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arXiv:1910.00344 · ACS Photonics

Persistent Currents in Half-Moon Polariton Condensates

Evgeny S. Sedov, V. A. Lukoshkin, Vladimir K. Kalevich, Z. Hatzopoulos, Pavlos G. Savvidis, Alexey Vitalievich Kavokin

Published Apr 16, 2020• 27 citations
arXivHTMLPDF

Abstract

We create exciton–polariton condensates in half-moon shapes by the nonresonant optical excitation of GaAs-based cylindrical pillar microcavities. In this geometry, the π-jump of the phase of the condensate wave function coexists with a gradual ±π phase variation between two horns of the half-moon. We switch between clockwise and counterclockwise phase currents by slightly shifting the excitation spot on the surface of the pillar. Half-moon condensates are expected to reveal features of two-level quantum systems similar to superconducting flux qubits.

DOI
10.1021/acsphotonics.9b01779

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We create exciton–polariton condensates in half-moon shapes by the nonresonant optical excitation of GaAs-based cylindrical pillar microcavities. In this geometry, the π-jump of the phase of the condensate wave function coexists with a gradual ±π phase variation between two horns of the half-moon. We switch between clockwise and counterclockwise phase currents by slightly shifting the excitation spot on the surface of the pillar. Half-moon condensates are expected to reveal features of two-level quantum systems similar to superconducting flux qubits.
Superposition states of circular currents of exciton-polaritons mimic the superconducting flux qubits. The current states are formed by a macroscopic number of bosonic quasiparticles that compose a single quantum state of a many-body condensate. The essential difference between a polariton fluid and a supercurrent comes from the fact that polaritons are electrically neutral, and the magnetic field would not have a significant effect on a polariton current. Nevertheless, the phase of a polariton condensate must change by an integer number of 2π2\pi2π, when going around the ring. If one introduces a π\piπ-phase delay line in the ring, the system is obliged to propagate a clockwise or anticlockwise circular current to reduce the total phase gained over one round-trip to zero or to build it up to 2π2\pi2π. We show that such a π\piπ-delay line can be provided by a dark soliton embedded into a ring condensate and pinned to a potential well created by the C-shape non-resonant pump-spot. The physics of resulting split-ring polariton condensates is essentially similar to the physics of flux qubits. In particular, they exhibit pronounced coherent oscillations passing periodically through clockwise and anticlockwise current states. We predict that these oscillations should persist far beyond the coherence time of the polariton condensates. As a consequence the qubits based on split-ring polariton condensates are expected to be characterized by a very high figure of merit that makes them a valuable alternative to superconducting qubits.
We predict the spontaneous symmetry breaking in a spinor Bose-Einstein condensate of exciton-polaritons (polaritons) caused by the coupling of its spin and orbital degrees of freedom. We study a polariton condensate trapped in a ring-shaped effective potential with a broken rotational symmetry. We propose a realistic scheme of generating controllable spinor azimuthal persistent currents of polaritons in the trap under the continuous wave optical pump. We propose a new type of half-quantum circulating states in a spinor system characterized by azimuthal currents in both circular polarizations and a vortex in only one of the polarizations. The spontaneous symmetry breaking in the spinor polariton condensate that consists in the switching from co-winding to opposite-winding currents in opposite spin states is revealed. It is characterized by the change of the average orbital angular momentum of the condensate from zero to non-zero values. The radial displacement of the pump spot and the polarization of the pump act as the control parameters. The considered system exhibits a fundamental similarity to a superconducting flux qubit, which makes it highly promising for applications in quantum computing.
A phase transition from a classical thermal mixed state to a quantum-mechanical pure state of exciton polaritons is observed in a GaAs multiple quantum-well microcavity from the decrease of the second-order coherence function. Supporting evidence is obtained from the observation of a nonlinear threshold behavior in the pump-intensity dependence of the emission, a polariton-like dispersion relation above threshold, and a decrease of the relaxation time into the lower polariton state. The condensation of microcavity exciton polaritons is confirmed.
The first-order spatial correlation function g (1)(r 12) and the polariton density distribution in the condensate of quasi-two-dimensional exciton polaritons formed in a high-Q semiconductor microcavity pillar under nonresonant optical pumping are investigated. It is found that the correlation function in certain regions of the micropillar decreases abruptly with increasing condensate density. It is shown that this behavior of the correlation function is caused by the formation of a localized dark soliton in these regions. A deep minimum of the polariton density and a shift in the phase of the condensate wavefunction by π occur within the soliton localization area.
We have performed real and momentum space spectroscopy of exciton polariton condensates in a GaAs-based microcavity under nonresonant excitation with an intensity-stabilized laser. An effective trapping mechanism is revealed, which is due to the stimulated scattering gain inside the finite excitation spot combined with the short lifetime. We observe several quantized modes while the lowest state shows Heisenberg-limited real and momentum space distributions. The experimental findings are qualitatively reproduced by an open dissipative Gross-Pitaevskii equation model.