Atomic coherence

About: Atomic coherence is a research topic. Over the lifetime, 877 publications have been published within this topic receiving 29395 citations.

Coherent sensing of magnetic waveforms with spin-squeezed atoms

Abstract: Optical magnetometers use magnetically-sensitive atomic ensembles and optical read-out to detect the amplitude of magnetic fields. They have become the most sensitive instruments for measuring low-frequency magnetic fields surpassing competing technologies like superconducting quantum interface devices (SQUIDs), and find applications in a variety of fields ranging from medicine, biology and geophysics, as well as tests of fundamental physics. However, their fundamental sensitivity is bounded by quantum mechanical behavior of the atoms, which gives rise to the standard quantum limit (SQL). As many instruments are approaching this fundamental limit, it becomes necessary to explore ways to overcome the SQL. Quantum metrology studies strategies to increase the sensitivity beyond the SQL by means of quantum engineering the atomic states. In this thesis, we investigate the quantum enhanced detection of time varying radio-frequency magnetic fields using a cold atomic ensemble of 87Rb atoms held in an optical dipole trap. We first theoretically develop a new measurement technique based on stroboscopic back-action evading measurements that takes advantage of the atomic coherence This measurement scheme is suitable for the detection of arbitrarily-chosen components of radio-frequency waveforms and includes radio-frequency magnetometry as a special case. Experimentally, we demonstrate the capabilities of this technique using a linearly chirped waveform as a test case. As a first experiment, we demonstrate the selective response of the method in the coherently accumulated signal by the atoms. For this, we dispersively probe the atoms via Faraday rotation and non-destructively measure the induced magnetization. In the last part of the thesis we demonstrate quantum enhanced magnetic field detection. In a measure-evolve-measure (MEM) sequence, a first stroboscopic quantum non-demolition (QND) measurement produces a state with reduced projection noise, followed by a period of free evolution where the atoms accumulate signal. A second QND measurement detects the change relative to thefirst measurement. We demonstrate entanglement-enhanced sensing of sinusoidal and linearly chirped waveforms, with metrologically-relevant noise reduction of \xi_m^2 =0.84(8) and \xi_m^2=0.80(3), respectively. We achieve volume-adjusted sensitivity \delta B\sqrt{V}=3.96 fT(cm3/Hz)^{1/2}, comparable to the best radio-frequency magnetometers.

Continuous-variable entanglement via an incoherent pump in a microwave-driven Λ-type single-atom laser

Abstract: We investigate the generation and evolution of continuous-variable (CV) entanglement via an incoherent pump in a single-atom cavity electrodynamics (CQED) system. The atomic coherence in such a ? configuration is introduced by driving the lower two levels with a strong classical field of Rabi frequency ?m. It is shown that the intensity of the driving field can influence effectively the period of the entanglement between the two cavity modes.

Maximal atomic coherence via selective trapping of dressed states

Abstract: We consider strong-field index enhancement in three-level systems in which there is a strong spontaneous decay from an auxiliary level into either of two states that the probe field couples. A control field is detuned resonant with the transition between the auxiliary level and one of the dressed states produced by the strong probe field. It is shown that it is possible to achieve maximal atomic coherence, which characterizes an ultralarge index of refraction and vanishing absorption. This scheme is based on selective dressed population trapping, which is established by relying on the control field and the strong decay to transfer population from one dressed state to another through the auxiliary level. An advantage of the present scheme is that the conditions for its realization are accessible experimentally.

Generation of entanglement via atomic coherence in a two-mode three-level cascade atomic system

Abstract: The generation of continuous variable entanglement via atomic coherence in a two-mode three-level cascade atomic system is discussed according to the entanglement criterion proposed by Duan et al. [Phys. Rev. Lett. 84, 2722 (2000)]. Atomic coherence between the top and bottom levels is induced with two photons of a strong external pump field. It shows that entanglement for the two-mode field in the cavity can be generated under certain conditions. Moreover, by means of the input-output theory, we show that the two-mode entanglement could also be approached at the output.