Quantum logic gate created using single atom and photon
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Professor Gerhard Rempe, director of the Max Planck Institute of Quantum Optics, and his team have created a quantum logic gate using a single photon and a single atom and say this hybrid device could be a major step in the development of quantum computers.
"Quantum communication, using flying photons, and data processing with atoms or ions have been regarded as separate research fields so far," said Prof Rempe. "In our, experiment we merge both techniques. In particular, our quantum gate could be implemented in a network in which atoms serve as stationary nodes for the storage of information, whereas photons transmit the information between these nodes, even over large distances. In this way, we hope to contribute to the realisation of a scalable quantum computer."
In their experiment, the binary states 0 and 1 are represented by the upwards and downwards spins of an atom and by two polarisation states of an optical photon (left or right circular).
In order to create a quantum gate, the atom is trapped inside a cavity made from two mirrors. The cavity's properties are chosen such that the atom and cavity form a strongly coupled system. The light quanta are prepared as faint laser pulses containing less than one photon on average.
A previous experiment showed that – by a proper choice of parameters –light quanta are always reflected. What matters is that, for certain combinations of atomic and photonic input states, the photons are reflected at the first mirror. For other combinations, they enter the cavity, then leave it on the same path, experiencing a phase shift of 180°.
"This conditional phase shift is the prerequisite for the implementation of a truth table assigning output signals to any combination of input bits in a deterministic way, similar to a classical logic gate," said Dr Stephan Ritter.
