High-dimensional quantum systems, known as qudits with dimension $d>2$, possess the capability to encode and process more information than traditional two-dimensional qubits. Entangled states serve as crucial resources for both quantum computation and quantum communication. On the basis of circuit quantum electrodynamics (QED) systems, we introduce an efficient scheme to deterministically generate high-dimensional multipartite entangled states of hybrid qutrits ($d=3$). Assuming a simple initial state has already been prepared with the use of local operations, our method requires only a single step to produce hybrid Greenberger-Horne-Zeilinger (GHZ) entangled states among circuit QED systems consisting of superconducting qutrits and a superconducting cavity. Remarkably, the number of steps and the operational time remain constant as the number of qutrits increases, highlighting the efficiency and scalability of our approach. Using numerical simulations for a three-dimensional hybrid GHZ state of seven qutrits, we demonstrate the feasibility of our scheme and study the effects of various parameters on the fidelity. The importance of this research is in directing the experimental creation of multipartite, high-dimensional, and hybrid GHZ states, which can then be used to enhance quantum communication protocols and computational processes.

Engineering high-order nonlinearities while suppressing lower-order terms is crucial for quantum error correction and state control in bosonic systems, yet it remains an outstanding challenge. Here, we introduce a general framework of a nonlinearity-engineered multiloop SQUID (NEMS) device, enabling the realization of arbitrary nonlinearities by tuning fluxes in multiple loops within superconducting circuits. We demonstrate specific examples of NEMS devices that selectively engineer cubic-, quartic-, and quintic-dominant interactions with suppressed parasitic couplings, showing great promise for realizing Kerr-cat bias-preserving cnot gates and stabilizing four-leg cat qubits. By opening avenues for tailoring nonlinear Hamiltonians of superconducting devices, this work enables sophisticated and precise manipulation of bosonic modes, with potential applications in quantum computation, simulation, and sensing.

Using an in-house Schrödinger-Poisson solver, we investigate the creation of a single hole-spin qubit inside a triple-gate triangular silicon fin field-effect transistor ($\mathrm{Si}$ FinFET) quantum device similar to experimental structures. The gate-induced formation of the required quantum dot is monitored based on the Luttinger-Kohn $6\times 6k\cdot p$ method, accounting for magnetic fields and strain to determine the qubit ground state. Strain arises from the inhomogeneous contraction of the different components of the FinFET when they are cooled to cryogenic temperatures. This leads to a renormalization of the qubit’s energy levels, thus impacting both the heavy-hole and light-hole populations, as well as their mixing. The dot length, band mixing, $g$-factor, and Larmor and Rabi frequencies of the considered device are extracted. In particular, we show that these metrics exhibit strong strain-dependent variations of their magnitude, thus underlining the importance of including realistic thermal-contraction scenarios when modeling hole-spin qubits.

Electrons trapped on the surface of superfluid helium have been thought of as a useful resource for quantum computing. Such electrons show long coherence of their surface-bound (Rydberg) states combined with their easy electrostatic manipulation. Recent proposals have explored the possibility of coupling the spin state of an electron and the state of its quantized motion, with the promise of a highly scalable two-dimensional architecture for a quantum computer. However, despite recent progress in the detection of quantized lateral motion of electrons using a circuit-QED setup, manipulation of a small number of electrons and their quantum state detection remains a challenging problem. Here, we report on the detection of the Rydberg transition of electrons on superfluid helium in an on-chip microchannel device in which electrons are moved and trapped by a set of electrostatic gates. A highly sensitive image-charge detection system allows us to not only resolve the transition spectra of such electrons, but also perform the device characterization. The demonstrated sensitivity shows the feasibility of detecting the Rydberg transition of a single electron, which can open an alternate pathway for a nondestructive spin-state readout.

Quantum error-correcting codes typically do not account for quantum state transitions—leakage—out of the computational subspace. Since these errors can last for multiple detection rounds, they can significantly contribute to logical errors. It is therefore important to understand how to numerically model them efficiently. Fully quantum simulations of leakage require more levels per leaked qubit, which substantially limits the system sizes that may be simulated. To address this, we introduce a subspace twirling approximation (STA) on quantum channels that preserves the incoherence between the computational and leakage subspaces. The assumption of incoherence enables the quantum simulation of leakage at little computational overhead. We motivate the approximation’s validity by showing that incoherence is achieved naturally during repeated stabilizer measurements. Additionally, we provide various simulation results which show that the STA yields accurate error-correction statistics in the repetition and surface codes with physical error parameters.

