Light-matter entanglement over 50 km of optical fibre

Abstract

When shared between remote locations, entanglement opens up fundamentally new capabilities for science and technology [1,2]. Envisioned quantum networks distribute entanglement between their remote matter-based quantum nodes, in which it is stored, processed and used [1]. Pioneering experiments have shown how photons can distribute entanglement between single ions or single atoms a few ten meters apart [3,4] and between two nitrogen-vacancy centres 1 km apart [5]. Here we report on the observation of entanglement between matter (a trapped ion) and light (a photon) over 50 km of optical fibre: a practical distance to start building large-scale quantum networks. Our methods include an efficient source of light-matter entanglement via cavity-QED techniques and a quantum photon converter to the 1550 nm telecom C band. Our methods provide a direct path to entangling remote registers of quantum-logic capable trapped-ion qubits [6][7][8], and the optical atomic clock transitions that they contain [9,10], spaced by hundreds of kilometers.Our network node consists of a 40 Ca + ion in a radiofrequency linear Paul trap with an optical cavity that enhances photon collection on the 854 nm electronic dipole transition. (Figure 1). A Raman laser pulse at 393 nm triggers emission, by the ion, of a photon into the cavity via a bichromatic cavity-mediated Raman transition (CMRT) [11]. Two indistinguishable processes are driven in the CMRT, each leading to the generation of a cavity photon and resulting in entanglement between photon polarisation and the electronic qubit state of the ion of the form 1/ √ 2 (|D J=5/2, mj =−5/2 , V + |D J=5/2, mj =−3/2 , H ), with horizontal (H) and vertical (V ) photon polarisation and two metastable Zeeman states of the ion (D J, mj ) [12]. The total probability of obtaining an on-demand free-space photon out of the ion vacuum chamber (entangled with the ion) is 0.5 ±0.1 [12].While the ∼ 3 dB/km losses suffered by 854 nm photons through state-of-the-art optical fibre allows for few km internode distances, transmission over 50 km would be 10 −15 . 854 nm photons are also frequencyincompatible with other examples of quantum matter, preventing the realisation of ion-hybrid quantum systems over any distance. Single photon frequency conversion to the telecom C band (1550 nm) offers a powerful general solution: this wavelength suffers the minimum fibre transmission losses (∼ 0.18 dB/km) and is therefore an ideal choice for a standard interfacing wavelength for quantum networking. Photons from solid-state memories [14], cold gas memories [15,16], quantum dots and nitrogen-vacancy centres [17] have been converted to telecom wavelengths. Frequency conversion of photons from ions has recently been performed, including to the telecom C band (without entanglement) [18], to the telecom * ben.lanyon@uibk.ac.at, † These authors contributed equally O band with entanglement over 80 m [19] and directly to an atomic Rubidium line at 780 nm [20].We inject single-mode fibre-coupled photons from the ion into a polarisat...

Quantum optical frequency up-conversion for polarisation entangled qubits: towards interconnected quantum information devices

“…To ensure that the wavelength conversion process does not deteriorate the polarisation state between the input and output photon, two requirements need to be satisfied [26][27][28][29][30]. First, the conversion efficiency in both arms needs to be equalised; second, the optical phase difference between upper and lower arms needs to be zero to avoid polarisation state rotation.…”

“…Such wavelength conversion interfaces are compatible with essentially all photonic observables, e.g. energy-time [16,17], time-bin [18][19][20][21], orbital angular momentum [22,23], squeezed states of light [24], and polarisation [25][26][27][28][29][30]. Concerning the very popular polarisation observable, a high-efficiency and versatile up-converter from telecom to the quantum memory band is still missing.…”

“…One attempt based on three-wave mixing in nonlinear crystals showed up-conversion from 810 nm to 532 nm, but the reported conversion efficiency is ∼ 0.04% [25]. Down-conversion experiments showed however, that higher efficiencies in the few 10 percent range should be feasible [26][27][28][29][30].…”

Entanglement and nonlocality between disparate solid-state quantum memories mediated by photons

“…Various materials, devices, and protocols are currently being studied towards this end [3][4][5], but, so far, there is no certainty about which elements will constitute its fundamental building blocks. However, it appears likely that they will operate within different wavelength regions, ranging from visible [6,7] via near-infrared [8][9][10][11][12][13], to telecommunication wavelengths [14,15]. This will allow leveraging the best properties of each device, and thereby offer heightened capabilities compared to a network consisting of identical quantum systems.…”

Towards long-distance quantum communication using trapped ions and frequency qubits

“…The quantum state of these photonic qubits is entangled with internal state of the atomic qubit and entanglement swapping operations can then be used to remotely entangle two atoms. Recently ion-photon entanglement was demonstrated through 50 km of fibre by encoding the quantum states in the polarisation degree of freedom 3 . This scheme is only feasible when the fibre is in a lab and isolated from temperature and pressure noises.…”