All-optical storage of a qubit encoded in an oscillator

The efficient and reliable storage of quantum states plays a crucial role for the realization of quantum computation and communication. For example, in linear optics quantum computation as represented by the KLM scheme [1], quantum storage enables one to store intermediate “results” or to boost scalability and reliability of the computation. To employ quantum storage for quantum computation, the storage should be applicable to superposition states, including phase information of the superposition as well as the amplitude information of the state's coefficients. Some schemes exist for such storage using electron or nuclear spins [2]. However, an all-optical storage without the use of atoms or ions has several attractive features such as the wavelength independence of the transition energy. We have already succeeded in storing and retrieving phase-insensitive single-photon states in all-optical way with high purity sufficient for observing non-classicality [3]. Here we demonstrate the all-optical storage of a phase-sensitive photon-number superposition as a next natural step. We can store arbitrary qubits α|0〉 + β|1〉.