Brillouin-based optomechanical systems with high-frequency acoustic modes provide a promising platform for implementing quantum-information processing and wavelength conversion applications, and for probing macroscopic quantum effects. Achieving strong coupling through electrostrictive Brillouin interaction is essential for coupling the massive mechanical mode to an optical field, thereby controlling and characterizing the mechanical state. However, achieving strong coupling at room temperature has proven challenging due to fast mechanical decay rates. While Brillouin strong coupling has been demonstrated, a comprehensive characterization of the system in this regime is still lacking. Here, we report an optomechanical system with independent control over pumping power and frequency detuning to achieve and characterize the strong-coupling regime of a bulk acoustic-wave resonator. Through spectral analysis of the cavity reflectivity, we identify clear signatures of strong coupling, namely, normal-mode splitting and an avoided crossing in the detuned spectra, while estimating the mechanical linewidth ${\mathrm{\Gamma }}_m/2\pi =7.13\mathrm{M}\mathrm{Hz}$ and the single-photon coupling rate $g_0/2\pi =7.76\mathrm{Hz}$ of our system. Our results provide valuable insights into the performance of room-temperature macroscopic mechanical systems and their applications in hybrid quantum devices.

Single-photon sources (SPSs) hold the potential to enhance the performance of quantum key distribution (QKD). QKD systems using SPS often require cryogenic cooling, while recent QKD attempts using SPS operating at room temperature have failed to achieve long-distance transmission due to the SPS not operating at telecommunication wavelength. In this work, we have successfully demonstrated QKD using a room-temperature SPS at telecommunication wavelength. The SPS used in this work is based on point defects hosted by gallium nitride ($\mathrm{Ga}\mathrm{N}$) thin films grown on sapphire substrates. We employed a time-bin and phase-encoding scheme to perform the BB84 and reference-frame-independent QKD protocols over a 33-km fiber spool, achieving a secure key rate of $7.58\times {10}^{-7}$ per pulse. Moreover, we also implemented a metropolitan QKD experiment over a 30-km deployed fiber, achieving a secure key rate of $6.06\times {10}^{-8}$ per pulse. These results broaden the prospects for future use of SPS in commercial QKD applications.

We experimentally demonstrate an effective interaction between telecommunication-wavelength optical photons and microwave photons, mediated by Rydberg atoms. A microwave-induced giant cross-Kerr effect is observed in a ${}^{85}\mathrm{Rb}$ vapor cell by our probing higher excited atomic states at the optical telecommunication band, and the measured third-order cross-Kerr coefficient ${\chi }^{(3)}$ can reach approximately $1\times {10}^{-10}{\mathrm{m}}^2/{\mathrm{V}}^2$. These results can be applied to information processing of the integration of wireless and fiber-optic networks in the classical regime or the quantum regime.

Privacy amplification (PA) is a vital step in the postprocessing stage of quantum key distribution, which can extract a secure key from a source partially leaked to the environment. We propose a PA scheme that uses the square hash (SQH)–modular arithmetic hash (MH) function. In consideration of a potential quantum attacker, we use the trace distance as a metric to measure the closeness between two classical-quantum states. We transform a raw key state to a processed state using this scheme and derive the trace distance between the processed state and the ideal target state. It is proven that the trace distance can be reached as small as desired. Additionally, our approach accelerates the SQH-MH function through the number-theoretic transform, reducing the computational complexity to $O(n\log n)$. On the basis of a CPU, a maximum throughput rate of 1.06 Gbit/s is achieved, and the results show that our algorithm demonstrates good stability of the throughput rate when handling tasks with different compression ratios.

Quantum information processing faces a significant hurdle: noise. Different noise sources induce varying errors in quantum operations depending on the underlying dynamics. To gain a deeper understanding of these error mechanisms, we introduce the concept of quantum error evolution diagrams (QEEDs). QEEDs establish a dual correspondence between driven noisy quantum dynamics and geometric space curves, providing quantitative geometric metrics to assess the severity of the noises. This framework enables the design of universal robust quantum gates that correct errors induced by generic noise. Additionally, we present a protocol for constructing a universal set of single- and two-qubit robust quantum gates with simple and smooth control pulses of arbitrary length and fidelities exceeding 99.99% across a wide range of noise strengths. Our work provides new insights into the geometric nature of noisy quantum dynamics and paves the way for developing strategies to dynamically correct quantum errors.

