High-efficiency single-photon detection in the microwave domain is a key enabling technology for various quantum applications. However, the extremely low energy of microwave photons presents a fundamental challenge, preventing direct photon-to-charge conversion as achieved in optical systems using semiconductors. Here, we demonstrate continuous microwave photon detection with an efficiency approaching 70% in the single-photon regime. We use a hybrid system comprising a gate-defined double quantum dot (DQD) charge qubit in a gallium arsenide/aluminum gallium arsenide heterostructure, coupled to a high-impedance Josephson junction array cavity. We systematically optimize the hybrid architecture to maximize the detection efficiency by leveraging strong charge-photon coupling, tunable DQD tunnel rates, and the frequency tunability of both subsystems. The system efficiency is characterized over a frequency range of 3 to 5.2 gigahertz. Our results establish semiconductor-based cavity–quantum electrodynamics architectures as a scalable and versatile platform for efficient microwave photon detection, opening promising avenues for quantum microwave optics and quantum information technologies.

Electrostatically tunable quantum confinement in nanoscale systems offers unprecedented opportunities for manipulating artificial quantum matter, positioning these platforms at the frontier of quantum science. The strategic integration of distinct confinement mechanisms could revolutionize quantum functionality by enabling novel states with enhanced coherence properties. Here, we demonstrate inherently interfacial potential engineering by combining lateral semiconductor 2D moiré potentials with vertical quantum confinement effects in semimetal bismuth nanofilms, creating localized periodic sites for quantum-confined charges. Using scanning tunneling microscopy, we observe moiré-mediated Wigner crystals with self-organized electron lattices arising from strong Coulomb interactions, exhibiting unexplored multiple energy quantization behaviors that can be manipulated by quantum well states in ultrathin bismuth films. These precisely localizable charge states provide a promising platform for van der Waals (vdW) charge qubits electrostatically confined within 2D materials. Our work demonstrates extended tunability of artificial atom states in phase, space, and energy regimes. With optimized designs, these 2D vdW architectures bridge fundamental Wigner crystallization phenomena with practical applications in advanced electronic systems and quantum state manipulation.

Cavity quantum electrodynamics (cQED) provides strong light-matter interactions that can be used for manipulating and detecting quantum states. The interaction can be enhanced by increasing the resonator's impedance, while approaching the quantum impedance (h/e2) remains challenging. Edge plasmons emergent as chiral bosonic modes in the quantum Hall channels provide high quantized impedance of h/νe2 that can exceed 10 kΩ for the Landau-level filling factor ν 2, well beyond the impedance of free space. Here, we apply such a high-impedance plasmon mode in a quantum-Hall plasmon resonator to demonstrate dispersive detection of a nearby charge qubit formed in a double quantum dot. The phase shift in microwave transmission through the plasmon resonator follows the dispersive shift associated with the qubit state, in agreement with the cQED theory. The high impedance allows us to perform dispersive detection of qubit spectroscopy with a plasmon resonator having a broad bandwidth. Leveraging these topological edge modes, our results establish two-dimensional topological insulators as a new platform of cQED.

Abstract Effect of statistical deviations of geometric sizes on operating parameters of an optically controlled semiconductor quantum dot (QD) charge qubit is studied. A numerical analysis of averaged spectra of a double QD and a photonic crystal microresonator (MR) is carried out in the presence of technological imperfections of their parameters. An estimate for correction electric field amplitudes that compensate for deviations of the real qubit parameters from the calculated ones is obtained. The possibility of correcting the optical spectrum of the MR by changing its temperature is shown.

Time fractional evolution of two superconducting charge qubits

Semiconductor-based spin qubits embedded into a superconducting microwave cavity constitute a fast-progressing and promising platform for realizing fast and fault-tolerant qubit control with long-range two-qubit coupling. The flopping-mode spin qubit consists of a single electron in a double quantum dot; it combines a charge qubit with a spin qubit. With its strong and tunable cavity coupling, the flopping-mode qubit is proven to be well suited for low-power qubit control and cavity-mediated long-range quantum gates. The singlet-triplet (ST) and exchange-only (EO) qubits are multielectron realizations that go without broadband control and are protected from some types of noise, but are challenging to couple to each other and to microwave cavities. We combine the flopping-mode concept with the ST and EO qubits and propose two new flopping-mode qubits that consist of three (four) quantum dots, occupied by two (three) electrons near the (1,0,1)↔(0,1,1)[(1,0,1,1)↔(0,1,1,1)] charge transition. The two-electron system augments the ST0 spin qubit with a charge qubit that interacts transversally and longitudinally with a cavity. Both couplings are highly tunable, and the longitudinal coupling distinguishes the flopping-mode ST qubit from the regular flopping-mode qubit. The longitudinal coupling allows for nondissipative universal control similar to superconducting transmon qubits. The EO flopping-mode qubit comprises four dots occupied by three electrons and opens a new possibility to perform two-qubit gates for EO qubits that are challenging to perform directly with the exchange coupling. We use input-output theory to provide means of extracting the coupling strengths from cavity transmission data.

