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Physical Review Letters

Persistent Self-Induced Larmor Precession Evidenced through Periodic Revivals of Coherence

Helgi Sigurðsson, Ivan Gnusov, Sergey Alyatkin, L. Pickup, Николай Алексеевич Гиппиус, Pavlos G. Lagoudakis, Alexis Askitopoulos

Published Oct 4, 2022• 19 citations
Publisher

Abstract

Interferometric measurements of an optically trapped exciton-polariton condensate reveal a regime where the condensate pseudo-spin precesses persistently within the driving optical pulse. For a single 20 μs optical pulse, the condensate pseudo-spin undergoes over 10^{5} full precessions with striking frequency stability. The emergence of the precession is traced to polariton nonlinear interactions that give rise to a self-induced out-of-plane magnetic field, which in turn drives the system spin dynamics. The Larmor precession frequency and trajectory are directly influenced by the condensate density, enabling the control of this effect with optical means. Our results accentuate the system's potential for the realization of magnetometry devices and can lead to the emergence of spin-squeezed polariton condensates.

DOI
10.1103/physrevlett.129.155301

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•Jan 1, 2016•0 citations
Interferometric measurements of an optically trapped exciton-polariton condensate reveal a regime where the condensate pseudo-spin precesses persistently within the driving optical pulse. For a single 20 μs optical pulse, the condensate pseudo-spin undergoes over 10^{5} full precessions with striking frequency stability. The emergence of the precession is traced to polariton nonlinear interactions that give rise to a self-induced out-of-plane magnetic field, which in turn drives the system spin dynamics. The Larmor precession frequency and trajectory are directly influenced by the condensate density, enabling the control of this effect with optical means. Our results accentuate the system's potential for the realization of magnetometry devices and can lead to the emergence of spin-squeezed polariton condensates.
First order coherence measurements of a polariton condensate, reveal a regime where the condensate pseudo-spin precesses persistently within the driving optical pulse. Within a single 20 μμμs optical pulse the condensate pseudo-spin performs over 10510^5105 precessions with striking frequency stability. The condensate maintains its phase coherence even after a complete precession of the spin vector, making the observed state by a definition a spin coherent state. The emergence of the precession is traced to the polariton interactions that give rise to a self-induced out-of-plane magnetic field that in turn drives the spin dynamics. We find that the Larmor oscillation frequency scales with the condensate density, enabling external tuning of this effect by optical means. The stability of the system allows for the realization of integrated optical magnetometry devices with the use of materials with enhanced exciton ggg-factor and can facilitate spin squeezing effects and active coherent control on the Bloch sphere in polariton condensates.
We demonstrate mutual synchronization of Larmor precessions between spatially separated, optically trapped polariton condensates. We identify the cross-condensate correlation g(2)1,2(τ) as a sensitive metric for condensate coupling. Synchronization of precessions in a condensate dimer is mediated by polariton outflow and resonantly depends on spectral overlap and precession winding. All-optical stirring of one site non-locally increases the pseudo-spin dephasing time of its static neighbor, prolonging phase-locking in a dimer by an order of magnitude. Our work establishes a scalable platform for studying non-equilibrium time-periodically driven systems and for high-speed spinoptronic networks.
External driving of spins by magnetic or optical fields in different systems underpins numerous applications ranging from magnetic resonance imaging to coherent state control in quantum computing. Here, we reveal the effect of an all-optically driven spin precession in microcavity polariton condensates. It is achieved through a radio frequency modulation of a spatially rotating, asymmetric exciton reservoir that both confines and actively replenishes the polariton condensate. The non-resonant stirring profile is realized by the beating note of two structured and frequency-detuned laser beams. We realize the GHz driven spin precession with striking phase stability, which occurs only in the resonance with the internal condensate self-induced Larmor precession frequency. From the shape of the revealed resonance, we estimate the spin coherence time ( T 2 ) for the polariton condensate. Our observations are supported by numerical simulations and evidence a quantum fluidic analogue of the nuclear magnetic resonance effect.
We demonstrate theoretically the spontaneous formation of a stochastic polarization in exciton-polariton Bose-Einstein condensates in planar microcavities under pulsed excitation. Below the threshold pumping intensity (dependent on the polariton lifetime), the average polarization degree is close to zero, while above threshold the condensate acquires a polarization described by a (pseudospin) vector with random orientation, in general. It is shown that the polariton-polariton interaction leads to suppression of the linear polarization degree of the condensate due to the self-induced Larmor precession of the pseudospin. We establish the link between the second-order coherence of the polariton condensate and the distribution function of its polarization. We examine also the mechanisms of polarization dephasing and relaxation.
Exciton-polariton Bose-Einstein condensates (BECs) are newly emerged quantum systems that are capable of showing macroscopic quantum phenomena with intrinsic open-dissipative nature. The spatial distribution of the polariton density, without any external potential, can be controlled by the geometric shape of the pumping laser, enabling the investigation of polariton dynamics with topologically non-trivial configurations. Meanwhile, exciton-polaritons have spin degrees of freedom inherited from excitons and photons, making it a candidate for the realization of quantum logic gates. In this thesis, we will investigate theoretically the nonlinear dynamics of exciton-polariton BECs involving both polaritons' spatial degrees of freedom and spin degrees of freedom, and interactions between them. This thesis is organised as follows: In Chapter 1, we will present an overall review of exiton-polariton systems and important properties of polariton BECs and then introduce the dynamical equations with various interactions that will serve as the main theoretical tool for subsequent chapters. Several polariton pumping and trapping techniques appearing in later chapters will also be introduced. In Chapter 2, we will investigate the superfluidity properties of a single-component polariton condensate under an incoherent annular pumping configuration. By studying the stability properties of polariton persistent currents, we find that the persistent currents can exhibit dynamical instability and energetic-like instability according to different parameter region. A stability phase diagram will be given and its relation with the Landau's criterion will be discussed. In Chapter 3, we will investigate the spin dynamics of a two-component polariton condensate under a homogeneous pumping configuration. Owing to the Josephson coupling, there exist multiple steady state solutions that allow of controlled spin state switching. A desynchronized region where there exists no stable steady solution is found. In the desynchronized region, a desynchronized state beating periodically over time can exist, which will serve as a building block of spin waves presented in the next chapter. In Chapter 4, by combining results from the previous two chapters we will investigate generally the nonlinear dynamics of polariton condensates under an annular pumping configuration. The spin-orbit interaction provided by the Josephson coupling supports azimuthon states that have simultaneous modulations in both amplitude and phase. The azimuthon states, when viewed in a different polarization basis, form rotating spin waves that can be referred to as the optical ferris wheel. In Chapter 5, results from previous chapters will be extended to micocavities that support the anisotropic TE-TM splitting interaction. Rotating singularities (small-scale vortices) are found as a result. Their properties and experimental observation techniques will be discussed. Chapter 2-5 provide a theoretical framework for the nonlinear dynamics of polariton condensates. They rely mostly on optical trapping techniques and are ready to be tested in experiments. In Chapter 6, polaritons trapped by an engineered periodic mesa potential will be discussed. We will investigate the band structure of polaritons under the influence of the periodic potential together with discussions on the phase-modulated interference pattern which corresponds to the polariton Talbot patterns observed in experiments.