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Chinese Physics B

Observation of Josephson effect in 23Na spinor Bose–Einstein condensates

Yong 永 Qin 秦, Xin Wang, Jun Jian, Wenliang Liu, Jizhou Wu, Yuqing Li, Jie Ma, Liantuan Xiao, Suotang Jia

Published Dec 31, 2024• 1 citation
Publisher

Abstract

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.

DOI
10.1088/1674-1056/ada42f

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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 explore the dynamics of a Bose–Einstein condensate created in the combined potential of a far-detuned laser standing wave superimposed to a 3D harmonic magnetic potential. We report the investigation of low-lying collective modes showing that the macroscopic dynamics along the optical lattice is strongly modified, resulting in a shift of the dipole and quadrupole mode frequencies depending on the height of the optical lattice, whereas the transverse breathing mode, occurring perpendicularly to the lattice axis, is not perturbed. The experimental findings are compared with the theoretical treatment that generalizes the hydrodynamic equation of superfluids for a weakly interacting Bose gas to include the effects of the periodic potential. We show that the array of condensates trapped in the optical wells and driven by the harmonic magnetic potential is equivalent to an array of Josephson junctions. In the regime of ‘small’ amplitude dipole oscillation the system performs a collective motion and we investigate the current–phase dynamics measuring the critical Josephson current. Increasing the amplitude of the dipole oscillation, we observe a transition from the coherent oscillation (superfluid regime) to a localization of the condensates in the harmonic trap (‘insulator’ regime). The onset of the coherent regime breakdown is interpreted as the result of a discrete modulational instability occurring when the velocity of the centre of mass of the system is larger than a critical velocity proportional to the tunnelling rate between adjacent wells.
Bose–Einstein condensates (BEC) of sodium atoms are transferred into one-dimensional (1D) optical lattice potentials, formed by two laser beams with a wavelength of 1064 nm, in a shallow optical trap. The phase coherence of the condensate in the lattice potential is studied by changing the lattice depth. A qualitative change in behavior of the BEC is observed at a lattice depth of ∼ 13.7 E r, where the quantum gas undergoes a transition from a superfluid state to a state that lacks well-to-well phase coherence.
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.
Josephson junctions are essential ingredients in the superconducting circuits used in many existing quantum technologies. Additionally, ultracold atomic quantum gases have also become essential platforms to study superfluidity. Here, we explore the analogy between superconductivity and superfluidity to present an intriguing effect caused by a thin finite barrier in a quasi-one-dimensional toroidal spinor Bose–Einstein condensate (BEC). In this system, the atomic current density flowing through the edges of the barrier oscillates, such as the electrical current through a Josephson junction in a superconductor, but in our case, there is no current circulation through the barrier. We also show how the nontrivial broken-symmetry states of spinor BECs change the structure of this Josephson-like current, creating the possibility to probe the spinor symmetry, solely using measurements of this superfluid current.
The Josephson effect is a prominent phenomenon of quantum supercurrents that has been widely studied in superconductors and superfluids. Typical Josephson junctions consist of two real-space superconductors (superfluids) coupled through a weak tunneling barrier. Here we propose a momentum-space Josephson junction in a spin-orbit coupled Bose-Einstein condensate, where states with two different momenta are coupled through Raman-assisted tunneling. We show that Josephson currents can be induced not only by applying the equivalent of "voltages," but also by tuning tunneling phases. Such tunneling-phase-driven Josephson junctions in momentum space are characterized through both full mean field analysis and a concise two-level model, demonstrating the important role of interactions between atoms. Our scheme provides a platform for experimentally realizing momentum-space Josephson junctions and exploring their applications in quantum-mechanical circuits.