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arXiv:1802.05741 · Scientific Reports

Implementation of an efficient linear-optical quantum router

Karol Bartkiewicz, Antonín Černoch, Karel Lemr

Published Sep 3, 2018• 17 citations

Abstract

For several decades, scientists have been aware of significant benefits allowing quantum information processing technologies to surpass their classical counterparts. Recent technological development allows these benefits to be tested experimentally and in some cases also implemented in practical devices. So far the majority of experimental quantum networks was limited to peer-to-peer communications between two parties. Practical implementation of quantum communications networks, however, needs to address the problem of scalability to serve large numbers of users. Similarly to classical computer networks, their quantum counterparts would require routing protocols to direct the signal from its source to destination. Devices implementing these routing protocols are called quantum routers and have recently been subject of an intense research. In this paper, we report on experimental implementation of a linear-optical quantum router. Our device allows single-photon polarization-encoded qubits to be routed coherently into two spatial output modes depending on the state of two identical control qubits. The polarization qubit state of the routed photon is maintained during the routing operation. The success probability of our scheme can be increased up to 25% making it the most efficient linear-optical quantum router developed to this date.

DOI: 10.1038/s41598-018-31273-0

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An all-linear-optical scheme for a fully featured quantum router is presented. This device directs the signal photonic qubit according to the state of one control photonic qubit. In the introduction we formulate the list of requirements imposed on a fully quantum router. Then we describe our proposal, showing the exact principle of operation on a linear-optical scheme. Subsequently we provide a generalization of the scheme in order to optimize the success probability by means of a tunable controlled-phase gate. Finally, we show how one can modify the device to route multiple signal qubits using the same control qubit.

Inspired by teleportation, we find that the GHZ state can be used to realize the quantum routing. This paper presents a scheme for optical implementation of a quantum router. The device directs the signal photonic qubit according to the control information. First we describe our proposal, showing the exact principle of operation on a optical scheme, the polarization state of the signal photon is undisturbed by control information under the routing operation, and the success probability can nearly reach up to 1/2. Subsequently, we discuss generalization of the scheme with multiple output ports, showing how to route signal qubit to any of the outputs.

The router is a key element for a network. We describe a scheme to realize genuine quantum routing of single-photon pulses based on cascading of conditional quantum gates in a Mach-Zehnder interferometer and report a proof-of-principle experiment for its demonstration using linear optics quantum gates. The polarization of the control photon routes in a coherent way the path of the signal photon while preserving the qubit state of the signal photon represented by its polarization. We demonstrate quantum nature of this router by showing entanglement generated between the initially unentangled control and signal photons, and confirm that the qubit state of the signal photon is well preserved by the router through quantum process tomography.

Abstract Motivated by the need for quantum computers to communicate between multiple, well separated qubits, we introduce the task of routing the quantum state from one input mode to a superposition of several output modes coherently. We report an experimental demonstration of a deterministic photonic routing protocol applied to an entangled state. We show in a quantum walk architecture, quantum networks perfectly route entangled states from an initial input mode to an arbitrary output mode coherently and deterministically. Our results demonstrate the key principle of a perfect router, opening a route toward data routing and transferring for quantum computing systems. The routing algorithm in our work can be applied to a wide range of physical systems, which provides a way for effective design of efficient routing protocols on practical quantum networks.

The quantum router is an elementary building block in quantum information processing and quantum communication. This work constitutes a step forward in this direction, i.e., an efficient scheme is proposed for the implementation of a nonreciprocal single-photon quantum router, allowing the routing of a single photon from one side but blocking it from the other side. Our model consists of two directly coupled coplanar-waveguide resonators and a superconducting ring resonator, which are connected together through a transmission line. The nonreciprocity is realized by unidirectionally interacting the rotating microwave modes of the ring resonator with a dissipative magnon mode in an yttrium iron garnet disk. We find that a single-photon signal can be delivered from a given input port to either of the two output ports, but is fully absorbed from the opposite one. The proposed scheme not only enriches the family of nonreciprocal quantum devices, but also finds useful applications in chiral quantum technologies.