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arXiv:2001.09828 · Physical Review Research

Controllable Josephson junction for photon Bose-Einstein condensates

Mario Vretenar, Ben Kassenberg, Shivan Bissesar, Chris Toebes, Jan Klaers

Published Jun 1, 2021• 35 citations
arXivHTMLPDF

Abstract

Josephson junctions are the basis for the most sensitive magnetic flux detectors, the definition of the unit volt by the Josephson voltage standard, and superconducting digital and quantum computing.They result from the coupling of two coherent quantum states, as they occur in superconductors, superfluids, atomic Bose Einstein Condensates (BECs), and exciton-polariton condensates.In their ground state, Josephson junctions are characterized by an intrinsic phase jump.Controlling this phase jump is fundamental for applications in computing.Here, we experimentally demonstrate controllable phase relations between photon BECs resulting from particle exchange in a thermo-optically tunable potential landscape.Our experiment realizes an optical analog of a controllable 0,π -Josephson junction.By connecting several junctions, we can study a reconfigurable 4-condensate system demonstrating the potential of our approach for analog spin-glass simulation.More generally, the combination of static and dynamic nanostructuring techniques introduced in our work offers a powerful platform for the implementation of adaptive optical systems for paraxial light in and outside of thermal equilibrium.

DOI
10.1103/physrevresearch.3.023167

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Josephson junctions are the basis for the most sensitive magnetic flux detectors, the definition of the unit volt by the Josephson voltage standard, and superconducting digital and quantum computing.They result from the coupling of two coherent quantum states, as they occur in superconductors, superfluids, atomic Bose Einstein Condensates (BECs), and exciton-polariton condensates.In their ground state, Josephson junctions are characterized by an intrinsic phase jump.Controlling this phase jump is fundamental for applications in computing.Here, we experimentally demonstrate controllable phase relations between photon BECs resulting from particle exchange in a thermo-optically tunable potential landscape.Our experiment realizes an optical analog of a controllable 0,π -Josephson junction.By connecting several junctions, we can study a reconfigurable 4-condensate system demonstrating the potential of our approach for analog spin-glass simulation.More generally, the combination of static and dynamic nanostructuring techniques introduced in our work offers a powerful platform for the implementation of adaptive optical systems for paraxial light in and outside of thermal equilibrium.
Josephson junctions are the basis for many important fields, such as ultrafast electronics with magnetic flux quanta and superconducting quantum computing. The physical predictions of Josephson junctions are highly universal and can be observed in systems as diverse as coupled superconductors, atomic Bose-Einstein condensates, and others. We experimentally demonstrate tunable tunneling between two photon Bose-Einstein condensates by a targeted shaping of the potential landscape acting on the photons during the tunneling process. The investigated device realizes an optical analogue of a 0,π-Josephson junction, which can act as a building block for an ultrafast all-optical spin glass simulator. The potential landscape in our photon Bose-Einstein condensate Josephson junctions is realized by a combination of direct laser writing for permanent mirror nanostructuring, and heating a thermosensitive polymer for runtime fine tuning of couplings.
In this thesis, I present the first experimental implementation of a Josephson junction for Bose-Einstein condensates. The weak link between two BECs constitutes the nonlinear generalization of the well known Josephson junction of weakly-coupled superconductors that are separated by a thin insulating barrier. In our experiment, the required overlap of two macroscopic wavefunctions is provided by loading a BEC into an optical double well potential. It is realized by a superposition of a one-dimensional optical lattice with a focused laser beam optical dipole trap. The tunneling dynamics between the two potential wells exhibits two distinct dynamical regimes. For small initial population imbalances of the two wells we observe nearly sinusoidal Josephson tunneling oscillations, which are characterized by an oscillating population and relative phase. The situation changes drastically, if the initial population imbalance is chosen above a critical value. In this case, resonant tunneling between the two wells is prohibited because the difference between the on-site particle interaction energies in the two wells exceeds the tunneling energy splitting. As a consequence, the atomic distribution becomes self-locked and the relative phase evolves unbound in time. This regime of prohibited tunneling, which has no analogon in superconducting Josephson junctions, is called “macroscopic quantum self-trapping”.
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 report the creation of a pair of Josephson junctions on a toroidal dilute gas Bose-Einstein condensate (BEC), a configuration that is the cold atom analog of the well-known dc superconducting quantum interference device (SQUID). We observe Josephson effects, measure the critical current of the junctions, and find dynamic behavior that is in good agreement with the simple Josephson equations for a tunnel junction with the ideal sinusoidal current-phase relation expected for the parameters of the experiment. The junctions and toroidal trap are created with the painted potential, a time-averaged optical dipole potential technique which will allow scaling to more complex BEC circuit geometries than the single atom-SQUID case reported here. Since rotation plays the same role in the atom SQUID as magnetic field does in the dc SQUID magnetometer, the device has potential as a compact rotation sensor.
We report the creation of a pair of Josephson junctions on a toroidal dilute gas Bose-Einstein condensate (BEC), a configuration that is the cold atom analog of the well-known dc superconducting quantum interference device (SQUID).We observe Josephson effects, measure the critical current of the junctions, and find dynamic behavior that is in good agreement with the simple Josephson equations for a tunnel junction with the ideal sinusoidal current-phase relation expected for the parameters of the experiment.The junctions and toroidal trap are created with the painted potential, a time-averaged optical dipole potential technique which will allow scaling to more complex BEC circuit geometries than the single atom-SQUID case reported here.Since rotation plays the same role in the atom SQUID as magnetic field does in the dc SQUID magnetometer, the device has potential as a compact rotation sensor.
Photon Bose-Einstein condensates (pBECs) are a platform of interest for both the study of fundamental phenomena in many-body physics and the implementation of next-generation analog simulators for solving optimization problems. In our physical system, the network of coupled pBECs in a microcavity setup is created by individually laser-pumping trap sites that are formed by permanent nanostructuring and connected via waveguides to their nearest neighbours. Due to the roughness and non-flat landscape of the microcavity mirrors, trap sites are excited at different frequencies, making the relative phases undefined. This is crucial for finding the ground state of a network of pBECs - a problem that is NP-complete, which makes this platform a promising candidate for analog optimization problem solver. We propose an approach for in-situ coherence tuning using a permanent nanostructuring technique with active feedback on excited trap site frequencies and report our preliminary results on the creation of an uncoupled coherent network of pBECs using this approach.