Supersonic Rotation of a Superfluid: A Long-Lived Dynamical Ring

Abstract

We present the experimental realization of a long-lived superfluid flow of a quantum gas rotating in an anharmonic potential, sustained by its own angular momentum. The gas is set into motion by rotating an elliptical deformation of the trap. An evaporation selective in angular momentum yields an acceleration of rotation until the density vanishes at the trap center, resulting in a dynamical ring with 350 angular momentum per particle. The density profile of the ring corresponds to the one of a quasi two-dimensional superfluid, with a linear velocity reaching Mach 18 and a rotation lasting more than a minute.Superfluidity is a rich quantum dynamical phenomenon first observed in low temperature helium that also occurs for interacting degenerate gases [1]. Among its most striking manifestations are the existence of a critical velocity for the creation of excitations [2] and the appearance of quantized vortices when set into rotation, as has been observed in liquid helium [3] and in dilute Bose-Einstein condensates (BEC) [4,5].Because of a formal analogy between the two Hamiltonians, a neutral gas in rotation is a natural candidate to simulate a quantum system of charged particles in a magnetic field, relevant for condensed matter problems such as type II superconductors or the quantum Hall effect [6,7]. For a quantum gas confined in a harmonic trap of radial frequency ω r and rotating at angular frequency Ω approaching ω r , the ground state of the system reaches the atomic analog of the Lowest Landau Level (LLL) relevant in the quantum Hall regime [8][9][10]. The LLL has been reached for a dilute BEC [11] rotating at Ω = 0.993 ω r where dynamical properties of the system are modified compared with the mean-field regime.For rotation rates even closer to ω r , the formation of highly correlated states is predicted, similar to the ones involved in the fractional quantum Hall regime, such as Laughlin states [12] or Moore-Read states [13]. Reaching these fast rotation rates is experimentally challenging in a harmonic trap because the radial effective trapping potential in the rotating frame vanishes due to the centrifugal force. To circumvent this limit, higher-order confining potentials have been developed [14], which allow to access the regime where Ω even exceeds ω r . Although the experimental study of highly correlated states seems very challenging [15][16][17], fast rotation in a two-dimensional harmonic-plus-quartic trap opens new physical regimes and has attracted a lot of attention [14,[18][19][20][21].In this situation, a zero-density area -a hole-grows at the trap center above a critical rotation frequency Ω h [22], leading to an annular two-dimensional density profile. Moreover, the velocity of the atomic flow is expected to be supersonic [18] i.e. exceeding by far the speed of 50 µm -2 0 2 4 6 8 10 Figure 1. (a) Computed density contour (red annulus) for a BEC rotating at 1.06 ωr in the shell trap (gray ellipsoid). (b) in situ integrated 2D density (in units of µm −2 ) of a dynamical ring rotating at Mach ...

A versatile ring trap for quantum gases

“…The linear velocity corresponding to this frequency is close to the speed of sound computed above. This method is well appropriate to produce fast rotating annular gases [50].…”

Persistent currents in rings of ultracold fermionic atoms

“…Progress in understanding transport phenomena in quantum fluids has often been made by considering spherical, cylindrical, toroidal, or more exotic geometries [6], and realizing experimentally viable quantum many-body systems in more exotic geometries is an important challenge in quantum engineering. Persistent currents have been observed in experiments with Bose-Einstein condensates (BECs) of ultracold atoms [7][8][9], quantized phase slips [10] have been observed in matterwave "circuits" with Josephson junctions [11,12], and ring-BECs have been used to explore other phenomena such as collective-mode precession [13], spontaneous [14] and quench-induced currents [15], and the stability of supersonic flows [16,17]. These previous experiments all used bosonic atoms with weak contact interactions.…”

Expansion of a quantum gas in a shell trap

“…They allow to access a variety of trapping geometries [23] from double wells [24][25][26][27] to bubble traps [28] and even reach the twodimensional regime [22]. Thanks to the high degree of control on all parameters they are ideally suited to study superfluid dynamics [29][30][31][32]. By time averaging [33,34] or multiple dressing [35] even more configurations can be realized, as smooth ring-shaped waveguides [36,37] or multiple wells [35].…”

“…Once combined with the possibility to tune dynamically the rf polarization it offers an interesting platform to study rotating superfluids in anharmonic traps, on Earth or in space. For example, starting from the annular gas, one can use a small change of the rf polarization to rotate the gas [32] and then reduce the gradient to reconnect the cloud. Such protocol offers the opportunity to produce correlated states [57].…”