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Journal of Physics B Atomic Molecular and Optical Physics

Motional dephasing of atomic clock spin waves in an optical lattice

S. D. Jenkins, T Zhang, T. A. B. Kennedy

Published Jun 8, 2012• 24 citations
Publisher

Abstract

In a cold atomic ensemble the weak Raman scattering of an incident laser beam writes a spin-wave grating by transferring an atom between ground-level hyperfine states. These spin-waves serve as a basis for a quantum memory. For clock states, where magnetic dephasing is suppressed, thermal motion of the atoms across the spin-wave is the principal source of dephasing on the sub-millisecond timescale, limiting the quantum memory time achievable. An investigation of the role of the optical lattice in reducing motional dephasing is presented, using Monte Carlo simulations to study the influence of ensemble temperature, trap depth and differential ac Stark shifts in the case of rubidium.

DOI
10.1088/0953-4075/45/12/124005

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In a cold atomic ensemble the weak Raman scattering of an incident laser beam writes a spin-wave grating by transferring an atom between ground-level hyperfine states. These spin-waves serve as a basis for a quantum memory. For clock states, where magnetic dephasing is suppressed, thermal motion of the atoms across the spin-wave is the principal source of dephasing on the sub-millisecond timescale, limiting the quantum memory time achievable. An investigation of the role of the optical lattice in reducing motional dephasing is presented, using Monte Carlo simulations to study the influence of ensemble temperature, trap depth and differential ac Stark shifts in the case of rubidium.
Increasing coherent interrogation times is central to advancing the precision of optical clocks. Synchronous differential optical clock comparisons have now demonstrated atomic coherence times that far exceed the coherence time of the clock laser. While atom coherence times are then primarily limited by errors induced by lattice Raman scattering, excited clock state radiative decay, and broadening from two-body collisions, many of these errors take the atoms out of the clock transition subspace, and can therefore be converted into “erasure” errors if the appropriate readout scheme is employed. Here we experimentally demonstrate a hyperfine-resolved readout technique for 87 Sr optical lattice clocks that mitigates decoherence from Raman scattering induced by the lattice as well as radiative decay. By employing hyperfine-resolved readout in synchronous differential comparisons between 87 Sr ensembles with both Ramsey and spin-echo spectroscopy sequences, we achieve enhanced atomic coherence times exceeding 100 and 150 s, respectively, enabling longer coherent measurements without a reduction in performance. We anticipate that this hyperfine-resolved readout technique will benefit applications of state-of-the-art optical lattice clock comparisons in which the coherence times are constrained by Raman scattering or radiative decay.
Accurately manipulating quantum states is a fundamental strategy for improving the performance of quantum metrology, computing, and simulation. However, the quantum state is susceptible to dephasing due to the temperature and density of the atomic ensembles. In this paper, we experimentally study the effect of Floquet engineering (FE) on the dephasing process in an 87Sr optical clock. By measuring the Rabi flopping process under different temperatures of the cold ensemble and numbers of atoms trapped in the lattice, our results show that the FE can suppress the dephasing due to high temperatures or a large number of atoms. Indeed, when the temperature and the number of atoms are 3.8 μK and 6300, respectively, the FE can obviously suppress the dephasing effect and improve the maximum excitation fraction of the Rabi spectrum by 15.4%.
Long-term storage of quantum information has diverse applications in quantum information science. This work presents an experimental realization of quantum memories with lifetimes greater then 0.1 s. The memories are based on cold rubidium atoms confined in one-dimensional optical lattices. First realization of lattice-based quantum memory and entanglement between a light field and a spin wave is presented in Chapter II. Chapter III describes two different methods (two-photon and magnetic) of compensation for inhomogeneous differential light shifts between the memory levels due to optical trapping potentials, and demonstration of entanglement between a telecom-band light field and a light-shift compensated memory qubit. Highly excited Rydberg atoms present a unique platform for study of strongly correlated systems and quantum information, because of their enormous dipole moments and consequent strong, long-range interactions. In the experiment described in Chapter IV single collective Rydberg excitations are created in a cold atomic gas. After a variable storage period the excitations are converted into light. As the principal quantum number n of the Rydberg level is increased beyond ~ 70, no more than a single excitation is retrieved from the entire mesoscopic ensemble of atoms. In Chapter V, by spatially selective conversion of the spin wave into a light field, we demonstrate that Rydberg-level interactions create long-range correlations of collective atomic excitations. These results hold promise for studies of dynamics and disorder in many-body systems with tunable interactions and for scalable quantum information networks. Chapter VI presents initial observations of coherent many-body Rabi oscillations between the ground level and a Rydberg level using several hundred cold rubidium atoms. The strongly pronounced oscillations indicate a nearly complete excitation blockade of the entire mesoscopic ensemble by a single excited atom.
We investigate the effects of stimulated scattering of optical lattice photons on atomic coherence times in a state-of-the art ^{87}Sr optical lattice clock. Such scattering processes are found to limit the achievable coherence times to less than 12 s (corresponding to a quality factor of 1×10^{16}), significantly shorter than the predicted 145(40) s lifetime of ^{87}Sr's excited clock state. We suggest that shallow, state-independent optical lattices with increased lattice constants can give rise to sufficiently small lattice photon scattering and motional dephasing rates as to enable coherence times on the order of the clock transition's natural lifetime. Not only should this scheme be compatible with the relatively high atomic density associated with Fermi-degenerate gases in three-dimensional optical lattices, but we anticipate that certain properties of various quantum states of matter-such as the localization of atoms in a Mott insulator-can be used to suppress dephasing due to tunneling.
The quantum metrology with ultra-high precision usually requires the atoms prepared in an ultra-stable environment with well-defined quantum states. Thus, the optical lattice clock system chooses deep lattice potential to trap the ultra-cold atoms. However, building atomic clock in shallow optical lattice can greatly reduce decoherence rate induced by Raman scattering as well as higher order light shift, but tunneling among the lattice sites will cause additional dephasing and strongly broadening of the Rabi spectrum. Here, we periodically drive the shallow 87^{87}87Sr optical lattice clock in the experiment. Counterintuitively, shaking the system can deform the wide broad spectral line into a sharp peak with 5.4Hz line-width. With careful comparison between the theory and experiment, we demonstrate the Rabi frequency and Bloch bands can be simultaneously and independently tuned by adjusting the driving frequency and amplitude. Our work not only provides a different idea for quantum metrology, such as building shallow optical lattice clock in outer space, but also paves the way for quantum simulation of new phase of matter, such as engineering exotic spin orbit coupling Hamiltonian.