Long-coherence-time qubit memory: Combining an efficient light-atom interface and low-decoherence atomic states

摘要

Summary form only given. Long-term storage of quantum information is a long standing goal since decades. It is particularly motivated by the fact that quantum network implementations strongly rely on quantum memories for light [1]. Long storage time and high efficiency are the main two figures of merit for a qubit memory. However, although quantum memories have been implemented in a large variety of physical systems, it turns out to be challenging to simultaneously fulfill those two main requirements.In our setup, a single 87Rb atom is trapped in a two-dimensional optical lattice inside a high-finesse cavity. Based on seminal work [2], the photonic polarization qubit a|L) + ß|R) described in the left/right polarization basis is mapped onto the atomic state |F = 2,= -1) + ß|F = 2, mf = +1) via a stimulated Raman adiabatic passage (STIRAP) as depicted in step I) of Fig. 1: The readout efficiency of this quantum memory protocol is basically constant for our typical coherence time. This is in contrast to systems based on atomic ensemble where decoherence affects the collective excitation reemission. However, fluctuations of the magnetic field deteriorate the coherence of the superposition. Indeed, the two levels we use are split by the Zeeman effect and thus the free evolution of the system will accumulate a random phase increasing with time. Typically, the coherence information of the qubit is lost after 200μs.