The decay of an excited atom undergoing spontaneous photon emission into the fluctuating quantum-electrodynamic vacuum is an emblematic example of the dynamics of an open quantum system. Recent experiments have demonstrated that the gapped photon dispersion in periodic structures, which prevents photons in certain frequency ranges from propagating, can give rise to unusual spontaneous-decay behaviour, including the formation of dissipative bound states. So far, these effects have been restricted to the optical domain. Here we demonstrate similar behaviour in a system of artificial emitters, realized using ultracold atoms in an optical lattice, which decay by emitting matter-wave, rather than optical, radiation into free space. By controlling vacuum coupling and the excitation energy, we directly observe exponential and partly reversible non-Markovian dynamics and detect a tunable bound state that contains evanescent matter waves. Our system provides a flexible platform for simulating open-system quantum electrodynamics and for studying dissipative many-body physics with ultracold atoms.
The understanding of how classical dynamics can emerge in closed quantum systems is a problem of fundamental importance. Remarkably, while classical behavior usually arises from coupling to thermal fluctuations or random spectral noise, it may also be an innate property of certain isolated, periodically driven quantum systems. Here, we experimentally realize the simplest such system, consisting of two coupled, kicked quantum rotors, by subjecting a coherent atomic matter wave to two periodically pulsed, incommensurate optical lattices. Momentum transport in this system is found to be radically different from that in a single kicked rotor, with a breakdown of dynamical localization and the emergence of classical diffusion. Our observation, which confirms a long-standing prediction for many-dimensional quantum-chaotic systems, sheds new light on the quantum-classical correspondence.
Behavior analogous to that of spontaneous emission in photonic band gap materials has been predicted for an atom-optical system consisting of an atom confined in a well of a state-dependent optical lattice that is coupled to free space through an internal-state transition [de Vega et al., Phys. Rev. Lett. 101, 260404 (2008)]. Using the Weisskopf-Wigner approach and considering a one-dimensional geometry, we analyze the properties of this system in detail, including the evolution of the lattice-trapped population, the momentum distribution of emitted matter waves, and the detailed structure of an evanescent matter-wave state below the continuum boundary. We compare and contrast our findings for the transition from Markovian to non-Markovian behaviors to those previously obtained for three dimensions.
Diffraction phenomena usually can be formulated in terms of a potential that induces the redistribution of a wave's momentum. Using an atomic Bose-Einstein condensate coupled to the orbitals of a state-selective optical lattice, we investigate a hitherto unexplored nonadiabatic regime of diffraction in which no diffracting potential can be defined, and in which the adiabatic dressed states are strongly mixed. We show how, in the adiabatic limit, the observed coupling between internal and external dynamics gives way to standard Kapitza-Dirac diffraction of atomic matter waves. We demonstrate the utility of our scheme for atom interferometry and discuss prospects for studies of dissipative superfluid phenomena.Diffraction, the bending of waves around obstacles, is one of the most fundamental and ubiquitous phenomena in optics, with a centuries-old history going back to the works of Grimaldi, Huygens, and Young on the wave nature of light. In the modern era it has led to the understanding of x-rays [1, 2], has provided direct proof for the wave nature of particles [3], and today finds many applications in physics and materials science, ranging from electron, x-ray and neutron diffraction, to applications in atom optics [4,5].Quite generally, diffraction is caused by a positiondependent potential with absorptive (imaginary) and/or dispersive (real) character. While the former includes material structures and diffraction gratings, an example for the latter is the ponderomotive potential exerted on electrons by an optical standing wave, as originally suggested by Kapitza and Dirac [6]. Kapitza-Dirac diffraction of electrons was first demonstrated only fairly recently [7], well after the first observation of an analogous effect on neutral atoms based on the ac Stark shift near an atomic resonance [8]. It has also been applied to Bose-Einstein condensates [9,10] and has become an important and often-used tool in atom interferometry and metrology [5].In this paper, we investigate a hitherto unexplored regime of diffraction, in which a diffracting potential cannot be defined. In our experiment we diffract an atomic matter wave from a microwave-dressed optical lattice, with a diffractive dynamics that is qualitatively different from Kapitza-Dirac diffraction of dressed matter waves from a periodic optical potential. We note that the coherent mixing of states interacting with an external field is often used for the engineering of dressed potentials [11][12][13][14][15][16][17][18][19]. Deviations from adiabaticity have previously been found to have deleterious effects on dressed-state lifetimes [20,21]. In our experiment, we enter the strongly nonadiabatic regime, in which coherent Landau-Zener transitions of the atomic wavefunction between the adiabatic dressed potentials (driven by zero-point motion) induce a strong coupling between internal and external degrees of freedom, leading to a breakdown of the usual BornOppenheimer (adiabatic) approximation [22]. Our experimental procedure, cf. Fig. 1a, uses microwave radiation ...
Precise control of magnetic fields is a frequent challenge encountered in experiments with atomic quantum gases. Here we present a simple method for performing in situ monitoring of magnetic fields that can readily be implemented in any quantum-gas apparatus in which a dedicated field-stabilization approach is not feasible. The method, which works by sampling several Rabi resonances between magnetically field sensitive internal states that are not otherwise used in a given experiment, can be integrated with standard measurement sequences at arbitrary fields. For a condensate of Rb atoms, we demonstrate the reconstruction of Gauss-level bias fields with an accuracy of tens of microgauss and with millisecond time resolution. We test the performance of the method using measurements of slow resonant Rabi oscillations on a magnetic-field sensitive transition and give an example for its use in experiments with state-selective optical potentials.
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