Multi-particle sources constitute an interesting new paradigm following the recent development of on-demand single-electron sources. Versatile devices can be designed using several single-electron sources, possibly of different types, coupled to the same quantum circuit. However, if combined non-locally to avoid cross-talk, the resulting architecture becomes very sensitive to electronic decoherence. To circumvent this problem, we here analyse two-particle sources that operate with several single-electron (or hole) emitters attached in series to the same electronic waveguide. We demonstrate how such a device can emit exactly two electrons without exciting unwanted electron-hole pairs if the driving is adiabatic. Going beyond the adiabatic regime, perfect two-electron emission can be achieved by driving two quantum dot levels across the Fermi level of the external reservoir. If a single-electron source is combined with a source of holes, the emitted particles can annihilate each other in a process which is governed by the overlap of their wave functions. Importantly, the degree of annihilation can be controlled by tuning the emission times, and the overlap can be determined by measuring the shot noise after a beam splitter. In contrast to a Hong-Ou-Mandel experiment, the wave functions overlap close to the emitters and not after propagating to the beam splitter, making the shot noise reduction less susceptible to electronic decoherence. a

The development of single-electron sources is paving the way for a novel type of experiment in which individual electrons are emitted into a quantum-coherent circuit. However, to facilitate further progress toward fully coherent on-chip experiments with electrons, a detailed understanding of the quantum circuits is needed. Here, it is proposed to perform time-domain spectroscopy of mesoscopic conductors by applying Lorentzian-shaped voltage pulses to an input contact. Specifically, it is shown how characteristic timescales of a quantum-coherent conductor can be extracted from the distribution of waiting times between charge pulses propagating through the circuit. To illustrate the idea, Floquet scattering theory is employed to evaluate the electron waiting times for an electronic Fabry-Pérot cavity and a Mach-Zehnder interferometer. The perspectives for an experimental realization of the proposal are discussed and possible avenues for further developments are identified.A central goal in quantum technology is to develop nanoscale circuits in which single charges are emitted on demand and manipulated coherently using beam splitters and interferometers as in quantum optics experiments with photons. [1][2][3] Advances in circuit design with edge states based on the quantum Hall effect [4,5] and, recently, the quantum spin Hall effect, [6] make these systems an excellent platform for such electronic solid-state experiments. However, to facilitate further progress toward fully quantum-coherent experiments with electrons, a detailed characterization of the on-chip circuits is required. As a first important step, the development of single-electron sources has enabled the controlled emission of individual electrons. [7][8][9][10][11][12][13][14][15][16] In one type of emitter, periodic voltage pulses are applied to an ohmic contact. [15,16] If the emitted charges are transmitted through a quantum point contact (QPC), the average current and its fluctuations can be directly related to the difference and

The development of dynamic single-electron sources has made it possible to observe and manipulate the quantum properties of individual charge carriers in mesoscopic circuits. Here, we investigate multi-particle effects in an electronic Mach–Zehnder interferometer driven by a series of voltage pulses. To this end, we employ a Floquet scattering formalism to evaluate the interference current and the visibility in the outputs of the interferometer. An injected multi-particle state can be described by its first-order correlation function, which we decompose into a sum of elementary correlation functions that each represent a single particle. Each particle in the pulse contributes independently to the interference current, while the visibility (given by the maximal interference current) exhibits a Fraunhofer-like diffraction pattern caused by the multi-particle interference between different particles in the pulse. For a sequence of multi-particle pulses, the visibility resembles the diffraction pattern from a grid, with the role of the grid and the spacing between the slits being played by the pulses and the time delay between them. Our findings may be observed in future experiments by injecting multi-particle pulses into a Mach–Zehnder interferometer.

A train of voltage pulses can be used for time‐domain spectroscopy of quantum‐coherent structures, as shown by Christian Flindt and co‐workers in article number 1900014. When single‐electron excitations are periodically injected into a mesoscopic circuit, the distribution of waiting times between the outgoing charges contains detailed information about the characteristic timescales of the device. The development of single‐particle emitters and detectors suggests that the proposal may soon be within reach.

Superconducting qubits 1, 2 are one of the most promising candidates to implement quantum computers, with projected applications in physics simulations 3 , optimization 4 , machine learning 5 , and chemistry 6 . The superiority of superconducting quantum computers over any classical device in simulating random but well-determined quantum circuits has already been shown in two independent experiments 7, 8 and important steps have been taken in quantum error correction 9-11 . However, the currently wide-spread qubit designs [12][13][14][15][16] do not yet provide high enough performance to enable practical applications or efficient scaling of logical qubits owing to one or several following issues: sensitivity to charge or flux noise leading to decoherence, too weak non-linearity preventing fast operations, undesirably dense excitation spectrum, or complicated design vulnerable to parasitic capacitance. Here, we introduce and demonstrate a superconducting-qubit type, the unimon, which combines the desired properties of high non-linearity, full insensitivity to dc charge noise, insensitivity to flux noise, and a simple structure consisting only of a single Josephson junction in a resonator. We measure the qubit frequency, ω 01 /(2π), and anharmonicity α over the full dc-flux range and observe, in agreement with our quantum models, that the qubit anharmonicity is greatly enhanced at the optimal operation point, yielding, for example, 99.9% and 99.8% fidelity for 13-ns single-qubit gates on two qubits with (ω 01 , α) = (4.49 GHz, 434 MHz) × 2π and (3.55 GHz, 744 MHz) × 2π, respectively. The energy relaxation time T 1 10 µs is stable for hours and seems to be limited by dielectric losses. Thus, future improvements of the design, materials, and gate time may promote the unimon to break the 99.99% fidelity target for efficient quantum error correction and possible quantum advantage with noisy systems.