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Volkoff, Tyler,Kwon, Yongkyung,Whaley, K. Birgitta American Physical Society 2016 Physical Review B Vol.94 No.14
<P>We present a path integral Monte Carlo study of the global superfluid fraction and local superfluid density in cylindrically symmetric reservoirs of liquid He-4 separated by nanoaperture arrays. The superfluid response to both translations along the axis of symmetry (longitudinal response) and rotations about the cylinder axis (transverse response) are computed, together with radial and axial density distributions that reveal the microscopic inhomogeneity arising from the combined effects of the confining external potential and the He-4-He-4 interatomic potentials. We make a microscopic determination of the length scale of decay of superfluidity at the radial boundaries of the system by analyzing the local superfluid density distribution to extract a displacement length that quantifies the superfluid mass displacement away from the boundary. We find that the longitudinal superfluid response is reduced in reservoirs separated by a septum containing sufficiently small apertures compared to a cylinder with no intervening aperture array, for all temperatures below T-lambda. For a single aperture in the septum, a significant drop in the longitudinal superfluid response is seen when the aperture diameter is made smaller than twice the empirical temperature-dependent He-4 healing length, consistent with the formation of a weak link between the reservoirs. Increasing the diameter of a single aperture or the number of apertures in the array results in an increase of the superfluid density toward the expected bulk value.</P>
Amplification of the quantum superposition macroscopicity of a flux qubit by a magnetized Bose gas
Volkoff, T. J.,Fischer, Uwe R. American Physical Society 2016 Physical Review A Vol.94 No.4
<P>We calculate a measure of superposition macroscopicity M for a superposition of screening current states in a superconducting flux qubit (SFQ), by relating M to the action of an instanton trajectory connecting the potential wells of the flux qubit. When a magnetized Bose-Einstein condensed (BEC) gas containing N-B similar to O (10(6)) atoms is brought into a O(1) mu m proximity of the flux qubit in an experimentally realistic geometry, we demonstrate the appearance of a twofold to fivefold amplification of M over the bare value without the BEC, by calculating the instanton trajectory action from the microscopically derived effective flux Lagrangian of a hybrid quantum system composed of the flux qubit and a spin-F atomic Bose gas. Exploiting the connection between M and the maximal metrological usefulness of a multimode superposition state, we show that amplification of M in the ground state of the hybrid system is equivalent to a decrease in the quantum Cramer-Rao bound for estimation of an externally applied flux. Our result therefore demonstrates the increased usefulness of the BEC-SFQ hybrid system as a sensor of ultraweak magnetic fields below the standard quantum limit.</P>
Quantum sine-Gordon dynamics on analogue curved spacetime in a weakly imperfect scalar Bose gas
Volkoff, T. J.,Fischer, Uwe R. American Physical Society 2016 Physical Review D Vol.94 No.2
<P>Using the coherent state functional integral expression of the partition function, we show that the sine-Gordon model on an analogue curved spacetime arises as the effective quantum field theory for phase fluctuations of a weakly imperfect Bose gas on an incompressible background superfluid flow when these fluctuations are restricted to a subspace of the single-particle Hilbert space. We consider bipartitions of the single-particle Hilbert space relevant to experiments on ultracold bosonic atomic or molecular gases, including, e.g., restriction to high-or low-energy sectors of the dynamics and spatial bipartition corresponding to tunnel-coupled planar Bose gases. By assuming full unitary quantum control in the low-energy subspace of a trapped gas, we show that (1) appropriately tuning the particle number statistics of the lowest-energy mode partially decouples the low-and high-energy sectors, allowing any low-energy single-particle wave function to define a background for sine-Gordon dynamics on curved spacetime and (2) macroscopic occupation of a quantum superposition of two states of the lowest two modes produces an analogue curved spacetime depending on two background flows, with respective weights continuously dependent on the corresponding weights of the superposed quantum states.</P>