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Physical Review Letters

Quantum Coherence in a Superfluid Josephson Junction

Supradeep Narayana, Yuki Sato

Published Feb 2, 2011• 8 citations
Publisher

Abstract

We report a new kind of experiment in which we take an array of nanoscale apertures that form a superfluid (4)He Josephson junction and apply quantum phase gradients directly along the array. We observe collective coherent behaviors from aperture elements, leading to quantum interference. Connections to superconducting and Bose-Einstein condensate Josephson junctions as well as phase coherence among the superfluid aperture array are discussed.

DOI
10.1103/physrevlett.106.055302

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We report a new kind of experiment in which we take an array of nanoscale apertures that form a superfluid (4)He Josephson junction and apply quantum phase gradients directly along the array. We observe collective coherent behaviors from aperture elements, leading to quantum interference. Connections to superconducting and Bose-Einstein condensate Josephson junctions as well as phase coherence among the superfluid aperture array are discussed.
We report on the direct observation of an oscillating atomic current in a one-dimensional array of Josephson junctions realized with an atomic Bose-Einstein condensate. The array is created by a laser standing wave, with the condensates trapped in the valleys of the periodic potential and weakly coupled by the interwell barriers. The coherence of multiple tunneling between adjacent wells is continuously probed by atomic interference. The square of the small-amplitude oscillation frequency is proportional to the microscopic tunneling rate of each condensate through the barriers and provides a direct measurement of the Josephson critical current as a function of the intermediate barrier heights. Our superfluid array may allow investigation of phenomena so far inaccessible to superconducting Josephson junctions and lays a bridge between the condensate dynamics and the physics of discrete nonlinear media.
Arrays of nanoscale apertures have been found to exhibit fascinating quantum phenomena such as the Josephson effect and collective quantized phase-slippage. To be in the Josephson regime, the aperture size must be comparable to the healing length of the 4He order parameter. For 50nm apertures this regime is attained ∼mK below T λ . Collective phase slippage occurs at considerably lower temperatures. Fabricating aperture arrays with appropriate properties (strength, temporal stability and reproducibility) at these length scales has been a long-standing goal for our group. Here, we present the techniques used thus far, based on recent work performed at the Cornell Nanoscale Facility. We discuss some issues that arise and their possible solutions.
Abstract The Josephson effect, an important quantum supercurrent phenomenon, has been extensively studied in superconductors and superfluids. In this paper, we investigate the rich physics of one-dimensional Josephson junctions in a red-detuned optical lattice with sodium (Na) quantum gas. A one-dimensional Josephson array is formed by setting up an optical lattice using a red-detuned laser. By characterizing the dependence of Josephson oscillations of the lattice depth, we experimentally demonstrate the Josephson current. The lattice depth is controlled by altering the lattice power, and our observations are consistent with theoretical predictions. These findings offer valuable insights into quantum coherent transport and the intricate dynamics inherent to superfluidity.
We report on the first realization of a single bosonic Josephson junction, implemented by two weakly linked Bose-Einstein condensates in a double-well potential. In order to fully investigate the nonlinear tunneling dynamics we measure the density distribution in situ and deduce the evolution of the relative phase between the two condensates from interference fringes. Our results verify the predicted nonlinear generalization of tunneling oscillations in superconducting and superfluid Josephson junctions. Additionally, we confirm a novel nonlinear effect known as macroscopic quantum self-trapping, which leads to the inhibition of large amplitude tunneling oscillations.
Experiments with small capacitance Josephson junction arrays are described that demonstrate the quantum behavior of the phase of the superconducting condensate. This quantum behavior is based on the complementarity of phase and number for a condensate state of many bosons. A key factor to observation of this quantum behavior is the coupling of the superconducting condensate state to dissipation in the electrodynamic environment of the Josephson junction. It is shown how one-dimensional Josephson junction arrays can be used to design an environment with very weak coupling to dissipation, allowing for measurement of condensate with well defined number. The well defined number is manifest in the Coulomb blockade of Cooper pair tunneling.