It is a universal empirical observation that socks become unpaired in the laundry. We propose a quasiparticle theory of sock dynamics in which individual socks are modelled as bosonic excitations of the agitated laundry condensate. The sock dispersion relation is material-dependent: nondispersive materials retain their shape, while dispersive materials give rise to the well-documented phenomenon of sock shrinkage. In the convex regions of the dispersive spectrum, socks undergo Beliaev decay and spontaneously split into two lower-momentum socks, while in the concave regions the dominant process is Landau-Khalatnikov scattering, which degrades socks into lint and loose threads. In addition, the rotating drum creates sock-antisock pairs from the laundry vacuum via the dynamical Casimir effect. The coexistence of these creation and destruction channels gives rise to a fundamental ambiguity: an unpaired sock at the end of a wash cycle is equally consistent with the destruction of its partner or the spontaneous creation of an entirely new sock.
It is a universal empirical observation that socks become unpaired in the laundry. We propose a quasiparticle theory of sock dynamics in which individual socks are modelled as bosonic excitations of the agitated laundry condensate. The sock dispersion relation is material-dependent: nondispersive materials retain their shape, while dispersive materials give rise to the well-documented phenomenon of sock shrinkage. In the convex regions of the dispersive spectrum, socks undergo Beliaev decay and spontaneously split into two lower-momentum socks, while in the concave regions the dominant process is Landau-Khalatnikov scattering, which degrades socks into lint and loose threads. In addition, the rotating drum creates sock-antisock pairs from the laundry vacuum via the dynamical Casimir effect. The coexistence of these creation and destruction channels gives rise to a fundamental ambiguity: an unpaired sock at the end of a wash cycle is equally consistent with the destruction of its partner or the spontaneous creation of an entirely new sock.
The description of spontaneous symmetry breaking that underlies the connection between classically ordered objects in the thermodynamic limit and their individual quantum-mechanical building blocks is one of the cornerstones of modern condensed-matter theory and has found applications in many different areas of physics. The theory of spontaneous symmetry breaking, however, is inherently an equilibrium theory, which does not address the dynamics of quantum systems in the thermodynamic limit. Here, we will use the example of a particular antiferromagnetic model system to show that the presence of a so-called thin spectrum of collective excitations with vanishing energy---one of the well-known characteristic properties shared by all symmetry-breaking objects---can allow these objects to also spontaneously break time-translation symmetry in the thermodynamic limit. As a result, that limit is found to be able, not only to reduce quantum-mechanical equilibrium averages to their classical counterparts, but also to turn individual-state quantum dynamics into classical physics. In the process, we find that the dynamical description of spontaneous symmetry breaking can also be used to shed some light on the possible origins of Born's rule. We conclude by describing an experiment on a condensate of exciton polaritons which could potentially be used to experimentally test the proposed mechanism.
We study the effects of quasiparticle interactions in a quasi-two-dimensional (quasi-2D), zero-temperature Bose-Einstein condensate of dipolar atoms, which can exhibit a roton-maxon feature in its quasiparticle spectrum. Our focus is the Beliaev damping process, in which a quasiparticle collides with the condensate and resonantly decays into a pair of quasiparticles. Remarkably, the rate for this process exhibits a highly nontrivial dependence on the quasiparticle momentum and the dipolar interaction strength. For weak interactions, low-energy phonons experience no damping, and higher-energy quasiparticles undergo anomalously weak damping. In contrast, the Beliaev damping rates become anomalously large for stronger dipolar interactions, as rotons become energetically accessible as final states. When the dipoles are tilted off the axis of symmetry, the damping rates acquire an anisotropic character. Surprisingly, this anisotropy does not simply track the anisotropy of the dipolar interactions, rather, the mechanisms for damping are qualitatively modified in the anisotropic case. Our study reveals the unconventional nature of Beliaev damping in dipolar condensates, and has important implications for ongoing studies of equilibrium and nonequilibrium dynamics in these systems. Further, our results are relevant for other 2D superfluids with roton excitations, including spin-orbit-coupled Bose gases, magnon condensates, and 4He films.
We present an analytical model for the theoretical analysis of spin dynamics and spontaneous symmetry breaking in a spinor Bose-Einstein condensate (BEC). This allows for an excellent intuitive understanding of the processes and provides good quantitative agreement with the experimental results of Scherer et al. [Phys. Rev. Lett. 105, 135302 (2010)]. It is shown that the dynamics of a spinor BEC initially prepared in an unstable Zeeman state mF=0 ($\left|0\right\ensuremath{\rangle}$) can be understood by approximating the effective trapping potential for the state $\left|\ifmmode\pm\else\textpm\fi{}1\right\ensuremath{\rangle}$ with a cylindrical box potential. The resonances in the creation efficiency of these atom pairs can be traced back to excitation modes of this confinement. The understanding of these excitation modes allows for a detailed characterization of the symmetry-breaking mechanism, showing how a twofold spontaneous breaking of spatial and spin symmetry can occur. In addition, a detailed account of the experimental methods for the preparation and analysis of spinor quantum gases is given.
