State-of-the-art noisy-intermediate-scale quantum processors are currently implemented across a variety of hardware platforms, each with their own distinct gatesets. As such, circuit compilation should not only be aware of but also deeply connect to the native gateset and noise properties of each. Trapped-ion processors are one such platform that provides a gateset that can be continuously parameterized across both one- and two-qubit gates. Here we use the Quantum Scientific Computing Open User Testbed to study noise-aware compilations focused on continuously parameterized two-qubit $ZZ$ gates (based on the Mølmer-Sørensen interaction) using superstaq, a quantum software platform for hardware-aware circuit compiler optimizations. We discuss the realization of $ZZ$ gates with arbitrary angle on the all-to-all connected trapped-ion system. Then we discuss a variety of different compiler optimizations that innately target these $ZZ$ gates and their noise properties. These optimizations include moving from a restricted maximally entangling gateset to a continuously parameterized one, swap mirroring to further reduce the total entangling angle of the operations, focusing the heaviest $ZZ$ angle participation on the best-performing gate pairs, and circuit approximation to remove the least impactful $ZZ$ gates. We demonstrate these compilation approaches on the hardware with randomized quantum volume circuits, observing the potential to realize a larger quantum volume as a result of these optimizations. Using differing yet complementary analysis techniques, we observe the distinct improvements in system performance provided by these noise-aware compilations and study the role of stochastic and coherent error channels for each compilation choice.

We investigate the superposition of coherent states, emphasizing quantum states with distinct Wigner phase-space features relevant to quantum information applications. In this study, we introduce generalized versions of the compass state, which display enhanced phase-space characteristics compared with the conventional compass state, typically a superposition of four coherent states. Our findings reveal that, unlike sub-Planckian structures and phase-space sensitivity of the compass state, these generalized states produce isotropic sub-Planckian structures and sensitivity to phase-space displacements. We demonstrate that these desirable phase-space characteristics are maintained in superpositions comprising at least six distinct coherent states. Furthermore, we show that increasing the number of coherent states in the superposition preserves these characteristics, provided the number remains even. We examine an optomechanical system capable of generating the proposed quantum states, resulting in optomechanical counterparts with nearly identical phase-space structures, thereby suggesting the feasibility of physically realizing these generalized compass states.

Realizing large-scale quantum networks requires the generation of high-fidelity quantum entanglement states between remote quantum nodes, a key resource for quantum communication, distributed computation, and sensing applications. However, entanglement distribution between quantum network nodes is hindered by optical transmission loss and local operation errors. Here, we propose a quantum repeater architecture that synergistically integrates Rydberg-atom quantum processors with optical cavities to overcome these challenges. Our scheme leverages cavity-mediated interactions for efficient remote entanglement generation followed by Rydberg-interaction-based entanglement purification and swapping. Numerical simulations, incorporating realistic experimental parameters, demonstrate the generation of Bell states with 99% fidelity at rates of 1.1 kHz between two nodes in a local-area network (distance $0.1\mathrm{km}$), and can be extended to metropolitan-area ($25\mathrm{km}$) or intercity ($250\mathrm{km}$, with the assistance of frequency converters) networks with a rate of 0.1 kHz. This scalable approach opens up near-term opportunities for exploring quantum network applications and investigating the advantages of distributed quantum information processing.

Achieving fast and high-fidelity qubit operations is crucial for unlocking the potential of quantum computers. In particular, reaching low gate errors in two-qubit gates has been a long-standing challenge in the field of superconducting qubits due to their typically long duration relative to coherence times. To realize fast gates, we use the hybridization between fixed-frequency superconducting qubits with a strongly interacting coupler mode that is tunable in frequency. To reduce population leakage during required adiabatic passages through avoided level crossings, we employ a sensitivity-adaptive closed-loop optimization method to design complex pulse shapes. We compare the performance of Gaussian-square, Fourier-series, and piecewise-constant-slope pulse parametrizations and are able to reach 99.9% controlled-$Z$-gate fidelity using a 64-ns-long Fourier-series pulse defined by only seven parameters. These high-fidelity values are achieved by analyzing the optimized pulse shapes to identify and systematically mitigate signal-line distortions in the experiment. To improve the convergence speed of the optimization, we implement an adaptive cost function, which continuously maximizes the sensitivity. The demonstrated method can be used for tune-up and recalibration of superconducting quantum processors.

