We generalize an approximation of eigenvalues associated with projection onto a rectangular patch of phase space so that it applies to patches with curved boundaries. This more general case is relevant to characterizing degrees of freedom and communication rates in multiple-input–multiple-output arrays and allows previous results derived under far-field assumptions to be applied to near-field scenarios. The underlying approximation is found by our relating these eigenvalues to tunneling probabilities in an associated quantum-mechanical Hamiltonian, which is generalized here to cope with deformed geometries by a semiclassical transformation of the problem to the traditionally studied rectangular one.

Superresolution imaging can be achieved through the exploration and implementation of optimal measurements that attain the fundamental quantum bound. To this end, degenerate optical-mode demultiplexing is proposed as a quantum optimal measurement for resolving two incoherent optical point sources in two-dimensional space. By projecting photons into degenerate subspaces of spatial modes, quantum-limited resolution is attained using quadratically fewer channels, thereby reducing the measurement complexity while preserving the quantum optimality. Universal quantum circuits based on Gouy phase interferometry are introduced to facilitate this degenerate projection. By employing fractional Fourier transform in a multipath interferometer, optical modes with distinct Gouy phases can be efficiently deviated, approaching the optimal resolution over longer distances. This scheme streamlines the implementation of quantum optimal measurement, offering a practical pathway for superresolution imaging across diverse applications.

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.

Rayleigh’s criterion states that resolving point sources below the width of the point spread function is impossible, with the error increasing at shorter distances. This is known as Rayleigh’s curse. While detection mode shaping solves this for equal sources, it fails for unbalanced sources with unknown brightness ratios. We propose BLESS, a technique using Beam moduLation and Examination of Shot Statistics, breaking Rayleigh’s curse for unbalanced sources. Classical and quantum Cramér-Rao bound calculations show the strong potential of BLESS for real imaging experiments.

On the path toward large-scale quantum information processing, the ability to implement diverse types of two-qubit gates is preferable for device structure designs and quantum circuit compilations. Here, taking advantage of the inherent Heisenberg interaction between spin qubits, we propose and verify a fast composite two-qubit gate scheme for spin qubits based on semiconductor quantum dots. Theoretical results indicate that the gate scheme facilitates the simultaneous implementation of various types of two-qubit gates, including five types of essential two-qubit gates, iswap-family gates, and fermionic simulation gates. Furthermore, we observe an excellent match between the measured and simulated results, representing the preliminary experimental demonstration of the composite gate scheme. We believe that, with this versatile composite two-qubit gate scheme, the available broad-spectrum two-qubit operations will allow us to efficiently utilize the hardware resources and accelerate the advancement of noisy intermediate-scale quantum computing.

Quantum state tomography of weak microwave signals is an important part of many protocols in the field of quantum information processing with superconducting circuits. This step typically relies on an accurate in-situ estimation of signal losses in the experimental setup and requires a careful photon-number calibration. Here, we present an improved method for the microwave loss estimation inside of a closed cryogenic system. Our approach is based on Planck’s law and makes use of independent temperature sweeps of individual parts of the cryogenic setup. Using this technique, we can experimentally resolve changes in microwave losses of less than 0.1 dB in the cryogenic environment. We discuss potential applications of this approach for the precise characterization of quantum limited superconducting amplifiers and in other prominent experimental settings.

We present the design and experimental characterization of a kinetic inductance traveling-wave parametric amplifier (KI-TWPA) for subgigahertz frequencies. KI-TWPAs amplify signals through mixing processes supported by the nonlinear kinetic inductance of a superconducting transmission line. The device described here uses a compactly meandered $\mathrm{Ti}\mathrm{N}$ microstrip transmission line to achieve the length needed to amplify subgigahertz signals. It is operated in a frequency-translating mode where the amplified signal tone is terminated at the output of the amplifier, and the idler tone at approximately 2.5 GHz is brought out of the cryostat. By varying the pump frequency, a gain of up to 22 dB was achieved in a tunable range from about 450 to 850 MHz. Using $\mathrm{Ti}\mathrm{N}$ as the nonlinear element allows for a reduction of the required pump power by roughly an order of magnitude relative to $\mathrm{Nb}\mathrm{Ti}\mathrm{N}$, which has been used for previous KI-TWPA implementations. This amplifier has the potential to enable high-sensitivity and high-speed measurements in a wide range of applications, such as quantum computing, astrophysics, and dark matter detection.