Abstract Emergent metastability in non-equilibrium systems is a subject that touches everything from the origins of life1 to the quantum nature of the universe2. In condensed quantum matter, a good topical example is the metastability of topologically inhibited self-organization of electronic domains in a correlated electronic crystal in the aftermath of a symmetry-breaking phase transition3. Understanding the dynamics of such systems is crucial for developing new quantum technologies and exploring fundamental aspects of many-body non-equilibrium quantum mechanics. Here, a non-equilibrium state is set up by carrier injection in the quantum material (M), resulting in a domain structure, whose time-evolution is investigated by scanning tunneling microscopy. Demonstrating a new approach to modelling of emergent non-equilibrium quantum behavior, we use an array of 2008 qubits in a programmable noisy superconducting quantum simulator (NSQS) without error correction (P), in which mutual qubit interconnections correspond directly to the electronic interactions on the triangular lattice of the electronic crystal material. The success of the simulations critically depends on both the faithfulness of the model correspondence between the two systems and careful management of the decoherence pathways. Importantly, decoherence of both P and M is driven by noise with a characteristic 1/ν frequency spectrum. The simulations reveal how an emergent false vacuum state arises, describing the time evolution and temperature dependence of the observed electronic domain dynamics of M solely on the basis of microscopic electronic interactions. The parallel experiments demonstrate the potential of NSQSs for studying emergent non-equilibrium dynamics in complex many-body quantum systems.
The non-equilibrium dynamics of trapped ultracold atomic gases, or mixtures thereof, is an extremely rich subject. Despite 20 years of studies, and remarkable progress mainly on the experimental front, numerous open question remain, related to the growth, relaxation and thermalisation of such systems, and there is still no universally accepted theory for their theoretical description. In this paper we discuss one of the state-of-the-art kinetic approaches, which gives an intuitive picture of the physical processes happening at the microscopic scale, being broadly applicable both below and above the critical region (but not within the critical region itself, where fluctuations become dominant and symmetry breaking takes place). Specifically, the 'Zaremba–Nikuni–Griffin' (ZNG) scheme provides a self-consistent description of the coupling between the condensate and the thermal atoms, including the collisions between these two subsystems. It has been successfully tested against experiments in various settings, including investigation of collective modes (e.g. monopole, dipole and quadrupole modes), dissipation of topological excitations (solitons and vortices) as well as surface evaporative cooling. Here, we show that the ZNG model can capture two important aspects of non-equilibrium dynamics for both single-component and two-component BECs: the Kohn mode (the undamped dipole oscillation independent of interactions and temperature) and (re)thermalisation leading to condensate growth following sudden evaporation. Our simulations, performed in a spherically symmetric trap reveal (i) an interesting two-stage dynamics and the emergence of a prominent monopole mode in the evaporative cooling of a single-component Bose gas, and (ii) the long thermalisation time associated with the sympathetic cooling of a realistic two-component mixture. Related open questions arise about the mechanisms and the nature of thermalisation in such systems, where further controlled experiments are needed for benchmarking.
Abstract We study the interactions among phonons and the phonon lifetime in a pair‐condensed Fermi gas in the BEC‐BCS crossover in the collisionless regime. To compute the phonon‐phonon coupling amplitudes we use a microscopic model based on a generalized BCS Ansatz including moving pairs, which allows for a systematic expansion around the mean field BCS approximation of the ground state. We show that the quantum hydrodynamic expression of the amplitudes obtained by Landau and Khalatnikov apply only on the energy shell, that is for resonant processes that conserve energy. The microscopic model yields the same excitation spectrum as the Random Phase Approximation, with a linear (phononic) start and a concavity at low wave number that changes from upwards to downwards in the BEC‐BCS crossover. When the concavity of the dispersion relation is upwards at low wave number, the leading damping mechanism at low temperature is the Beliaev‐Landau process 2 phonons ↔ 1 phonon while, when the concavity is downwards, it is the Landau‐Khalatnikov process 2 phonons ↔ 2 phonons. In both cases, by rescaling the wave vectors to absorb the dependence on the interaction strength, we obtain a universal formula for the damping rate. This universal formula corrects and extends the original analytic results of Landau and Khalatnikov [ZhETF 19, 637 (1949)] for the 2↔2 processes in the downward concavity case. In the upward concavity case, for the Beliaev 1↔ 2 process for the unitary gas at zero temperature, we calculate the damping rate of an excitation with wave number q including the first correction proportional to q7 to the q5 hydrodynamic prediction, which was never done before in a systematic way.