Self-assembled $\mathrm{In}\mathrm{Ga}\mathrm{As}$/$\mathrm{Ga}\mathrm{As}$ quantum dots (QDs) are of particular importance for the deterministic generation of spin-photon entanglement. One promising scheme relies on the Larmor precession of a spin in a transverse magnetic field, which is governed by the in-plane $g$ factors of the electron and the valence-band heavy hole. We probe the origin of heavy-hole $g$-factor anisotropy with respect to the in-plane magnetic field direction and uncover how it impacts the entanglement generated between the spin and the photon polarization. First, using polarization-resolved photoluminescence measurements on a single QD, we determine that the impact of valence-band mixing dominates over effects due to a confinement-renormalized cubic Luttinger $q$ parameter. From this, we construct a comprehensive hole $g$-tensor model. We then use this model to simulate the concurrence and fidelity of spin-photon entanglement generation with anisotropic hole $g$ factors, which can be leveraged by tuning the magnetic field angle and the excitation polarization. The results demonstrate that postgrowth control of the hole $g$ factor can be used to improve spin-photon cluster-state generation.

Andreev-spin qubits are a new qubit platform that merges superconductivity with semiconductor physics. The mechanisms dominating observed energy relaxation remain unidentified. We report here on three steps taken to address these questions in an $\mathrm{In}\mathrm{As}$ nanowire weak link. First, we designed a microwave readout circuit tuned to be directly sensitive to the spin-dependent inductance of the weak link so that higher orbital states are not necessary for readout—this resulted in larger windows in parameter space in which the spin-state properties can be probed. Second, we implemented a successful gap-engineering strategy to mitigate quasiparticle poisoning. Third, the weak link was fabricated by in situ shadow evaporation, which has been shown to improve atomic-scale disorder. We show how our design allows characterization of the spin stability and coherence over the full range of magnetic flux and gate voltage of an odd-parity bias point. The spin-relaxation and dephasing rates are comparable with those of the best devices previously reported, suggestive that surface atomic-scale disorder and quasiparticle poisoning are not linked to spin relaxation in $\mathrm{In}\mathrm{As}$ nanowires. Our design strategies are transferrable to novel materials platforms for Andreev qubits, such as germanium and carbon.

We propose and demonstrate two cavity-assisted thermometry methods that exploit the sensitivity of cavity radiation to the position of the ion in the cavity mode to achieve highly sensitive temperature measurements. Our analysis allows us to determine the relative positions of the rf null and the cavity mode by the change of cavity emission at different positions of the ion in the cavity mode. By analyzing the second-order correlation function of the cavity emission, temperature measurements from near the motional ground state to over tens of millikelvin can be achieved and motion patterns of different frequencies can be characterized. We demonstrate our approach by measuring the temperature of a single ${}^{138}{\mathrm{Ba}}^{+}$ ion trapped in a fiber microcavity with a length of 230 $\text{μ}\mathrm{m}$ using both proposed methods, achieving comparable results.

We develop a method for entangling operations on nuclear spins surrounding a negatively charged boron-vacancy (V$\mathrm{B}$-center) point defect in hexagonal boron nitride (h$\mathrm{BN}$). To this end, we propose to employ the electron spin of a V$\mathrm{B}$ center as a control qubit. We show that, in the presence of a background magnetic field and by applying control pulses, one can collectively manipulate the state of the nuclei with ${\hat{U}}_z$ and ${\hat{U}}_x$ rotations. These rotations can serve for implementing the synchronous three-qubit $X$, $Z$, and Hadamard gates. Through our numerical analyses considering realistic system parameters and decoherence effects, we demonstrate that these gates can be executed with high fidelities. Furthermore, as an example for the application of our toolbox, we use these collective gates to prepare the highly entangled Greenberger-Horne-Zeilinger states among the three nuclear spins with a fidelity of 0.99. By including electron decoherence effects, we find that the relative deviations of the gate fidelities from the noisy terms are negligibly small, demonstrating the noise resilience of our protocols. Our work can serve as the foundation for exploiting the nuclear spins in h$\mathrm{BN}$ in future quantum technological applications.