Abstract Surface acoustic wave (SAW) resonators offer distinct advantages for coupling to semiconductor qubits, including low loss, high stability, and compatibility with magnetic fields. However, the integration of SAW resonators with double quantum dots (DQDs) which host charge and spin qubits remains largely unexplored. In this work, we propose a flip-chip architecture that enables three-dimensional integration of a semiconductor DQD with a SAW resonator. Taking experimental feasibility into account, we estimate the coupling strength between a DQD and a SAW resonator. The results suggest that strong coupling regime can be reached in our design. This study provides theoretical insight and practical guidance for experimental exploration of phonon-electron coupling in hybrid SAW-DQD quantum systems.

Intrinsic Decoherence-Induced Generation of Quantum Information Resources in Coupled Semiconductor Charge Qubits

In this paper we investigate the thermal quantum correlations in a semiconductor double-quantum-dot system. The device comprises a single electron in a double quantum dot subjected to a longitudinal-magnetic-field and a transverse-magnetic-field gradient. The thermal entanglement of the single electron is driven by the charge and spin qubits. Employing the density-matrix formalism, we derive analytical expressions for thermal concurrence and correlated coherence, providing a comprehensive analysis of how temperature and various system parameters influence quantum coherence. Our results demonstrate that the transverse magnetic field serves as a tunable parameter for controlling thermal entanglement and quantum coherence. We also highlight the roles of thermal entanglement and correlated coherence in generating quantum correlations, noting that thermal correlated coherence is consistently more robust than thermal entanglement. This suggests that quantum algorithms based solely on correlated coherence might be more resilient than those relying on entanglement.

Electrons trapped above the surface of helium provide a means to study many-body physics free from the randomness that comes from defects in other condensed-matter systems. Localizing an electron in an electrostatic quantum dot makes its energy spectrum discrete, with controlled level spacing. The lowest two states can act as charge qubit states. In this paper, we study how the coupling to the quantum field of capillary waves on helium-known as ripplons-affects electron dynamics. As we show, the coupling can be strong. This bounds the parameter range where electron-based charge qubits can be implemented. The constraint is different from the conventional relaxation time constraint. The electron-ripplon system in a dot is similar to a color center formed by an electron defect coupled to phonons in a solid. In contrast to solids, the coupling in the electron on helium system can be varied from strong to weak. This enables a qualitatively new approach to studying color center physics. We analyze the spectroscopy of the pertinent synthetic color centers in a broad range of the coupling strength.

We suggest a nanoelectromechanical setup for designing a quantum-nanomechanical motion of a charge qubit in two-dimensional space. The quantum character of such motion comes through the fact that states of the qubit are entangled with nanomechanical cat states. The setup is based on mesoscopic terminal utilizing the ac Josephson effect between voltage-biased superconducting electrodes and mesoscopic superconducting grain—the charge qubit, vibrating in plane fixed on a nanomechanical cantilever. Required functionality is achieved by operating two external parameters: bias voltage between superconducting electrodes and controlling voltage applied over the gate electrodes. While the resonant tunneling of Cooper pairs between the superconductors and charge qubit builds nanomechanical coherent states comprising the cat state, gate voltage has a specific role of tuning their two-dimensional character in the form of the in-plane linear motion under the voltage-controlled direction. Controlling the spatial motion of a charge qubit opens a possibility of quantum communication between different terminals located in space. Directional control of nanomechanical motion entangled with charge qubit states we call the “quantum switch”.

Abstract Quantum operations on a charge qubit, which is represented by a single-electron double quantum dot (QD) with structure asymmetry, are considered. It is shown that with the help of an electric field created by voltage gates the tunnel coupling of the QD excited states is possible to control. An algorithm for performing single-qubit rotations based on a sequence of tunnel and optical transitions is described. The dependences of the correction electric field on geometric parameters of the gates are calculated.

In this article, we introduce a simple variational model describing the ground state of a superconducting charge qubit. The model gives rise to a shape optimization problem that aims at maximizing the number of qubit states at a given gating voltage. We show that for small values of the charge, optimal shapes exist and are C 2, α -nearly spherical sets. In contrast, we prove that balls are not minimizers for large values of the charge and conjecture that optimal shapes do not exist, with the energy favoring disjoint collections of sets.

We present a detailed mathematical description, both an analytical model and a numerical simulation, of a physical system based on a superconducting nanoelectromechanical setup that generates nanomechanical cat-states entangled with charge qubit states. The system consists of a superconducting grain in a regime of the Cooper pair box (the charge qubit) that performs mechanical vibrations between two bulk superconductors. Operation of the device is based on the AC Josephson effect, i.e., the phase difference between superconducting electrodes is controlled by a DC bias voltage following the operational switch on/off protocol. We compare an analytical idealised solution with numerical simulation using experimentally feasible parameters, different decoherence processes, as well as imperfections of experimental procedures such as time-control of the bias voltage, to get insight into how they influence the time-evolution of the realistic system, deteriorate the quantum coherence, and affect the formation of the cat-states.

