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Measuring the internal state of a single atom without energy exchange
One prerequisite for quantum information processing on atom chips is the efficient, non-destructive readout of the qubit state.
However, real quantum measurements almost always cause a much stronger back action than required by the laws of quantum mechanics. This causes irreversible energy exchange and heating, a limitation that obviates straightforward reuse of the qubit. No such energy exchange is required by quantum mechanics.
We have recently demonstrated the optical readout of the hyperfine state of a single rubidium atom with significantly less than one scattering event (Volz et al., arXiv : 1106.1854). This detection without energy exchange is possible by measuring transmission and reflection of a high-finesse optical cavity with the atom inside. The thereby obtained knowledge on the qubit state exceeds the maximum knowledge accessible in an ideal fluorescence detection.
The experimental principle
For an atom in the dark state |0> (top), probe light is either transmitted, reflected or lost by mirror imperfections. For the bright state |1> (bottom), most incident photons are reflected. In both cases, only a small fraction is scattered by the atom.
The graph below shows a typical detection trace of cavity transmission (blue) and reflection (red) for an atom initially in|1> performing a quantum jump to |0> owing to spontaneous emission.
Results
Using the cavity detection setup, we can read out the qubit state with a detection error below 10% while scattering less than 0.2 photons on average. The graph below shows the detection error vs. the number of scattered photons. The grey area is the range accessible to free-space detection schemes. This limit is overcome using a cavity. The green solid line shows the minimum detection error by the cavity measurement assuming maximum detection efficiency. The red dotted line shows the minimal detection error obtainable with the detection efficiency in our experiment. The blue circles denote the detection error actually obtained from the experiment with one s.d. error bars. This error is below the free-space limit in spite of experimental imperfections and fits well to the theoretical model (red line).
