Bloch oscillations in the absence of a lattice

Abstract

We experimentally investigate the quantum motion of an impurity atom that is immersed in a strongly interacting one-dimensional Bose liquid and is subject to an external force. We find that the momentum distribution of the impurity exhibits characteristic Bragg reflections at the edge of an emergent Brillouin zone. While Bragg reflections are typically associated with lattice structures, in our strongly correlated quantum liquid they result from the interplay of short-range crystalline order and kinematic constraints on the many-body scattering processes in the one-dimensional system. As a consequence, the impurity exhibits periodic dynamics that we interpret as Bloch oscillations, which arise even though the quantum liquid is translationally invariant. Our observations are supported by large-scale numerical simulations.A skydiver accelerated by the gravitational force approaches a constant drift velocity due to friction with the surrounding medium. In the quantum realm, dynamics can be significantly richer. For example, a quantum particle accelerated in a periodic crystal potential does not move at all on average but rather undergoes a periodic motion known as Bloch oscillations [1, 2]. Such an oscillatory motion is a direct consequence of the periodic momentum-dependence of the eigenstates in a lattice potential and arises from continuous translational symmetry breaking. Bloch oscillations have been observed for electrons in solid state systems [3] and have been investigated in detail with ultracold atoms in optical lattices [4][5][6][7][8]. One might expect that a quantum liquid, which is fully translational invariant, or in other words does not have an imprinted lattice structure, would preclude such striking dynamics. However, recent theoretical studies [9, 10] suggest that Bloch oscillations can emerge also in the presence of a continuous translational symmetry. In particular, for impurity atoms immersed in one-dimensional (1D) quantum liquids such dynamics is expected to arise due to strong quantum correlations, which lead to effective crystal-like properties. Yet, suitable conditions for that phenomenon are debated [11]. Ultracold quantum gases provide an ideal setting to experimentally study the dynamics of impurity particles coupled to host environments [12][13][14][15][16] due to excellent parameter control, precise initial state preparation, and decoupling from the environment. FIG. 1: Concept of the experiment. (A)We realize an ensemble of 1D Bose gases in tubes formed by two pairs of counter-propagating and interfering laser beams. In each tube, a single strongly interacting impurity (green sphere) is immersed in the correlated host gas (black spheres) and is accelerated by gravity (green arrow). Inset: Scattering length as for collisions between the atoms in the host gas (dashed line) and between the impurity and the host atoms (solid line) as a function of the magnetic field B. (B) The excitation spectrum of the impurity coupled to the 1D Bose liquid is a 2kF periodic function of the system's t...

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