Atomtronics has the potential for engineering new types of functional devices, such as Josephson junctions (JJs). Previous studies have mainly focused on JJs whose ground states have zero or $\ensuremath{\pi}$ superconducting phase difference across the junctions, while arbitrary phase-tunable JJs may have important applications in superconducting electronics and quantum computation. Here we show that a phase-tunable JJ can be implemented in a spin-orbit-coupled cold atomic gas with the magnetic tunneling barrier generated by a spin-dependent focused laser beam. We consider the JJ confined in either a linear harmonic trap or a circular ring trap. In the ring trap, the magnetic barrier induces a spontaneous mass current for the ground state of the JJ, demonstrating the magnetoelectric effects of cold atoms.
Atomtronics has the potential for engineering new types of functional devices, such as Josephson junctions (JJs). Previous studies have mainly focused on JJs whose ground states have zero or \ensuremathπ superconducting phase difference across the junctions, while arbitrary phase-tunable JJs may have important applications in superconducting electronics and quantum computation. Here we show that a phase-tunable JJ can be implemented in a spin-orbit-coupled cold atomic gas with the magnetic tunneling barrier generated by a spin-dependent focused laser beam. We consider the JJ confined in either a linear harmonic trap or a circular ring trap. In the ring trap, the magnetic barrier induces a spontaneous mass current for the ground state of the JJ, demonstrating the magnetoelectric effects of cold atoms.
Josephson junctions (JJs) and junction arrays are well-studied devices in superconductivity. With external magnetic fields one can modulate the phase in a long junction and create traveling, solitonic waves of magnetic flux, called fluxons. Today, it is also possible to devise two different types of junctions: depending on the sign of the critical current density , they are called 0- or π-junctions. In turn, a 0–π junction is formed by joining two of these junctions. As a result, one obtains a pinned Josephson vortex of fractional magnetic flux, at the 0–π boundary. Here, we analyze this arrangement of superconducting junctions in the context of an atomic bosonic quantum gas, where two-state atoms in a double well trap are coupled in an analogous fashion. There, an all-optical 0–π JJ is created by the phase of a complex valued Rabi frequency and we derive a discrete four-mode model for this situation, which qualitatively resembles a semifluxon.
We theoretically investigate a possible spontaneous current state in a polarized superfluid Fermi gas confined in a toroidal trap. When one puts a weak nonmagnetic potential barrier in this system, this barrier is known to be magnetized in the sense that some of excess ↑-spin atoms are localized around it (where the ↑-spin describes the majority component of the polarized Fermi gas). Using this unique property, we show that this magnetized barrier, or the pseudo-ferromagnetic junction, induces a spontaneous current circulating along the toroidal trap. While the ordinary supercurrent state appears as a metastable state, this spontaneous current state is realized as the most stable state, originating from the phase twist of the superfluid order parameter by the magnetized potential barrier.
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.
We discuss an idea to realize a spontaneous current in a superfluid Fermi gas. When a polarized Fermi superfluid (${N}_{\ensuremath{\uparrow}}>{N}_{\ensuremath{\downarrow}}$, where ${N}_{\ensuremath{\sigma}}$ is the number of atoms in the hyperfine state described by pseudospin $\ensuremath{\sigma}=\ensuremath{\uparrow},\ensuremath{\downarrow}$) is loaded onto a ring-shaped trap with a weak potential barrier, some excess atoms ($\ensuremath{\Delta}N={N}_{\ensuremath{\uparrow}}\ensuremath{-}{N}_{\ensuremath{\downarrow}}$) are localized around the barrier. As shown in our previous paper [T. Kashimura, S. Tsuchiya, and Y. Ohashi, Phys. Rev. A 82, 033617 (2010)], this polarized potential barrier works as a $\ensuremath{\pi}$ junction in the sense that the superfluid order parameter changes its sign across the barrier. Because of this, the phase of the superfluid order parameter outside the junction is shown to be twisted by $\ensuremath{\pi}$ along the ring, which naturally leads to a circulating supercurrent. While the ordinary supercurrent state is obtained as a metastable state, this spontaneous current state is shown to be more stable than the case with no current. Our results indicate that localized excess atoms would be useful for the manipulation of the superfluid order parameter in cold Fermi gases.
Atomtronics, a technique to control atoms for an application of devices, has been studied in recent years.The purpose of this thesis is to propose a novel method to control atoms for the application to the atomtronics.To achieve this purpose, we use a spin current in a spin-orbit (SO) coupled cold atoms.The spin current, that is a counterflow of particles with different spins, has attracted attention as a method to control electrons.On the other hand, the artificial SO coupled cold atoms were experimentally realized in 2011 and have been studied theoretically and experimentally.We can expect that a spin current can be formed in the SO coupled cold atoms because the SO coupling is known to induce a spin current and controlled because the cold atoms have various experimentally controllable parameters.However, little has been studied so far on the spin current in the cold atom.Therefore, we studied novel controllability of the spin current in the SO coupled cold atom.An SO coupled Bose-Einstein condensate (BEC) was experimentally realized in 2011 and then an SO coupled degenerate fermions became available in 2012.In these systems, since the SO coupling was engineered in a neutral atom by dressing two atomic spin states with a pair of Raman lasers and an external magnetic field, it is called artificial SO coupling.In addition, some parameters such as intensity of the SO coupling can be experimentally tuned by the lasers and the external magnetic field in the artificial SO coupled cold atoms.Therefore, by investigating parameter dependence of the spin currents, we reveal controllability of the atoms in the artificial SO coupled Bose and Fermi systems.As a result, in the SO coupled Bose system, the spin current changes discontinuously and non-smoothly across the phase boundaries, while in the SO coupled Fermi system, the spin current changes continuously and smoothly at zero temperature.On the other hand, Bose-Fermi mixture was experimentally realized and have been studied from different viewpoints.Therefore, we can expect to realize an artificial SO coupled Bose-Fermi mixture in near future.In this system, since atoms of different quantum statistics coexist under an influence of the SO coupling, novel phenomena can be expected.Based on these ideas, we investigate a ground state and a spin current in a mixture of fermions and SO coupled bosons.The ground states are calculated in the SO coupled Bose-Fermi mixture with high density or low density of the atoms by using a variational method and some approximations such as tight-binding approximation.For the high density, the fermions have a large influence on the Bose system, while, for the low density, the fermions have little influence although the relative amounts of the bosons and the fermions are comparable in the two cases.In addition, we show that the spin current of the i
A Josephson junction (JJ) is a key device for developing superconducting circuits, wherein a supercurrent in the JJ is controlled by the phase difference between the two superconducting electrodes. When two JJs sharing one superconducting electrode are coherently coupled and form the Andreev molecules, a supercurrent of one JJ is expected to be nonlocally controlled by the phase difference of another JJ. Here, we evaluate the supercurrent in one of the coupled two JJs as a function of local and nonlocal phase differences. Consequently, the results exhibit that the nonlocal phase control generates a finite supercurrent even when the local phase difference is zero. In addition, an offset of the local phase difference giving the JJ ground state depends on the nonlocal phase difference. These features demonstrate the anomalous Josephson effect realized by the nonlocal phase control. Our results provide a useful concept for engineering superconducting devices such as phase batteries and dissipationless rectifiers.