Yoav Sagi

Observation of anomalous diffusion and fractional self-similarity in one dimension.

We experimentally study anomalous diffusion of ultracold atoms in a one dimensional polarization optical lattice. The atomic spatial distribution is recorded at different times and its dynamics and shape are analyzed. We find that the width of the cloud exhibits a power-law time dependence with an exponent that depends on the lattice depth. Moreover, the distribution exhibits fractional self-similarity with the same characteristic exponent. The self-similar shape of the distribution is found to be well fitted by a Lévy distribution, but with a characteristic exponent that differs from the temporal one. Numerical simulations suggest that this is due to long trapping times in the lattice and correlations between the atoms velocity and flight duration.

Process Tomography of Dynamical Decoupling in a Dense Cold Atomic Ensemble

Atomic ensembles have many potential applications in quantum information science. Owing to collective enhancement, working with ensembles at high densities increases the efficiency of quantum operations, but at the same time also increases the collision rate and leads to decoherence. Here we report on experiments with optically trapped 87Rb atoms demonstrating a 20-fold increase of the coherence time when a dynamical decoupling sequence with more than 200 pi pulses is applied. Using quantum process tomography we demonstrate that a dense ensemble with an optical depth of 230 can be used as an atomic memory with coherence times exceeding 3 seconds.

Measurement of the Homogeneous Contact of a Unitary Fermi Gas

By selectively probing the center of a trapped gas, we measure the local, or homogeneous, contact of a unitary Fermi gas as a function of temperature. Tans contact, C, is proportional to the derivative of the energy with respect to the interaction strength and is thus an essential thermodynamic quantity for a gas with short-range correlations. Theoretical predictions for the temperature dependence of C differ substantially, especially near the superfluid transition, T(c), where C is predicted to either sharply decrease, sharply increase, or change very little. For T/T(F)>0.4, our measurements of the homogeneous gas contact show a gradual decrease of C with increasing temperature, as predicted by theory. We observe a sharp decrease in C at T/T(F)=0.16, which may be due to the superfluid phase transition. While a sharp decrease in C below T(c) is predicted by some many-body theories, we find that none of the predictions fully account for the data.

Direct measurement of the system–environment coupling as a tool for understanding decoherence and dynamical decoupling

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 ultra-cold 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 some well-known dynamical decoupling sequences. 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.

Correlated fermions in nuclei and ultracold atomic gases

Background: The high momentum distribution of atoms in two spin-state ultra-cold atomic gases with strong short-range interactions between atoms with different spins, which can be described using Tans contact, are dominated by short range pairs of different fermions and decreases as

Scheme for generating Greenberger-Horne-Zeilinger-type states of n photons

k^{-4}

Many-body localization in system with a completely delocalized single-particle spectrum

. In atomic nuclei the momentum distribution of nucleons above the Fermi momentum (

The Experimental Setup

k>k_F \approx 250

Ramsey-like measurement of the decoherence rate between Zeeman sublevels

Mev/c) is also dominated by short rangecorrelated different-fermion (neutron-proton) pairs. Purpose: Compare high-momentum unlike-fermion momentum distributions in atomic and nuclear systems. Methods: We show that, for

Probing local quantities in a strongly interacting Fermi gas

k>k_F