Abstract

It has recently been theoretically shown that Quantum Memories (QM) could enable truly global quantum networking when deployed in space [1, 2] thereby surpassing the limited range of land-based quantum repeaters. Furthermore, QM in space could enable novel protocols and long-range entanglement and teleportation applications suitable for Deep-Space links and extended scenarios for fundamental physics tests. In this white paper we will make the case for the importance of deploying QMs to space, and also discuss the major technical milestones and development stages that will need to be considered.

Introduction

A quantum memory (QM) is a device with the ability to store the quantum state of an incoming light pulse and release it on-demand at a later time [3]. QM are required in several protocols in quantum communication, quantum computing and optics. In the context of long-range links, the main function of QMs is to synchronize otherwise probabilistic two (or more)-photon transfer and detection events in order to speed up protocols, such as quantum repeaters [4, 5] and deterministic creation of multiphoton states [6, 7].

There are several figures of merit that characterize the performance of QMs: • Fidelity (F): the retrieved quantum state, ρ 0 should be as close as possible to the input quantum state ρ. The fidelity is given by the overlap between these states, and can be calculated as: F(ρ) = Trp√ ρ 0ρ √ ρ 0 . • Efficiency (η): The memory efficiency is ratio of probability of detecting the output photon to that of the input photon1 ; η = Pout/Pin. For ensemble based memories, it is usually limited by optical depth of the atomic ensemble and strength and temporal/spectral profile of the control pulses, mediating the storage. • Storage time (τ ): in principle a QM should store the input quantum state as long as possible.

This is usually limited by interatomic interactions, thermal effects, external magnetic or electric field noises and can be mitigated with several means. Today, QMs are pushing towards 1 s threshold [9], while classical pulse storage for up to 1 h has been recently demonstrated [10]. 2 Quantum memories for space applications It has recently been theoretically shown that QMs could enable truly global quantum networking when deployed in space [1, 2] thereby surpassing the limited range of land-based quantum repeaters. When combined with a set of tools such as high-fidelity entangled photon pair sources, entanglement swapping [11] and teleportation [12] schemes, QMs will also enhance the distances for very-large scale entanglement and teleportation implementations in space, and enable novel applications and tests of gravity. Specifically, QMs could also find use in long-distance Bell tests [13, 14] where they ensure the space-like separation of the detection events in order to close the locality loophole.

This is achieved by delaying one of the pairs by a certain amount τ before the detection event. While short fibre-based delay lines (∼ µs) are sufficient for some experiments such as optical COW tests as envisioned in [13], QMs are definitely required in space-based experiments that require adjustable delay times of many milli-seconds to seconds, which is in particular the case for scenarios that involve large link distances such as for geostationary orbit to Earth (120 ms), or Earth to Moon (1.3 s). Long-lived QMs achieving high efficiencies (η > 70% for τ ∼ 200 ms) have already been demonstrated in the lab [9]. Hypothetically, a fiber delay line for such a storage time2 comes with a prohibitively high loss of > 5600 dB. 2.1 Ultra-long distance entanglement and teleportation experiments Long-range entanglement and teleportation experiments are fundamentally interesting for several reasons. Such experiments would open up the path towards long-distance quantum communication [1, 2, 16, 17], all the way towards enabling tests of quantum mechanics across vast distances [13, 14].