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arXiv:1404.5685 · Physical Review Letters

Quantum Mass Acquisition in Spinor Bose-Einstein Condensates

Nguyen Thanh Phuc, Yuki Kawaguchi, Masahito Ueda

Published Dec 2, 2014• 7 citations
arXivHTMLPDF

Abstract

Quantum mass acquisition, in which a massless (quasi)particle becomes massive due to quantum corrections, is predicted to occur in several subfields of physics. However, its experimental observation remains elusive since the emergent energy gap is too small. We show that a spinor Bose-Einstein condensate is an excellent candidate for the observation of such a peculiar phenomenon as the energy gap turns out to be 2 orders of magnitude larger than the zero-point energy. This extraordinarily large energy gap is a consequence of the dynamical instability. The propagation velocity of the resultant massive excitation mode is found to be decreased by the quantum corrections as opposed to phonons.

DOI
10.1103/physrevlett.113.230401

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Quantum mass acquisition, in which a massless (quasi)particle becomes massive due to quantum corrections, is predicted to occur in several subfields of physics. However, its experimental observation remains elusive since the emergent energy gap is too small. We show that a spinor Bose-Einstein condensate is an excellent candidate for the observation of such a peculiar phenomenon as the energy gap turns out to be 2 orders of magnitude larger than the zero-point energy. This extraordinarily large energy gap is a consequence of the dynamical instability. The propagation velocity of the resultant massive excitation mode is found to be decreased by the quantum corrections as opposed to phonons.
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In this paper, we bring out several potential signatures of negative mass regimes while investigating an expanding spin-orbit-coupled spinor F=1F=1F=1 Bose-Einstein condensate by analyzing the dispersion relation of the single-particle quantum system. In spin-orbit-coupled spinor condensates, a negative-mass parameter generates a wave packet that propagates in the opposite direction of the momentum. We analyze the dynamics of spin waves analytically and present a simple approach to investigate the expansion of spinor condensates. In particular, we examine the dynamics when both masses are negative, which results in the spinor condensates splitting into two counterpropagating self-interfering packets. Using numerical simulations of the coupled Gross-Pitaevskii equations, we demonstrate the density expansion and self-interference patterns with and without magnetization for repulsive and attractive interactions with different coupling parameters. The highlight of our investigation is that we are able to unearth several phenomena observed in experiments, such as self-interfering packets, pileup, modulation instability, slow down, self-trapping, and gap solitons. In particular, the gap soliton exists at the gap created by the intersection of two negative masses.
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{m}_{F}=0mF​=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.
The Scharnhorst effect in quantum electrodynamics predicts a superluminal velocity shift δc>0δc>0δc>0 at low photon frequencies due to vacuum fluctuations between two mirrors. Although theoretically robust (no causality violation), the effect is extraordinarily small (δc/c≈10−32δc/c\approx 10^{-32}δc/c≈10−32 for mirrors 1 μm1\,μ{\rm m}1μm apart) and outside current experimental reach. Here we propose a condensed matter analogue, replacing the photons in the electromagnetic vacuum by phonon excitations of a Bose-Einstein condensate. In an idealized geometry, the two-dimensional surface of a cylinder (circumference LLL) at zero temperature, the axial speed of sound is increased due to zero-point fluctuations of the condensate by δc/c≃5×10−3 g~δc/c\simeq 5\times 10^{-3}\,\tilde{g}δc/c≃5×10−3g~​ at L≃2 ξL\simeq 2\,ξL≃2ξ, to first order in the dimensionless interaction constant g~≃10−1\tilde{g}\simeq 10^{-1}g~​≃10−1 of the Bose gas of healing length ξ≃1 μmξ\simeq 1\,μ{\rm m}ξ≃1μm. This is 28 orders of magnitude larger than the photonic effect, basically because phonons have a self-interaction while photons only interact via massive electrons. We discuss the prospects for observation of the acoustic Scharnhorst effect, the main restriction being the low-temperature requirement.