The dynamics of many-body systems spanning condensed matter, cosmology, and beyond are hypothesized to be universal when the systems cross continuous phase transitions. The universal dynamics are expected to satisfy a scaling symmetry of space and time with the crossing rate, inspired by the Kibble-Zurek mechanism. We test this symmetry based on Bose condensates in a shaken optical lattice. Shaking the lattice drives condensates across an effectively ferromagnetic quantum phase transition. After crossing the critical point, the condensates manifest delayed growth of spin fluctuations and develop antiferromagnetic spatial correlations resulting from the sub-Poisson distribution of the spacing between topological defects. The fluctuations and correlations are invariant in scaled space-time coordinates, in support of the scaling symmetry of quantum critical dynamics.
The dynamics of many-body systems spanning condensed matter, cosmology, and beyond are hypothesized to be universal when the systems cross continuous phase transitions. The universal dynamics are expected to satisfy a scaling symmetry of space and time with the crossing rate, inspired by the Kibble-Zurek mechanism. We test this symmetry based on Bose condensates in a shaken optical lattice. Shaking the lattice drives condensates across an effectively ferromagnetic quantum phase transition. After crossing the critical point, the condensates manifest delayed growth of spin fluctuations and develop antiferromagnetic spatial correlations resulting from the sub-Poisson distribution of the spacing between topological defects. The fluctuations and correlations are invariant in scaled space-time coordinates, in support of the scaling symmetry of quantum critical dynamics.
In recent years there has been an explosion of interest in the field of quantum many-body physics. Understanding the complex and often unintuitive behavior of systems containing interacting quantum constituents is not only fascinating but also crucial for developing the next generation of quantum technology, including better materials, sensors, and computers. Yet understanding such systems remains a challenge, particularly when considering the dynamics which occur when they are excited far from equilibrium. Ultracold atomic gases provide an ideal system with which to study dynamics by enabling clean, well-controlled experiments at length- and time-scales which allow us to observe the dynamics directly. ,This thesis describes experiments on the many-body dynamics of ultracold, bosonic cesium atoms. Our apparatus epitomizes the versatility of ultracold atoms by providing extensive control over the quantum gas. In particular, we will discuss our use of a digital micromirror device to project arbitrary, dynamic external potentials onto the gas; our development of a powerful new scheme for optically controlling Feshbach resonances to enable spatiotemporal control of the interactions between atoms; and our use of near-resonant shaking lattices to modify the kinetic energy of atoms.,Taking advantage of this flexible apparatus, we have been able to test a longstanding conjecture based on the Kibble-Zurek mechanism, which says that the dynamics of a system crossing a quantum phase transition should obey a universal scaling symmetry of space and time. After accounting for this scaling symmetry, critical dynamics would be essentially independent of the rate at which a system crossed a phase transition. We tested the universal scaling of critical dynamics by using near-resonant shaking to drive Bose-Einstein condensates across an effectively ferromagnetic quantum phase transition. After crossing the phase transition, condensates divide themselves spatially into domains with finite quasimomentum. We measured the growth of these domains over time and the correlation functions describing their spatial distribution by directly reconstructing the quasimomentum distribution. We observed the expected scaling laws across more than an order of magnitude in the crossing rate, aside from which the observed critical dynamics were indeed independent of the crossing rate. These experiments provide strong support for the universal scaling symmetry of space and time and the extension of the Kibble-Zurek mechanism to quantum phase transitions.,We also present the first observation of Bose Fireworks: the sudden emission of many bright, narrow jets of atoms from condensates with oscillating interaction strength. Even though the underlying inelastic s-wave collisions induced by oscillating interactions are isotropic, the collective nature of collisions in the condensate causes the outgoing bosonic atoms to bunch into narrow jets in the horizontal plane. This bunching results from runaway stimulated collisions, which we find can only occur above a threshold oscillation amplitude. The observed atom number in the jets suggests that they are seeded by quantum fluctuations. Moreover, in azimuthal correlation functions we observe forward correlations consistent with theory, which saturate the limit from the uncertainty principle. We also observe partial correlation between counterpropagating jets. Bose Fireworks provide a well-controlled platform for understanding the diverse class of systems in which a coherent source rapidly emits pairs of counterpropagating bosons.
We investigate the universal dynamics of a continuous magnetic phase transition from a unmagnetized phase to a ferromagnetic phase in a spin-1 spin-orbit-coupled Bose-Einstein condensate. When the system approaches the critical point, the Landau critical velocity gradually decreases to zero and so the correlation length diverges. Therefore, during the slow passage through the critical point, ferromagnetic domains are spontaneously created according to the Kibble-Zurek mechanism. Through calculating the Bogoliubov excitations, we give the Landau critical velocity and the correlation length, from which we derive the Kibble-Zurek scalings. Furthermore, we numerically simulate the critical spatial-temporal dynamics of the bifurcation delay and spontaneous domain formation, and extract the universal scaling exponents. The numerical scalings extracted from the critical spatial-temporal dynamics are very consistent with the scalings derived from the correlation length.
