Communication over proven-secure quantum channels is potentially one of the most wide-ranging applications of currently developed quantum technologies. It is generally envisioned thatin future quantum networks, separated nodes containing stationary solid-state or atomic qubits are connected via the exchange of optical photons over large distances. In this work we explore an intriguing alternative for quantum communication via all-microwave networks. To make this possible, we describe a general protocol for sending quantum states through thermal channels, even when the number of thermal photons in the channel is much larger than one. The protocol can be implemented with state-of-the-art superconducting circuits and enables the transfer of quantum states over distances of ~100 m via microwave transmission lines cooled to only T=4K. This opens up completely new possibilities for quantum communication within and across buildings, and consequently, for the implementation of intra-city quantum networks based on microwave technology only.
We study effective light-matter interactions in a circuit QED system consisting of a single LC resonator, which is coupled symmetrically to multiple superconducting qubits. Startingfrom a minimal circuit model, we demonstrate that in addition to the usual collective qubit-photon coupling the resulting Hamiltonian contains direct qubit-qubit interactions, which prevent the otherwise expected superradiant phase transition in the ground state of this system. Moreover, these qubit-qubit interactions are responsible for an opposite mechanism, which at very strong couplings completely decouples the photon mode and projects the qubits into a highly entangled ground state. These findings shed new light on the controversy over the existence of superradiant phase transitions in cavity and circuit QED systems, and show that the physics of ultrastrong light-matter interactions in two- or multi-qubit settings differ drastically from the more familiar one qubit case.
We propose a spin-orbit qubit in a nanowire quantum dot on the surface of a multiferroic insulator with a cycloidal spiral magnetic order. The spiral exchange field from the multiferroicinsulator causes inhomogeneous Zeeman-like interaction on the electron spin in the quantum dot, assisting the realization of a spin-orbit qubit. The absence of an external magnetic field benefits the integration of such spin-orbit qubit into high-quality superconducting resonators for manipulation. By exploiting the Rashba spin-orbit coupling in the quantum dot via a gate voltage, one can obtain an effective spin-photon coupling with an efficient on/off switching. This makes the proposed device promising for hybrid quantum communications.
We propose how to realize high-fidelity quantum storage using a hybrid
quantum architecture including two coupled flux qubits and a nitrogen-vacancy
center ensemble (NVE). One of theflux qubits is considered as the quantum
computing processor and the NVE serves as the quantum memory. By separating the
computing and memory units, the influence of the quantum computing process on
the quantum memory can be effectively eliminated, and hence the quantum storage
of an arbitrary quantum state of the computing qubit could be achieved with
high fidelity. Furthermore the present proposal is robust with respect to
fluctuations of the system parameters, and it is experimentally feasibile with
currently available technology.
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 qubitused as a data bus. The resulting coupling can be
used to transfer quantum information between the spin ensemble and the
resonator. More importantly, 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., 3 to 4 orders of magnitude less).
This drastic reduction of the number of spins in the ensemble can greatly
improve the quantum coherence of the spin ensemble. This proposed hybrid
quantum circuit could enable a long-time quantum memory when storing
information in the spin ensemble.
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 phenomenaand 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). We discuss how
these circuits combine elements from atomic physics, quantum optics, condensed
matter physics, and nanoscience, and we present different possible approaches
for integrating various systems into a single circuit. 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.