An ensemble of two level quantum systems coupled to a fluctuating external environment (“bath”) is a common paradigm in many fields of study. This coupling leads to decoherence that limits the usefulness of these systems, e.g. as qubits in quantum computation systems. In cold atomic ensembles, which have many potential applications in quantum information, this problem is intrinsic since the fluctuations arise due to collisions which are inherent to the high densities required to achieve a good overall efficiency of quantum operations. The understanding and characterization of this coupling is an important step towards improving the coherence of the system.

Here we present a direct measurement of the collisional bath spectrum in optically trapped cold atomic ensemble. We perform this measurement by applying a long spectrally sharp external field, and record the atomic response. The measured spectrum is a Lorentzian at lower frequencies [1], but exhibit also non-monotonic features at higher frequencies due to the oscillatory motion of the atoms in the trap. These features are well understood within our analytical models and numerical Monte Carlo simulations of the collisional bath. In addition, we present a simple method to circumvent the inherent problem of measuring the spectrum in the so called “strong coupling” regime, namely when the coherence time is shorter than the correlation time of the bath.

[1]. Sagi, Y., Almog, I., and Davidson, N. Phys. Rev. Lett. 105, 053201 (2010).