Fluxonium qubits are recognized for their high coherence times and high operation fidelities. These are attributed to their unique design incorporating a superinductor, which is typically implemented using an array of over 100 Josephson junctions; however, this complexity poses significant fabrication challenges, particularly in achieving high yield and junction uniformity with traditional methods. Here, we introduce an overlap process for Josephson-junction fabrication that achieves nearly 100% yield and maintains uniformity across a 2-inch (50.8-mm) wafer with less than 5% variation for the phase-slip junction and less than 2% for the entire junction array. We use a compact junction array design that achieves state-of-the-art dielectric loss tangents and flux noise levels, as confirmed by multiple devices. This enables fluxonium qubits to reach energy relaxation times exceeding 1 ms at the flux-frustration point. This work paves the way for scalable, high-coherence fluxonium processors using processes compatible with complementary metal-oxide semiconductor manufacturing, marking a significant step toward practical quantum computing.

Charge noise is a major obstacle to improved gate fidelities in silicon spin qubits. Numerous methods exist to mitigate charge noise, including improving device fabrication, dynamical decoupling, and real-time parameter estimation. In this work, we demonstrate a class of techniques to mitigate charge noise in semiconductor quantum dots by controlling the noise sources themselves. Using two different classical feedback methods, we stabilize an individual charged two-level fluctuator in a $\mathrm{Si}$/$\mathrm{Si}$-$\mathrm{Ge}$ quantum dot by exploiting sensitive gate-voltage dependence of the switching times. These control methods reduce the low-frequency component of the noise power spectrum by an order of magnitude. These techniques also enable stabilization of the fluctuator in either of its states. In the future, such techniques may enable improved coherence times in quantum dot spin qubits.

We propose a method to realize microwave-activated cz gates between two remote spin qubits in quantum dots using a charge-sensitive superconducting coupler. The qubits are longitudinally coupled to the coupler, such that the transition frequency of the coupler depends on the logical qubit states; a capacitive network model using first-quantized charge operators is developed to illustrate this. Driving the coupler transition then implements a conditional phase shift on the qubits. Two pulsing schemes are investigated: a rapid, off-resonant pulse with constant amplitude, and a pulse with envelope engineering that incorporates dynamical decoupling to mitigate charge noise. We develop non-Markovian time-domain simulations to accurately model gate performance in the presence of $1/f^{\beta }$ charge noise. Simulation results indicate that a cz gate fidelity exceeding 90% is possible with realistic parameters and noise models.

The dominant contribution to the energy relaxation of state-of-the-art superconducting qubits is often attributed to their coupling to an ensemble of material defects which behave as two-level systems. These defects have varying microscopic characteristics which result in a wide range of observable defect properties such as resonant frequencies, coherence times, and coupling rates to qubits $g$. Here, we investigate strategies to mitigate losses to the family of defects that strongly couple to qubits ($g/2\pi \ge 0.5\mathrm{M}\mathrm{Hz}$). Such strongly coupled defects occur more rarely and are particularly detrimental to the qubit coherence when resonant with the qubit, and to the fidelities of operations relying on frequency excursions, such as flux-activated two-qubit gates. To assess their impact, we perform swap spectroscopy on 92 frequency-tunable qubits and quantify the spectral density of these strongly coupled modes. We show that the frequency configuration of the defects is rearranged by warming the sample to room temperature, whereas the total number of defects on a processor tends to remain constant. We then explore methods for fabricating qubits with a reduced number of strongly coupled defect modes by systematically measuring their spectral density for decreasing Josephson junction dimensions and for various surface cleaning methods. Our results provide insights into the properties of strongly coupled defect modes and show the benefits of minimizing Josephson junction dimensions to improve qubit properties.

The weak spin-orbit coupling (SOC) in bilayer graphene (BLG) is essential for encoding spin qubits while bringing technical challenges for extracting the opened small SOC gap ${\mathrm{\Delta }}_{\mathrm{SO}}$ in experiments. Moreover, in addition to the intrinsic Kane-Mele term, extrinsic mechanisms also contribute to SOC in BLG, especially under experimental conditions including encapsulation of BLG with hexagonal boron nitride (h-BN) and applying an external out-of-plane electric displacement field D. Although measurements of ${\mathrm{\Delta }}_{\mathrm{SO}}$ in h-BN-encapsulated BLG have been reported, the relatively large experimental variations and existing experimental controversy make it difficult to fully understand the physical origin of ${\mathrm{\Delta }}_{\mathrm{SO}}$. Here, we report a combined experimental and theoretical study on ${\mathrm{\Delta }}_{\mathrm{SO}}$ in h-BN-encapsulated BLG. We use an averaging method to extract ${\mathrm{\Delta }}_{\mathrm{SO}}$ in gate-defined single quantum dot devices. Under D fields as large as 0.57–0.90 V/nm, ${\mathrm{\Delta }}_{\mathrm{SO}}$ ≈ 53.4–61.8 $\text{μ}$eV is obtained from two devices. Benchmarked with values reported at lower D field regime, our results support a D-field-independent ${\mathrm{\Delta }}_{\mathrm{SO}}$. This behavior is confirmed by our first-principles calculations, based on which ${\mathrm{\Delta }}_{\mathrm{SO}}$ is found to be independent of D field, regardless of different h-BN/BLG/h-BN stacking configurations. Our calculations also suggest a weak proximity effect from h-BN, indicating that SOC in h-BN-encapsulated BLG is dominated by the intrinsic Kane-Mele mechanism. Our results offer insightful understanding of SOC in BLG, which benefits SOC engineering and spin manipulations in BLG.

