A Single-Atom Quantum Memory

The faithful storage of a quantum bit of light is essential for long-distance quantum communication, quantum networking and distributed quantum computing. The required optical quantum memory must, first, be able to receive and recreate the photonic qubit and, second, store an unknown quantum state of light better than any classical device. These two requirements have so far been met only by ensembles of material particles storing the information in collective excitations. Recent developments, however, have paved the way for a new approach in which the information exchange happens between single quanta of light and matter. This single-particle approach allows one to address the material qubit and thus has fundamental advantages for realistic implementations: First, to combat inevitable losses and finite efficiencies, it enables a heralding mechanism that signals the successful storage of a photon by means of state detection. Second, it allows for individual qubit manipulations, opening up avenues for in situ processing of the stored quantum information. Here we demonstrate the most fundamental implementation of such a quantum memory by mapping arbitrary polarization states of light into and out of a single atom trapped inside an optical cavity. The memory performance is analyzed through full quantum process tomography. The average fidelity is measured to be 93% and low decoherence rates result in storage times exceeding 180\mu s. This makes our system a versatile quantum node with excellent perspectives for optical quantum gates and quantum repeaters.