the magnetic field induced by the piezomagnetic effect. By taking a strip SAW waveguide made of a piezomagnetic medium as an example, we further study the coupling strengths between different magnetic quantum systems with magnetic moments and the quantized single-mode SAW in the waveguide. Based on this, we discuss the interaction between magnetic quantum systems mediated by a quantized multimode SAW in a piezomagnetic waveguide. Our study provides a convenient way to directly control magnetic quantum systems by quantized SAWs, and offers potential applications to on-chip information processing based on solid-state quantum systems via quantized acoustic waves.

A scalable and programmable quantum computer holds the potential to solve computationally intensive tasks that classical computers cannot accomplish within a reasonable time frame, achieving quantum advantage. However, the vulnerability of the current generation of quantum processors to errors poses a significant challenge towards executing complex and deep quantum circuits required for practical problems. Quantum error-correction codes such as stabilizer codes offer a promising path forward for fault-tolerant quantum computing; however, their realization on quantum hardware is an on-going area of research. In particular, fault-tolerant quantum processing must employ logical gates on logical qubits with error suppression with realistically large size codes. This work has implemented a transversal controlled-not (cnot) gate between two logical qubits constructed using the repetition code with flag qubits, and demonstrated error suppression with increasing code size under multiple rounds of error detection and correction. Our results conducted on IBM quantum devices through cloud access show that despite the potential for error propagation among logical qubits during the transversal cnot gate operation, scaling the code distance as we increase the number of physical qubits from 21 to 39 and 57 can suppress errors, which persists over ten rounds of error detection and correction. Our work establishes the feasibility of employing logical cnot gates alongside error detection on a superconductor-based processor using current generation quantum hardware.

In superconducting qubits, suppression of spontaneous emission is essential to achieve fast dispersive measurement and reset without sacrificing qubit lifetime. We show that resonator-mediated decay of the qubit mode to the feedline can be suppressed using destructive interference, where the readout resonator is coupled to the feedline at two points. This “interferometric Purcell filter” does not require dedicated filter components or impedance mismatch in the feedline, making it suitable for applications such as all-pass readout. We design and fabricate a device with the proposed scheme and demonstrate suppression of resonator-mediated decay that exceeds 2 orders of magnitude over a bandwidth of $400\mathrm{M}\mathrm{Hz}$ for a resonator linewidth of 13.8 MHz.

The conditional displacement (CD) gate between an oscillator and a discrete-variable auxiliary qubit plays a key role in quantum information processing tasks, such as enabling universal control of the oscillator and longitudinal readout of the qubit; however, the gate is unprotected against the propagation of auxiliary qubit decay errors, meaning it is not fault tolerant. Here, we propose a CD gate scheme with fluxonium as the auxiliary qubit, which has been experimentally demonstrated to have a large noise bias and millisecond-level lifetimes. The proposed gate is applied cross-resonantly by modulating the external flux of the fluxonium at the frequency of the target oscillator, which requires minimal hardware overhead and does not increase sensitivity to decoherence mechanisms such as dephasing. We further provide a perturbative description of the gate mechanism and identify the error budget. Additionally, we develop an approximate procedure for choosing device and gate parameters that optimize gate performance. Following the procedure for multiple sets of fluxonium parameters from the literature, we numerically demonstrate CD gates with unitary fidelity exceeding 99.9% and gate times of hundreds of nanoseconds.