Nonreciprocal currents arise in a broad range of systems, from magnons and phonons to supercurrents, due to an interplay between spatial and temporal symmetry breakings. These find applications in devices, such as circulators and rectifiers, as well as in probing the interactions and states that underlie the nonreciprocity. An established symmetry argument anticipates the emergence of nonreciprocal currents along a direction perpendicular to the applied magnetic field that breaks the time-reversal symmetry. Here, motivated by recent experiments, we examine the emergence of nonreciprocity in vortex-limited superconducting critical currents along an applied magnetic field. Employing London’s equations for describing the Meissner response of a superconducting film, we find that an additional symmetry breaking due to a preferred vortex axis enables nonreciprocal critical currents along the applied magnetic field, consistent with the so far unexplained experimental observation. Building on our concrete theoretical model for supercurrents, we discuss a possible generalization of the prevailing symmetry consideration to encompass nonreciprocal currents along the time-reversal symmetry-breaking direction.

We propose how to achieve a backscattering-immune quantum system containing a cavity with defects. We find that, by placing a nanoparticle in the evanescent field of the whispering-gallery-mode resonator with proper relative position and size, the impact of backscattering induced by the defect can be almost eliminated completely. This makes it possible to realize backscattering immunity for both quantum entanglement and classical information transmission. We also show that both the degree of optomechanical entanglement and the transmissivity of the resonator can be manipulated by tuning the relative position of the nanoparticle. This work provides insights into the protection and manipulation of quantum and classical resources, which is meaningful for building noise-tolerant optomechanical quantum processors, achieving ideal quantum precision measurements and backscattering-immune integrated quantum photonic circuits.

Multicolored optical switching phenomena are crucial for a broad range of applications, such as telecommunications and optical computing. However, most materials typically exhibit single-colored optical nonlinearity under intense laser illumination. Herein, our femtosecond time-resolved transient transmission measurements reveal the coexistence of the transient Pauli blocking effect in both the $\mathrm{\Gamma }$ and $L$ valleys following intense optical excitation in $\mathrm{Ge}$ films, demonstrating their feasibility as multicolored optical switching materials based on multivalley scattering. Additionally, a split-off energy of 240 meV at the $L$ point, attributed to the spin-orbit coupling effect, is directly observed from time-resolved transient transmission spectra. Based on an accurately calculated band structure, a scheme has been developed to interpret pump-probe transient transmission traces, effectively avoiding overinterpretation of overlapping transient signals caused by multiple-transition pathways. Furthermore, analysis of transmission transients using a three-level model indicates that the intervalley scattering time between the $\mathrm{\Gamma }$ and $L$ valleys is within 196 fs, and the intravalley scattering time is within 1.10 ps in the $\mathrm{\Gamma }$ valley, highlighting the potential for achieving an ultrafast multicolored optical switching device for key applications in multiband communications and optical computing.

For superconducting quantum processors, stable high-fidelity two-qubit operations depend on precise flux control of the tunable coupler. However, the pulse distortion poses a significant challenge to the control precision. Current calibration methods, which often rely on microwave crosstalk or additional readout resonators for coupler excitation and readout, tend to be cumbersome and inefficient, especially when couplers only have flux control. Here, we introduce and experimentally validate a pulse calibration scheme that exploits the strong coupling between qubits and couplers, eliminating the need for extra coupler readout and excitation. Our method directly measures the short-time and long-time step responses of the coupler flux pulse transient, enabling us to apply predistortion to subsequent signals using fast Fourier transformation and deconvolution. This approach not only simplifies the calibration process, but also significantly improves the precision and stability of the flux control. We demonstrate the efficacy of our method through the implementation of diabatic controlled-$Z$ (cz) and $i$swap gates with fidelities of $99.61\mathrm{\%}\pm 0.04\mathrm{\%}$ and $99.82\mathrm{\%}\pm 0.02\mathrm{\%}$, respectively, as well as a series of diabatic controlled-phase gates with high fidelities characterized by cross-entropy benchmarking. The consistency and robustness of our technique are further validated by the reduction in pulse distortion and phase error observed across multilayer cz gates. These results underscore the potential of our calibration and predistortion method to enhance the performance of two-qubit gates in superconducting quantum processors.

A cylindrical ${\mathrm{TM}}_{0,1,0}$ mode microwave cavity resonator was excited using a balanced interferometric configuration that allowed manipulation of the electric field and potential within the resonator by adjusting the phase and amplitude of the interferometer arms driving the resonator. With precise tuning of the phase and amplitude, 25-dB suppression of the electric field at the resonance frequency was achieved while simultaneously resonantly enhancing the time-varying electric-scalar potential. Under these conditions, the system demonstrated electromagnetically induced absorption in the cavity response due to the annulment of the electric field at the resonance frequency. This phenomena can be regarded as a form of extreme dispersion, and led to a sharp increase in the cavity phase versus frequency response by an order of magnitude when compared to the cavity $Q$ factor. This work presents an experimental setup that will allow the electric-scalar Aharonov-Bohm effect to be tested under conditions involving a time-varying electric-scalar potential, without the presence of an electric field or magnetic vector potential, an experiment that has not yet been realized.