We propose a superconducting circuit that supports the propagation of Korteweg--de Vries (KdV)-type voltage solitons. By employing Cooper-pair boxes as nonlinear capacitors, we realize a dual system to the previously reported current solitons based on nonlinear inductors. The proposed voltage solitons manifest as propagating capacitors, carrying a velocity-dependent induced charge. This unique characteristic facilitates local electrostatic interactions, which are dual to the magnetic interactions mediated by fluxons.

Investigating quantum characteristics in a solid-state system is a crucial focus for advanced exploration. Within this scenario, double quantum dots emerge as a flexible and promising framework with the capability of bringing about significant progress in the fields of quantum computation and nanotechnology. The study presented in this work conducts a comprehensive examination of the capabilities embedded in two charge qubits residing within quantum dots, particularly focusing on their ability to generate essential quantum information resources within a thermal environment. The investigation delves into the assessment of local quantum Fisher information at the individual qubit level, local quantum uncertainty, and the quantification of entanglement through negativity. Through this meticulous exploration of quantum characteristics, the study aims to provide valuable insights into the complex interplay of thermal dynamics and quantum correlations inherent in the charge qubits within the semiconductor framework. A noteworthy revelation from our study is the discernible dependence of non-classical resources on intrinsic parameters, such as Coulomb interaction parameters, as well as extrinsic factors, including temperature. This highlights the nuanced influence of both intrinsic and extrinsic variables on the quantum properties, opening avenues for a more profound comprehension of the complex interactions that mold the quantum landscape in solid-state systems.

We derive a mesoscopic theory of the Josephson junction from nonrelativistic scalar electrodynamics. Our theory reproduces the Josephson relations with the canonical current phase relation acquiring a weak second harmonic term, and it improves the standard lumped-element descriptions employed in circuit quantum electrodynamics by providing spatial resolution of the superconducting order parameter and electromagnetic field. By providing an ab initio derivation of the charge qubit Hamiltonian that relates traditionally free qubit parameters to geometric and material properties, we progress toward the quantum engineering of superconducting circuits at the subnanometer scale.

The transmon, which has a short gate time and remarkable scalability, is the most commonly utilized superconducting qubit, based on the Cooper pair box as a qubit or coupler in superconducting quantum computers. Lattice and heavy-hexagon structures are well-known large-scale configurations for transmon-based quantum computers that classical computers cannot simulate. These structures share a common feature: a resonator coupler that connects two transmon qubits. Although significant progress has been made in implementing quantum error correction and quantum computing using quantum error mitigation, fault-tolerant quantum computing remains unachieved due to the inherent vulnerability of these structures. This raises the question of whether the transmon-resonator-transmon structure is the best option for constructing a transmon-based quantum computer. To address this, we demonstrate that the average fidelity of CNOT gates can exceed 0.98 in a structure where a resonator coupler mediates the coupling of three transmon qubits. This result suggests that our novel structure could be a key method for increasing the number of connections among qubits while preserving gate performance in a transmon-based quantum computer.

We propose a superconducting nanomechanical device that can actuate and probe an oscillating flying qubit facilitating quantum information transfer over a distance. The flying qubit is formed by a movable Cooper-pair box (CPB) consisting of a superconducting dot and a bulk superconductor, which are entangled by lifting the Coulomb blockade of Cooper-pair tunneling electrostatically. We show that flying qubit states formed on a movable CPB can be observed in electron transport to a normal electrode via Andreev reflections. The charge transfer due to periodic mechanical motion of CPB leads to the occurrence of a nonzero current at zero bias voltage, and its coherence can be identified through the oscillatory current dependence of the current on a gate voltage. The proposed device provides a building block for dissipationless quantum information circuits.

In this study, we provide a full experimental characterization of the parametric longitudinal coupling between a complementary metal oxide semiconductor (CMOS) charge qubit and an off-chip rf resonator. Following Corrigan et al., Phys. Rev. Appl. 20, 064005 (2023), we activate parametric longitudinal coupling by driving the charge qubit at the resonator frequency. Managing the crosstalk between the drive applied to the qubit and the resonator allows for the systematic study of the dependence of the longitudinal and dispersive charge-photon couplings on the qubit-resonator detuning and the applied drive. Our experimental estimations of the charge-photon couplings are perfectly reproduced by theoretical simple formulas, without relying on any fitting parameter. We go further by showing a parametric displacement of the resonator's steady state, conditional on the qubit state, and the insensitivity of the longitudinal coupling constant to the photon population of the resonator. Our results pave the way for the exploration of the photon-mediated longitudinal readout and coupling of multiple and distant spins, with long coherent times, in hybrid CMOS circuit quantum electrodynamics architectures.