Direct Measurement of the System-Environment Coupling as a Tool For Understanding Decoherence and Dynamical Decoupling

TL;DR

This paper presents a method to directly measure the environment's coupling spectrum in ultracold atoms, enabling better design of dynamical decoupling sequences to mitigate decoherence in quantum systems.

Contribution

It introduces a spectrally narrow control field technique to measure the bath spectrum directly, validating the overlap integral formalism for dynamical decoupling.

Findings

Measured bath spectrum shows Lorentzian shape at low frequencies.

Non-monotonic features at higher frequencies match models and simulations.

Experimental results agree with predictions for dynamical decoupling performance.

Abstract

Decoherence is a major obstacle to any practical implementation of quantum information processing. One of the leading strategies to reduce decoherence is dynamical decoupling --- the use of an external field to average out the effect of the environment. The decoherence rate under any control field can be calculated if the spectrum of the coupling to the environment is known. We present a direct measurement of the bath coupling spectrum in an ensemble of optically trapped ultracold atoms, by applying a spectrally narrow-band control field. The measured spectrum follows a Lorentzian shape at low frequencies, but exhibits non-monotonic features at higher frequencies due to the oscillatory motion of the atoms in the trap. These features agree with our analytical models and numerical Monte-Carlo simulations of the collisional bath. From the inferred bath-coupling spectrum, we predict the performance of well-known dynamical decoupling sequences: CPMG, UDD and CDD. We then apply these sequences in experiment and compare the results to predictions, finding good agreement in the weak-coupling limit. Thus, our work establishes experimentally the validity of the overlap integral formalism, and is an important step towards the implementation of an optimal dynamical decoupling sequence for a given measured bath spectrum.