Single Atoms and Single Photons: Retrieving Information

Both simple atoms and photons can be described by two quantum states only, e.g. two selected internal states for atoms, two polarization states for photons. In a new language, introduced over the past two decades, such objects are now considered carriers of quantum information where in addition to the pure states ǀ↑〉 and ǀ↓〉, available for classical bits, too, superposition states play a central role. Trapped atoms store qubits, photons transport qubits from one node to another. A single atom interacting with a single photon is thus not only an elementary process of light matter interaction but also of quantum information science.

The interaction of an atom with the light field of a high finesse resonator can be understood in terms of two coupled oscillators: at resonance, the modes are split and the transmission of the empty resonator is strongly suppressed by the factor 1/(1+C)², where C = g²/κγ is the so called cooperativity quantifying the coupling of the atom-resonator field (rate g) in terms of the loss rates of atom (γ) and field (κ). At C = 25, very strong resonant suppression occurs. [1]

In atoms, two different long lived ground states make good qubit states, e.g. the hyperfine states of the Cs atomic clock. Transitions between those states are induced by micro-waves or by Raman two photon transitions. Resonant interaction with the cavity field occurs for only one of the two qubits states: For one state the cavity looks empty and hence transmits the full probe laser light; for the other one strong atom cavity coupling suppresses transmission. We observe a random telegraph signal exhibiting quantum jumps between the two qubit states caused by incidental excitation from the probe and a weak repumping laser. This measurement approaches a QND (quantum non-demolition) measurement since it continuously monitors the systemquantum state, ideally without scattering photons. [2] We detect single photons (“clicks”) (lower trace), and by suitable binning we can straightforwardly assign quantum states. The information content carried by a single detected photon can be used in an optimal way using Bayes’ rule of conditional probabilities: every photon click “updates” our knowledge about the state of the system, provided a suitable model is available. [3]

References

[1] H.J. Kimble in P. Berman, Cavity quantum electrodynamics, academic press, Boston, 1994.

[2] M. Khudaverdyan et al., Phys. Rev. Lett. 103, 2009, 123006; J. Volz et al., 475, 2011, 210.

[3] S. Reick et al., J. Opt. Soc. Am., B 27, A152, 2010.

[4] M. Mücke, Nature (London), 465, 2010, 755; T. Kampschulte et al., Phys. Rev. Lett., 105, 2010, 153603.

[5] A. Rauschenbeutel et al., Phys. Rev. Lett., 83, 1999, 5166.

[6] T. Wilk et al., Phys. Rev. Lett., 104, 2010, 010502; L. Isenhower et al., Phys. Rev. Lett., 104, 2010, 010503.

[7] A. Sorensen et al., Phys. Rev. Lett., 91, 2003, 097905.

[8] S. Brakhane et al., Phys. Rev. Lett., 109, 2012, 173601.