We introduce several probabilistic quantum algorithms that overcome the normal unitary restrictions in quantum machine learning by leveraging the linear combination of unitaries (LCU) method. We cover three distinct topics, beginning with quantum native implementations of residual networks (ResNets). We demonstrate that while residual connections between layers of a variational Ansatz can prevent barren plateaus in models, this approach is accompanied by a trade-off in success probability. Second, we implement a quantum analogue of average-pooling layers from convolutional networks using single-qubit-controlled basic arithmetic operators and show that the LCU success probability remains stable for the Modified National Institute of Standards and Technology (MNIST) database. This method can be further generalized to convolutional filters, while using exponentially fewer controlled unitaries than previous approaches. Finally, we propose a general framework for applying a linear combination of irreducible-subspace projections on quantum encoded data for any finite group. This enables a quantum state to remain within an exponentially large space, while selectively amplifying specific subspaces relative to others, alleviating simulability concerns that arise when fully projecting to a polynomially sized subspace. We demonstrate improved classification performance for partially amplified permutation-invariant encoded point-cloud data when compared to noninvariant or fully permutation-invariant encodings. We also demonstrate a novel rotationally invariant encoding for point-cloud data via Schur-Weyl duality. These quantum computing frameworks are all constructed using the LCU method, suggesting that further novel quantum machine-learning (QML) algorithms could be created by utilizing the LCU technique.

The control of flying qubits carried by itinerant photons is ubiquitous in quantum networks. In addition to their logical states, the shapes of flying qubits must also be tailored for high-efficiency information transmission. In this paper, we introduce quantum optimal control theory to the shaping of flying qubits. Building on the flying-qubit control model established in our previous work, we design objective functionals for the generation of shaped flying qubits under practical constraints on the emitters and couplers. Numerical simulations employing gradient-descent algorithms demonstrate that the optimized control can effectively mitigate unwanted level and photon leakage caused by these nonidealities. Notably, while coherent control offers limited shaping capacity with a fixed coupler, it can significantly enhance the shaping performance when combined with a tunable coupler that has restricted tunability. The proposed optimal control framework provides a systematic approach to achieving high-quality control of flying qubits using realistic quantum devices.

Quantum computing has garnered attention for its potential to solve complex computational problems considerably more rapidly than classical computing. Despite notable advancements in the field, achieving meaningful scalability and noise control in quantum hardware remains challenging. Incoherent errors caused by decoherence restrict the total computation time, often making it very short. While hardware advancements continue to progress, quantum software specialists seek to minimize quantum circuit latency to mitigate dissipation; however, at the pulse level, fast quantum gates often lead to leakage, leaving minimal room for further optimization. Recent advancements have demonstrated the effectiveness of quantum control techniques in generating quantum gates that are robust to coherent error sources. Nevertheless, these techniques come with a trade-off—extended gate durations. In this paper, we introduce an alternative approach to pulse scheduling that enables the use of both fast and robust quantum gates within the same quantum circuit. The time-optimization framework models the quantum circuit as a dependency graph, implements the fastest quantum gates on the critical path, and uses idle periods outside the critical path to optimally implement longer, more robust gates from the gate set, without increasing latency. Experiments conducted on IBMQ Brisbane show that this approach improves the absolute success probability of quantum circuit execution by more than 25%, with performance gains scaling as the number of qubits increases.

Quantum entanglement detection and characterization are crucial for various quantum information processes. Most existing methods for entanglement detection rely heavily on a complete description of the quantum state, which requires numerous measurements and complex setups. This makes these theoretically sound approaches costly and impractical, as the system size increases. In this work, we propose a multiview neural-network model to generate representations suitable for entanglement structure detection. The number of required quantum measurements shows a significant reduction compared to conventional quantum state tomography approaches as the qubit number increases. This remarkable reduction in resource costs makes it possible to detect specific entanglement structures in large-scale systems. Numerical simulations show that our method achieves over 95% detection accuracy for up to 19 qubit systems. By enabling a universal, flexible, and resource-efficient analysis of entanglement structures, our approach enhances the capability of utilizing quantum states across a wide range of applications.