论文标题
由于光子没有吸收，测量原子在激发状态下花费的时间原子花费
Measuring the time atoms spend in the excited state due to a photon they don't absorb
论文作者
论文摘要
当共振光子遍历吸收原子样品时，原子在激发态下花费了多少时间？答案是否取决于光子最终被吸收还是传播？特别是，如果是$ \ textit {not} $吸收，它是否会导致原子在激动的状态下花费任何时间，如果是，多少？在对超冷rubidium原子的实验中，我们同时记录了原子是否被入射（信号”光子激发，以及这些光子是否被传输。我们通过使用单独的，非谐振的“探针”激光器来测量原子在激发态中所花费的时间来监视样品的折射指数 - 也就是说，我们测量了由信号脉冲在此探针梁上编写的非线性相移 - 并使用直接检测来隔离单个传输光子的效果。对于短脉冲（将10 ns与26 ns原子寿命进行比较）和光学较厚的培养基（峰od = 4，鉴于我们的带宽宽，导致60％的吸收），我们发现，由于一个传输光子不是零，而是兴奋的（77美元$ \ pm \ pm \ pm \ p pm \ p pm 16），平均水平的时间是，他们的平均含量是在兴奋的状态下花费的平均水平。我们将这种对“激发无损”的观察结果归因于相干的正向发射，当瞬时的Rabi频率（脉冲包膜）拾取相位翻转时可能会产生 - 当宽带脉冲通过频率依赖性吸收的光学较厚培养基传播时，这种情况就会自然发生[1]。这些结果明确地揭示了光子通过吸收介质传播的复杂历史，并说明了利用后选择的力量，以实验研究观察到的量子系统的过去行为。
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 the photon is ultimately absorbed or transmitted? In particular, if it is $\textit{not}$ absorbed, does it cause atoms to spend any time in the excited state, and if so, how much? In an experiment with ultra-cold Rubidium atoms, we simultaneously record 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 - that is, 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 average time atoms spend in the excited state due to one transmitted photon is not zero, but rather (77 $\pm$ 16)% of the time the average incident photon causes them to spend in the excited state. We attribute this observation of "excitation without loss" to coherent forward emission, which can arise when the instantaneous Rabi frequency (pulse envelope) picks up a phase flip - this happens naturally when a broadband pulse propagates through an optically thick medium with frequency-dependent absorption [1]. These results unambiguously reveal the complex history of photons as they propagate through an absorbing medium and illustrate the power of utilizing post-selection to experimentally investigate the past behaviour of observed quantum systems.