The heavy holes in $\mathrm{Ge}$/$\mathrm{Ge}\text{−}\mathrm{Si}$ heterostructures show highly anisotropic gyromagnetic response with in-plane $g$ factors $g_{x,y}^{\ast }\lesssim 0.3$ and out-of-plane $g$ factor $g_z^{\ast }\gtrsim 10$. As a consequence, Rabi hot spots and dephasing sweet lines are extremely sharp and call for a careful alignment of the magnetic field in $\mathrm{Ge}$ spin-qubit devices. We investigate how the $g$ factors can be engineered by strains. We show that uniaxial strains can raise in-plane $g$ factors above unity while leaving $g_z^{\ast }$ essentially constant. We discuss how the etching of an elongated mesa in a strained buffer can actually induce uniaxial (but inhomogeneous) strains in the heterostructure. This broadens the operational magnetic field range and enables spin manipulation by shuttling holes between neighboring dots with different $g$ factors.

Both orbital angular momentum and radial momentum describe the transverse momentum of a light field. Efficient discrimination between and sorting of the two kinds of momentum lie at the heart of further applications. Here, we propose a parabolalike lens that can transform the orbital angular momentum and radial momentum into different positions in the parabolas. We experimentally characterize the performance of our implementation by separating individual angular and radial momenta as well as multiple superposition states. The reported scheme can achieve two kinds of transverse momentum identification and thus can provide a possible way to complete the characterization of the full transverse momentum of an optical field. The proposed device can readily be used in multiplexing and demultiplexing of optical information, and, in principle, achieve unit efficiency, and thus may be suitable for applications that involve quantum states of light.

Designing quantum systems with the measurement speed and accuracy needed for quantum error correction using superconducting qubits requires iterative design and testing informed by accurate models and characterization tools. We introduce a single protocol, with few prerequisite calibrations, to measure the dispersive shift, resonator linewidth, and drive power used in the dispersive readout of superconducting qubits. We find that the resonator linewidth is poorly controlled, with a factor of 2 variation between the maximum and minimum measured values, and this variation is likely to require focused attention in future quantum error correction experiments. We also introduce a protocol for measuring the efficiency of the readout system using the same power levels as are used in typical qubit readout, and without the need to measure the qubit dephasing. We routinely run these protocols on chips with tens of qubits to a hundred qubits, driven by automation software with little human interaction. Using the extracted system parameters, we find that a model based on these parameters predicts the readout signal-to-noise ratio to within 10% over a device with 54 qubits.

Low-phase-noise microwave-frequency integrated oscillators provide compact solutions for various applications in signal processing, communications, and sensing. Surface acoustic waves (SAWs), featuring orders-of-magnitude shorter wavelength than electromagnetic waves at the same frequency, enable integrated microwave-frequency systems with much smaller footprint on chip. SAW devices also allow higher-quality (Q) factors than electronic components at room temperature. Here, we demonstrate a low-phase-noise gigahertz-frequency SAW oscillator on 128°Y-cut lithium niobate, where the SAW resonator occupies a footprint of $0.05{\mathrm{mm}}^2$. Leveraging phononic crystal band-gap-edge modes to balance between Q factors and insertion losses, our 1-GHz SAW oscillator features a low phase noise of −132.5 dBc/Hz at a 10-kHz offset frequency and an overlapping Hadamard deviation of 6.5 × ${10}^{-10}$ at an analysis time of 64 ms. The SAW resonator-based oscillator holds high potential in developing low-noise sensors and acousto-optic integrated circuits.

Traumatic events can initiate cavitation within the cerebrospinal fluid surrounding the human brain. While the brain is soft [Young’s modulus $E\sim O(1)$ kPa], a thin membrane named cranial pia mater [$E\sim O(1)$ MPa] separates the cerebral cortex from the liquid. Here, we reveal the effect of this composite boundary on the damage potential of cavitation through well-controlled experiments. Therefore, a soft hydrogel is covered by a thin biomembrane and is exposed to cavitation bubbles created at various distances with a pulsed laser. High-speed imaging reveals a perforation of the membrane and penetration of a liquid jet deep into the composite structure from a single event, and its strong dependence on the stand-off distance of the bubble to the boundary. When the stand-off distance is around 0.65, needlelike liquid jets in excess of $1000$ m/s can be formed that pass through the bubble without atomization. The later visible shock-wave emission is related to the phenomenon of shock-wave self-focusing.