Remote entanglement of distant, non-interacting quantum entities is a key primitive for quantum information processing. We present a new protocol to remotely entangle two stationaryqubits by first entangling them with propagating ancilla qubits and then performing a joint two-qubit measurement on the ancillas. Subsequently, single-qubit measurements are performed on each of the ancillas. We describe two continuous variable implementations of the protocol using propagating microwave modes. The first implementation uses propagating Schro¨dinger cat-states as the flying ancilla qubits, a joint-photon-number-modulo-2 measurement of the propagating modes for the two-qubit measurement and homodyne detections as the final single-qubit measurements. The presence of inefficiencies in realistic quantum systems limit the success-rate of generating high fidelity Bell-states. This motivates us to propose a second continuous variable implementation, where we use quantum error correction to suppress the decoherence due to photon loss to first order. To that end, we encode the ancilla qubits in superpositions of Schr\“odinger cat states of a given photon-number-parity, use a joint-photon-number-modulo-4 measurement as the two-qubit measurement and homodyne detections as the final single-qubit measurements. We demonstrate the resilience of our quantum-error-correcting remote entanglement scheme to imperfections. Further, we describe a modification of our error-correcting scheme by incorporating additional individual photon-number-modulo-2 measurements of the ancilla modes to improve the success-rate of generating high-fidelity Bell-states. Our protocols can be straightforwardly implemented in state-of-the-art superconducting circuit-QED systems.
Concurrent remote entanglement of distant, non-interacting quantum entities is a crucial function for quantum information processing. In contrast with the existing protocols which employaddition of signals to generate entanglement between two remote qubits, the protocol we present is based on multiplication of signals. This protocol can be straightforwardly implemented by a novel Josephson junction mixing circuit. Our scheme would be able to generate provable entanglement even in presence of practical imperfections: finite quantum efficiency of detectors and undesired photon loss in current state-of-the-art devices.
While dissipation is widely considered as being harmful for quantum coherence, it can, when properly engineered, lead to the stabilization of non-trivial pure quantum states. We proposea scheme for continuous generation and stabilization of Schr\“{o}dinger cat states in a cavity using dissipation engineering. We first generate non-classical photon states with definite parity by means of a two-photon drive and dissipation, and then stabilize these transient states against single-photon decay. The single-photon stabilization is autonomous, and is implemented through a second engineered bath, which exploits the photon number dependent frequency-splitting due to Kerr interactions in the strongly dispersive regime of circuit QED. Starting with the Hamiltonian of the baths plus cavity, we derive an effective model of only the cavity photon states along with analytic expressions for relevant physical quantities, such as the stabilization rate. The deterministic generation of such cat states is one of the key ingredients in performing universal quantum computation.
We demonstrate full frequency conversion in the microwave domain using a
Josephson three-wave mixing device pumped at the difference between the
frequencies of its fundamental eigenmodes.By measuring the signal output as a
function of the intensity and phase of the three input signal, idler and pump
tones, we show that the device functions as a controllable three-wave
beam-splitter/combiner for propagating microwave modes, in accordance with
theory. Losses at the full conversion point are found to be less than 10^-2.
Potential applications of the device include quantum information transduction
and realization of an ultra-sensitive interferometer with controllable
feedback.