Quantum crosstalk, which stems from unwanted interference of quantum operations with nearby qubits, is a major source of noise or errors in a quantum processor. In the context of shared quantum computing, it is challenging to mitigate the crosstalk effect between quantum computations being simultaneously run by multiple users since the exact spatiotemporal gate distributions are not apparent due to privacy concerns. It is therefore important to develop techniques for accurate detection and mitigation of crosstalk to enable high-fidelity quantum computing. Assuming prior knowledge of crosstalk parameters, we propose a time-adaptive detection method leveraging spectator qubits and multiple quantum coherence to amplify crosstalk-induced perturbations. We demonstrate its utility in detecting random sparsely distributed crosstalk within a time window. Our work evaluates its performance in two scenarios: simulation using an artificial noise model with gate-induced crosstalk and always-on idlings channels; and the simulation using noise sampled from an IBM quantum computer parameterized by the reduced Hamiltonian, stochastic, and affine (HSA) error model. The presented results show our method’s efficacy hinges on the dominance of single-qubit coherent noise across channels, and the effect of angle mismatching is suppressed as spectator qubits increase. From simulation using real-device noise parameters, our strategy outperforms the previous constant-frequency detection method of Harper et al. [arXiv:2402.02753] in the detection success rate, achieving an average detection success probability of $0.852\pm 0.022$ (equally scaled noise channels) and $0.933\pm 0.024$ (asymmetrically scaled noise channels) from one to seven crosstalk counts.

Quantum networks hold promise for key distribution, private and distributed computing, and quantum sensing, among other applications. The scale of such networks for ground users is currently limited by one’s ability to distribute entanglement between distant locations. This can in principle be carried out by transmitting entangled photons through optical fibers or satellites. The former is limited by fiber-optic attenuation, while the latter is limited by atmospheric extinction and diffraction. Here, we propose a hybrid network and protocol that outperform both ground- and satellite-based designs and lead to high-fidelity entanglement at a continental or even global scale.

This paper assesses the numerical accuracy of the Harrow-Hassidim-Lloyd (HHL) algorithm in solving a finite-difference approximation of the Poisson equation, with the Thomas algorithm used as a comparative benchmark. In the one-dimensional setting with homogeneous boundary conditions, the HHL algorithm exhibits numerical accuracy comparable to the Thomas algorithm; however, the HHL algorithm exhibits increased sensitivity to variations in input parameters for simulations with nonhomogeneous boundary conditions. While the current implementation of the HHL algorithm is limited to solving the one-dimensional Poisson equation, we propose a hybrid framework to extend the applicability of the HHL algorithm to solving the two-dimensional Poisson equation. Our results show that although both algorithms perform similarly for small systems of $8\times 8$ mesh nodes, the HHL algorithm shows increased input-parameter sensitivity for systems of $16\times 16$ mesh nodes, indicating that further development is needed to scale the hybrid approach effectively.

In this work, we study the tomography of the spatial structure of light. We develop a simple technique that allows one to perform tomography over the space of fixed-order modes. The technique is based on two spatially resolved intensity measurements, the second of which is performed after the light field has undergone an astigmatic transformation implemented by a tilted lens. We demonstrate that this method is informationally complete within the considered subspaces, and this is experimentally verified in both the intense and photocount regimes. We also study the effect of obstructions on our ability to reconstruct the states. The work presented is expected to help the development of characterization techniques in the field of structured light and in its application to both classical and quantum information protocols.

Reservoir computing is an effective method for predicting chaotic systems by the use of a high-dimensional dynamic reservoir with fixed internal weights, while the learning phase is kept linear, which simplifies training and reduces computational complexity compared with fully trained recurrent neural networks. Quantum reservoir computing uses the exponential growth of Hilbert spaces in quantum systems, allowing greater information processing, memory capacity, and computational power. We present a hybrid neuromorphic quantum-classical approach that implements memory through classical postprocessing of quantum measurements, thus avoiding the need for multiple coherent input injections (as in the original proposal). We tested our model on two physical platforms—a fully connected Ising model and a Rydberg-atom array—and evaluated it on various benchmark tasks, including the chaotic Mackey-Glass time series prediction, where it demonstrates significantly enhanced predictive capabilities and achieves a substantially longer prediction time, outperforming previously reported approaches.

The development of working cryogenic memories is very important for the scalability and feasibility of quantum computing. In this context, $\mathrm{Fe}\mathrm{Co}\mathrm{B}$/$\mathrm{Mg}\mathrm{O}$-based spin-transfer-torque magnetic random-access memories emerge as promising candidates to address this challenge. At low temperatures, the reduction in thermal noise allows for a lower energy barrier while retaining sufficient memory performance. Since the effective anisotropy of the system is directly proportional to the energy barrier, understanding how it varies with material parameters and temperature is crucial. In this work, we present an experimental method for extrapolating the thin-film magnetic properties measured at room temperature to the electrical properties of nanofabricated devices at various targeted temperatures. We employ magneto-optical Kerr-effect (MOKE) magnetometry to calculate the effective anisotropy energy density across the entire 100-mm wafer surface at room temperature. This method is entirely adapted to probe wafers of various sizes, and it is found to be faster and equivalent to conventional methods, such as vibrating sample magnetometry and superconducting quantum interference device (SQUID) magnetometry. The temperature evolution of magnetic anisotropy is further investigated using SQUID magnetometry from 300 to 10 K. Finally, the critical switching voltage from 300 to 40 K is measured for various nanofabricated devices with different $\mathrm{Fe}\mathrm{Co}\mathrm{B}$ storage-layer thicknesses. A model correlating the switching voltage of these devices with the effective anisotropy at the thin-film level is used to explain their temperature and thickness dependencies. These results confirm the validity of the MOKE method presented in this work and its potential in selecting devices across the wafer to function at a given operational target temperature.

