Radio-frequency-induced ground-state degeneracy in a Bose-Einstein condensate of chromium atoms

Abstract

We study the effect of strong radio-frequency (rf) fields on a chromium Bose-Einstein condensate (BEC), in a regime where the rf frequency is much larger than the Larmor frequency. We use the modification of the Landé factor by the rf field to bring all Zeeman states to degeneracy, despite the presence of a static magnetic field of up to 100 mG. This is demonstrated by analyzing the trajectories of the atoms under the influence of dressed magnetic potentials in the strong field regime. We investigate the problem of adiabaticity of the rf dressing process, and relate it to how close the dressed states are to degeneracy. Finally, we measure the lifetime of the rf dressed BECs, and identify a new rf-assisted two-body loss process induced by dipole-dipole interactions. [2]. These systems are known as spinor quantum gases. To explore these new features, it is important that differences in interaction energies between different total spin states are larger than their relative Zeeman energy, which requires magnetically shielded environments.Up to now experiments on spin-1 [3] and spin-2 [4] spinor condensates were typically performed starting with atoms in the |m = 0 magnetic state, with emphasis on spin dynamics and coherent oscillations between the spin components. Since spin dynamics is driven by spin exchange collisions, which do not modify the total spin angular momentum, one therefore works in a subspace insensitive to first order Zeeman effect, but the spinor ground state is not obtained [5]. A subspace insensitive to magnetic fields to first order is also used for quantum computing purposes with cold atoms in optical lattices, to reduce decoherence during quantum gate operations [6]. More generally, a very accurate control of the magnetic fields is required for precision measurements (e.g. atomic clocks use both magnetic shielding and a transition insensitive to the Zeeman effect to first order).To ease the constraints on magnetic field control, we suggest to use strong off resonant linearly polarized rf fields, to bring all Zeeman states to degeneracy despite a non-zero magnetic field. We demonstrate this idea by sending strong rf fields to optically trapped Bose condensed chromium atoms. We analyze the trajectories of atoms in dressed magnetic potentials and we show that, as expected from [7], the Landé factor is modifed, and can even be set to zero. At this point, all Zeeman states are degenerate. We show that the adiabaticity criterion for ramping up the rf power strongly depends on such degeneracy. Finally, we discuss inelastic losses measured in the dressed sample, and attribute them to an exoenergetic rf-assisted dipolar coupling to higher partial waves.Before describing our experimental results, let us give a physical insight into the modification of the atoms eigenenergies by the rf field. As shown in [7] using first order perturbation theory, when the rf frequency ω is much larger than the Larmor frequency ω ⊥ the Landé factor g J perpendicular to the rf field axis is modified by the rf dressing of t...

Long-Lived Spin-Orbit-Coupled Degenerate Dipolar Fermi Gas

“…The relaxation process may also be viewed as a transition between two molecular potentials [57]. As illustrated in Fig.…”

Magic Radio-Frequency Dressing of Nuclear Spins in High-Accuracy Optical Clocks

“…For dressing by a large number of rf photons, a perturbative quantum analysis predicts a dressed Landé g-factor dependent on the zeroth-order Bessel function of the first kind g d j (x j ) = g j J 0 (x j ), with x j = Ω j /ω rf , Ω j = g j Ω/g S , (j = P, S) [21]. That dependence was verified in experiments on atoms [17,18,22], neutrons [23] and chromium Bose-Einstein condensate [24]. It is valid for whatever spin value and equally spaced Zeeman levels [25].…”

Microwave spectroscopy of radio-frequency-dressed Rb87

“…This problem can be studied in the context of the response of continuously driven quantum systems, which has been the subject of theoretical and experimental study over several decades [32][33][34]. The experimental observations of the spectrum of off-resonant RF-dressed states made by Haroche and Cohen-Tannoudji can be understood using perturbative expansions of driven twolevel systems (TLS) [35][36][37][38][39]. In addition, more recent experiments demonstrate that the modified response of resonantly RF-dressed alkali atoms to MW fields enables the encoding and manipulation of qudits exploiting the full complexity of the hyperfine manifold [29,40], and going beyond the TLS paradigm.…”