Universal dynamics of spontaneous symmetry breaking is central to understanding the universal behavior of spontaneous defect formation in various systems from the early universe, condensed-matter systems to ultracold atomic systems. We explore the universal real-time dynamics in an array of coupled binary atomic Bose-Einstein condensates in optical lattices, which undergo a spontaneous symmetry breaking from the symmetric Rabi oscillation to the broken-symmetry self-trapping. In addition to Goldstone modes, there exist gapped Higgs modes whose excitation gap vanishes at the critical point. In the slow passage through the critical point, we analytically find that the symmetry-breaking dynamics obeys the Kibble-Zurek mechanism. From the scalings of bifurcation delay and domain formation, we numerically extract two Kibble-Zurek exponents, b 1 = ν / ( 1 + ν z ) and b 2 = 1 / ( 1 + ν z ) , which give the static correlation-length critical exponent ν and the dynamic critical exponent z . Our approach provides an efficient way for the simultaneous determination of the critical exponents ν and z for a continuous phase transition.
Universal dynamics of spontaneous symmetry breaking is central to understanding the universal behavior of spontaneous defect formation in various system from the early universe, condensed-matter systems to ultracold atomic systems. We explore the universal real-time dynamics in an array of coupled binary atomic Bose-Einstein condensates in optical lattices, which undergo a spontaneous symmetry breaking from the symmetric Rabi oscillation to the broken-symmetry self-trapping. In addition to Goldstone modes, there exist gapped Higgs mode whose excitation gap vanishes at the critical point. In the slow passage through the critical point, we analytically find that the symmetry-breaking dynamics obeys the Kibble-Zurek mechanism. From the scalings of bifurcation delay and domain formation, we numerically extract two Kibble-Zurek exponents b1=ν/(1+νz) and b2=1/(1+νz), which give the static correlation-length critical exponent ν and the dynamic critical exponent z. Our approach provides an efficient way to simultaneous determination of the critical exponents ν and z for a continuous phase transition.
Abstract Symmetry-breaking quantum phase transitions lead to the production of topological defects or domain walls in a wide range of physical systems. In second-order transitions, these exhibit universal scaling laws described by the Kibble–Zurek mechanism, but for first-order transitions a similarly universal approach is still lacking. Here, we propose a spinor Bose–Einstein condensate as a testbed system where critical scaling behaviour in a first-order quantum phase transition can be understood from generic properties. We demonstrate the applicability of the Kibble–Zurek mechanism for this transition to determine the critical exponents for: (1) the onset of the decay of the metastable state on short times scales, and (2) the number of resulting phase-separated ferromagnetic domains at longer times, as a one-dimensional spin-1 condensate is ramped across a first-order quantum phase transition. The predictions are in excellent agreement with mean-field numerical simulations and provide a paradigm for studying the decay of metastable states in experimentally accessible systems.
We study the equilibrium and dynamical properties of a ferromagnetic spinor atomic Bose-Einstein condensate. In the vicinity of the critical point for a continuous quantum phase transition, universal behaviors are observed both in the equilibrium state and in the dynamics when the quadratic Zeeman shift is swept linearly. Three distinct dynamical regions are identified for different sweeping time scales ($\ensuremath{\tau}$) when compared to the time scale ${\ensuremath{\tau}}_{\mathrm{KZ}}\ensuremath{\sim}{N}^{(1+\ensuremath{\nu}z)/\ensuremath{\nu}d}$ decided by external driving in a system with finite size N ($\ensuremath{\nu},z$ are critical exponents and d the dimensionality). They are manifested by the excitation probability P and the excess heat density Q. The adiabatic region of $\mathcal{P}\ensuremath{\sim}\mathcal{Q}\ensuremath{\sim}{\ensuremath{\tau}}^{\ensuremath{-}2}$ follows from the adiabatic perturbation theory when $\ensuremath{\tau}>{\ensuremath{\tau}}_{\mathrm{KZ}}$, while the nonadiabatic universal region of $\mathcal{P}\ensuremath{\sim}\mathcal{Q}\ensuremath{\sim}{\ensuremath{\tau}}^{\ensuremath{-}1}$ in the thermodynamic limit is described by the Kibble-Zurek mechanism when ${\ensuremath{\tau}}_{\mathrm{KZ}}>\ensuremath{\tau}>$ the time scale given by initial gap. The Kibble-Zurek scaling hypothesis is augmented with finite-size scaling in the latter region and several experimentally falsifiable features for the finite system we consider are predicted. The region of the fastest sweeping is found to be nonuniversal and far from equilibrium, with P and Q essentially being constants independent of $\ensuremath{\tau}$.