Matthew Boguslawski

Quantum Kibble-Zurek Mechanism in a Spin-1 Bose-Einstein Condensate.

The dynamics of a quantum phase transition are explored using slow quenches from the polar to the broken-axisymmetry phases in a small spin-1 ferromagnetic Bose-Einstein condensate. Measurements of the evolution of the spin populations reveal a power-law scaling of the temporal onset of excitations versus quench speed as predicted from quantum extensions of the Kibble-Zurek mechanism. The satisfactory agreement of the measured scaling exponent with the analytical theory and numerical simulations provides experimental confirmation of the quantum Kibble-Zurek model.

Adiabatic quenches and characterization of amplitude excitations in a continuous quantum phase transition

Spontaneous symmetry breaking occurs in a physical system whenever the ground state does not share the symmetry of the underlying theory, e.g., the Hamiltonian [1, 2]. It gives rise to massless Nambu-Goldstone modes [1, 3] and massive Anderson-Higgs modes [4, 5]. These modes provide a fundamental understanding of matter in the Universe and appear as collective phase/amplitude excitations of an order parameter in a many-body system. The amplitude excitation plays a crucial role in determining the critical exponents governing universal non-equilibrium dynamics in the KibbleZurek mechanism (KZM). Here, we characterize the amplitude excitations in a spin-1 condensate and measure their energy gap for different phases of the quantum phase transition [6, 7]. At the quantum critical point of the transition, finite size effects lead to a non-zero gap. Our measurements are consistent with this prediction, and furthermore, we demonstrate an adiabatic quench through the phase transition, which is forbidden at the mean field level. This work paves the way toward generating entanglement through an adiabatic phase transition [8].Significance Symmetry-breaking phase transitions play important roles in many areas of physics, including cosmology, particle physics, and condensed matter. The freezing of water provides a familiar example: The translational and rotational symmetries of water are reduced upon crystallization. In this work, we investigate symmetry-breaking phase transitions of the magnetic properties of an ultracold atomic gas in the quantum regime. We measure the excitations of the quantum magnets in different phases and show that the excitation energy (gap) remains finite at the phase transition. We exploit the nonzero gap to demonstrate an adiabatic (reversible) quench across the phase transition. Adiabatic quantum quenches underlie proposals for generating massively entangled spin states and are fundamental to the ideas of adiabatic quantum computation. Spontaneous symmetry breaking occurs in a physical system whenever the ground state does not share the symmetry of the underlying theory, e.g., the Hamiltonian. This mechanism gives rise to massless Nambu–Goldstone modes and massive Anderson–Higgs modes. These modes provide a fundamental understanding of matter in the Universe and appear as collective phase or amplitude excitations of an order parameter in a many-body system. The amplitude excitation plays a crucial role in determining the critical exponents governing universal nonequilibrium dynamics in the Kibble–Zurek mechanism (KZM). Here, we characterize the amplitude excitations in a spin-1 condensate and measure the energy gap for different phases of the quantum phase transition. At the quantum critical point of the transition, finite-size effects lead to a nonzero gap. Our measurements are consistent with this prediction, and furthermore, we demonstrate an adiabatic quench through the phase transition, which is forbidden at the mean field level. This work paves the way toward generating entanglement through an adiabatic phase transition.