Quantum phase slips: from condensed matter to ultracold quantum gases

Abstract

Quantum phase slips are the primary excitations in one-dimensional superfluids and superconductors at low temperatures. They have been well characterized in most condensed-matter systems, and signatures of their existence has been recently observed in superfluids based on quantum gases too. In this review we briefly summarize the main results obtained on the investigation of phase slips from superconductors to quantum gases. In particular we focus our attention on recent experimental results of the dissipation in one-dimensional Bose superfluids flowing along a shallow periodic potential, which show signatures of quantum phase slips. I. QUANTUM PHASE SLIPS IN CONDENSED MATTERThe ability to carry charge or mass currents with zero dissipation is the hallmark of superconductivity and superfluidity. Superconductors and superfluids are characterized by a macroscopic wave function Ψ(r) = |Ψ(r)|e iφ(r) , the order parameter. The amplitude and phase coherence of the order parameter vanish at the transition temperature or at the critical current, where the system goes back to the normal phase. However, also below the critical temperature and the critical current, the phase coherence of the system can be affected by the so-called phase slips, i.e. phase fluctuations of the order parameter, which induce a finite resistance and therefore lead to a destruction of persistent currents.Phase slips have been historically predicted by Little [1] as thermally activated topological defects, but it is now assessed that they may occur even at zero temperature, due to quantum tunneling events [2]. A phase slip event, in fact, is an elementary excitation of the order parameter due to thermal or quantum fluctuations, corresponding to a local suppression of its amplitude and a simultaneous jump of the phase by 2π. As a consequence of the phase slip event, the superfluid metastable state, i. e. a local minimum of the Ginzburg-Landau free energy F [3] with velocity v ∝ ∇φ(x), decays into a state with lower velocity, since the phase has locally unwound [1]. A phase slip can be thermally activated (TAPS) [4][5][6] when the temperature is higher than the free-energy barrier δF between two neighbouring metastable states and the system may overcome the barrier via thermal fluctuations. The nucleation rate of TAPS follows the Arrhenius law Γ ∝ e −δF/kB T [4, 5], making these events extremely improbable for T < ∼ δF/k B . In this latter regime, a second mechanism for the activation of phase slips becomes dominant: quantum tunneling below the free-energy barrier, triggered by quantum fluctuations. This second type of excitations has been called quantum phase slips (QPS) [2]. Since the role of both thermal and quantum fluctuations becomes increasingly important when reducing the dimensionality, these phenomena are particularly relevant for one-dimensional systems.QPS have been observed in different condensed-matter systems, such as superconducting nanowires [7][8][9][10][11] and Josephson junction arrays [12]. In these systems it is typic...

Critical Transport and Vortex Dynamics in a Thin Atomic Josephson Junction

“…Re-cent experiments with inhomogeneous three-dimensional Fermi superfluids [24,25] revealed the intimate connection between phase slippage and dissipation arising from vortices created within the barrier and shed into the superfluid. Similar effects have been studied in ringshaped bosonic condensates [26][27][28][29], mesoscopic structures [30,31] and lower-dimensional geometries [32][33][34]. While vortices crossing the weak link are known to yield a finite resistance [25,27,30], the relation between microscopic vortex nucleation, dynamics and macroscopic dissipative flow is still poorly understood.…”

Breakdown of ergodicity in quantum systems: from solids to synthetic matter

“…They observe a long-lived local memory of the out-of-equilibrium initial conditions and signatures of time crystal in the presence of quenched disorder. On the cold atom side, D'Errico et al [23] review recent work on microscopic dissipation by the creation of phase slips in 1D Bose-Hubbard systems.…”

Collective quantum phase slips in multiple nanowire junctions

“…There has been a growing research interest on quantum phase slips in not only condensed matters but also ultracold quantum gas [1]. In the solid-state context such as superconducting nanowires, the phase of the superconducting order parameter ψ = ψ 0 e iφ is allowed to change (i.e., slip) rapidly by ±2π if its amplitude tends to zero, due to the requirement that ψ 2 0 ∇φ gives a constant [2][3][4].…”