Group Seminar via Zoom: Measuring the time atoms spend in the excited state due to a photon they don't absorb

August 18, 2020

Josiah Sinclair, Univ. of Toronto
Group meeting via video conference (Zoom)
Tuesday, August 18th, 16:00 (MEZ)

Dealing with the unique situation of partial lock downs worldwide and home office solutions at our Institute due to the current spreading of the Covid 19 virus, we are now holding our group seminars and journal clubs via video conference. This procedure enables us to continue our research, enhance discussions and exchange important information.


When a resonant photon traverses a sample of absorbing atoms, how much time do atoms spend in the excited state?  Does the answer depend on whether or not the photon is eventually transmitted? If the photon is not absorbed, do atoms spend any time in the excited state, and if so, how much? In an experiment with ultra-cold Rubidium atoms, we simultaneously measure whether atoms are excited by incident (``signal'') photons and whether those photons are transmitted. We measure the time spent by atoms in the excited state by using a separate off-resonant probe laser to monitor the index of refraction of the sample (in other words, we measure the nonlinear phase shift written by a signal pulse on this probe beam), and use direct detection to isolate the effect of single transmitted photons. For short pulses (10 ns, to be compared to the 26 ns atomic lifetime) and an optically thick medium (peak OD = 4, leading to 60% absorption given our broad bandwidth), we find that the time atoms spend in the excited state due to transmitted photons is not zero, but rather 0.79 +/- 0.16 of the time spent in the excited state due to the average incident photon. We attribute this observation of ``excitation without loss'' to coherent forward emission, which arises for instance due to the sign-flip the instantaneous Rabi frequency (pulse envelope) picks up when a broadband pulse propagates through an optically thick medium with highly frequency-dependent absorption. These results raise intriguing new questions about the history of post selected photons, which motivate further experiments and cry out for a fully quantized theoretical treatment of the correlations between the final state of the incident photon and the time spent by atoms in the excited state.

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