Hybrid Quantum Circuit Research Articles

Quantum Controlled-Not Gate Operation and Complete Bell-State Analysis Using Hybrid Quantum Circuits

Here we propose a hybrid quantum circuit for achieving the quantum controlled-not (CNOT) gate operation on a photon-spin hybrid state. The hybrid quantum circuit consists of a nitrogen-vacancy (N-V) center and microtoroidal resonator coupling system, and a single photon waveguide. We implement the complete Bell state analysis using the proposed circuit. This proposed hybrid quantum circuit could enable a high fidelity of qubit manipulation and allows the feasible with the current experimental technologies.

Quantum dynamics in the partial Wigner picture

Recently we have shown how the partial Wigner representation of quantum mechanics can be used to study hybrid quantum models where a system with a finite number of energy levels is coupled to linear or nonlinear oscillators (Beck and Sergi 2013 Phys. Lett. A 377 1047). The purpose of this work is to provide a detailed derivation of the partially Wigner-transformed quantum equations of motion for nonlinear oscillator subsystems under the action of general polynomial potentials. Such equations can be written in terms of a propagator, which can then be expanded in a power series. The linear terms of the series describe quantum-classical dynamics while the nonlinear terms provide the corrections needed to restore the fully quantum character of the evolution. In the case of polynomial potentials and position dependent couplings, the number of nonlinear terms is finite and the corrections can be calculated explicitly. In this work we show how to implement numerically the above scheme where, in principle, no assumption about the strength of the coupling must be taken. We illustrate the formalism by studying a two-level system interacting with an asymmetric quartic oscillator. We integrate the quantum dynamics of the total system and provide a comparison with the case of the quantum-classical dynamics of the quartic oscillator. The approach presented here is expected to be effective for studying hybrid quantum circuits in quantum information theory and for witnessing the quantum-to-classical transition in nano-oscillators coupled to pseudo-spins.

Hybrid quantum circuit consisting of a superconducting flux qubit coupled to a spin ensemble and a transmission-line resonator

We propose an experimentally realizable hybrid quantum circuit for achieving a strong coupling between a spin ensemble and a transmission-line resonator via a superconducting flux qubit used as a data bus. The resulting coupling can be used to transfer quantum information between the spin ensemble and the resonator. In particular, in contrast to the direct coupling without a data bus, our approach requires far less spins to achieve a strong coupling between the spin ensemble and the resonator (e.g., three to four orders of magnitude less). This proposed hybrid quantum circuit could enable a long-time quantum memory when storing information in the spin ensemble, and allows the possibility to explore nonlinear effects in the ultrastrong-coupling regime.

Hybrid quantum circuits: Superconducting circuits interacting with other quantum systems

Hybrid quantum circuits combine two or more physical systems, with the goal of harnessing the advantages and strengths of the different systems in order to better explore new phenomena and potentially bring about novel quantum technologies. This article presents a brief overview of the progress achieved so far in the field of hybrid circuits involving atoms, spins and solid-state devices (including superconducting and nanomechanical systems). How these circuits combine elements from atomic physics, quantum optics, condensed matter physics, and nanoscience is discussed, and different possible approaches for integrating various systems into a single circuit are presented. In particular, hybrid quantum circuits can be fabricated on a chip, facilitating their future scalability, which is crucial for building future quantum technologies, including quantum detectors, simulators, and computers.

Electron spin resonance detected by a superconducting qubit

A new method for detecting the magnetic resonance of electronic spins at low temperature is demonstrated. It consists in measuring the signal emitted by the spins with a superconducting qubit that acts as a single-microwave-photon detector, resulting in an enhanced sensitivity. We implement this new type of electron-spin resonance spectroscopy using a hybrid quantum circuit in which a transmon qubit is coupled to a spin ensemble consisting of NV centers in diamond. With this setup we measure the NV center absorption spectrum at 30mK at an excitation level of \thicksim15\,\mu_{B} out of an ensemble of 10^{11} spins.

Hybrid Quantum Circuit with a Superconducting Qubit Coupled to a Spin Ensemble

We report the experimental realization of a hybrid quantum circuit combining a superconducting qubit and an ensemble of electronic spins. The qubit, of the transmon type, is coherently coupled to the spin ensemble consisting of nitrogen-vacancy centers in a diamond crystal via a frequency-tunable superconducting resonator acting as a quantum bus. Using this circuit, we prepare a superposition of the qubit states that we store into collective excitations of the spin ensemble and retrieve back into the qubit later on. These results constitute a proof of concept of spin-ensemble based quantum memory for superconducting qubits.

Submicron YBaCuO biepitaxial Josephson junctions: d-wave effects and phase dynamics

We report a systematic study of the transport properties of high critical temperature superconductor (HTS) biepitaxial Josephson junctions in the submicron range. Junction performances point to more uniform and reproducible devices and to better control of d-wave intrinsic properties. Outcomes promote novel insights into the transport mechanisms across grain boundaries and encourage further developments in the control of dissipation in HTS devices. The application of nanotechnology to HTS could be an additional tool to properly engineer the junction properties to match specific circuit design also in view of the integration into hybrid quantum circuits.

Switchable coupling between charge and flux qubits

We propose a hybrid quantum circuit with both charge and flux qubits connected to a large Josephson junction that gives rise to an effective interqubit coupling controlled by the external magnetic flux. This switchable interqubit coupling can be used to transfer back and forth an arbitrary superposition state between the charge qubit and the flux qubit working at the optimal point. The proposed hybrid circuit provides a promising quantum memory because the flux qubit at the optimal point can store the tranferred quantum state for a relatively long time.

Hybrid Quantum Processors: Molecular Ensembles as Quantum Memory for Solid State Circuits

We investigate a hybrid quantum circuit where ensembles of cold polar molecules serve as long-lived quantum memories and optical interfaces for solid state quantum processors. The quantum memory realized by collective spin states (ensemble qubit) is coupled to a high-Q stripline cavity via microwave Raman processes. We show that, for convenient trap-surface distances of a few microm, strong coupling between the cavity and ensemble qubit can be achieved. We discuss basic quantum information protocols, including a swap from the cavity photon bus to the molecular quantum memory, and a deterministic two qubit gate. Finally, we investigate coherence properties of molecular ensemble quantum bits.