For quantum computation using photons, performing deterministic quantum gate operations is a challenge due to the probabilistic nature of the photon-photon interaction. Encoding qubits in multiple degrees of freedom of photons and controlling operations between them is one of the promising ways to navigate the probabilistic behavior. Using the single-photon discrete-time quantum walk in combination with polarization and path degrees of freedom, we experimentally demonstrate the realization of a universal set of quantum gates with high fidelity at room temperature. The deterministic realization of quantum gates through the photonic quantum walk are characterized via quantum state tomography. For a three-qubit system using a single photon, the first qubit is encoded using polarization information, and the other two qubits are encoded using path information, closely resembling a Galton-board setup. To generate a six-qubit Greenberger-Horne-Zeilinger state, entangled photon pairs are used to entangle the two three-qubit modules on which gate operations are performed. We also provide insights into the mapping of photonic quantum walk operations to quantum circuit representation and propose method to resourcefully scale. This demonstration marks significant progress towards using quantum walks for quantum computing and provides a framework for using fewer photons in combination with different degrees of freedom of photons to scale the number of qubits.

Integrated microring resonators have a broad range of applications in diverse fields with the potential to design compact, robust, energy-efficient devices crucial for quantum applications. Degenerate optical parametric oscillations (DOPOs) realized in dual-pumped microring resonator with third-order optical nonlinearity are of special interest. They demonstrate both bistability of the phase of the excited signal mode and generation of nonclassical light, which can be used for coherent photonic computing. Using coupled mode equations, we perform a comprehensive numerical analysis of DOPO conditions with normal group velocity dispersion and with bichromatic pumping. Through analytical and numerical approaches, we identify optimal setup parameter values that minimize the threshold power, highlighting the importance of considering the full spectrum of mode interactions. Additionally, we show that dispersion engineering, achievable in photonic molecules or photonic crystal microresonators, may provide a targeted frequency shift of specific microresonator modes resulting in pump power threshold reduction.

The presence of quasiparticles typically degrades the performance of superconducting microwave circuits. The readout signal can generate nonequilibrium quasiparticles, which lead to excess microwave loss and decoherence. To understand this effect quantitatively, we measure quasiparticle fluctuations and extract the quasiparticle density across different temperatures, readout powers, and resonator volumes. We find that microwave power generates a higher quasiparticle density as the active resonator volume is reduced and show that this effect sets a sensitivity limit on kinetic inductance detectors. We compare our results with theoretical models of direct microwave photon absorption by quasiparticles and conclude that an unknown, indirect mechanism plays a dominant role in quasiparticle generation. These results provide a route to mitigate quasiparticle generation due to readout power in superconducting devices.

Non-Hermitian dynamics in quantum systems have unveiled novel phenomena, yet the implementation of valid non-Hermitian quantum measurement remains a challenge, because a feasible quantum projective mechanism on the complete but skewed non-Hermitian eigenstates is not explicit in experiments. This limitation hinders the direct acquisition of non-Hermitian observable statistics (e.g., non-Hermitian population dynamics), and also constrains investigations of non-Hermitian quantum measurement properties such as the uncertainty relation. Here we address these challenges by presenting a non-Hermitian projective protocol and investigating the non-Hermitian uncertainty relation. We derive the uncertainty relation for pseudo-Hermitian (PH) observables under the specific non-Hermitian metric. We then investigate the projective measurement properties for general quantum states which are projected onto the complete non-Hermitian eigenvectors, and present a quantum simulation method to apply the valid non-Hermitian projective measurement on a direct-sum dilated space. Subsequently, we experimentally construct a quantum simulator in the quantum optical circuit and realize a proof-of-principle experiment involving three-dimensional non-Hermitian quantum measurement on single-photon qutrits. Using this platform, we explore the uncertainty relation experimentally with different PH metrics. Our non-Hermitian quantum measurement method is state independent and outputs directly the non-Hermitian quantum projective statistics, paving the way for studies of extensive non-Hermitian observables in the quantum domain.