We observe a spontaneous parity breaking bifurcation to a ferromagnetic state in a spatiallytrapped exciton-polariton condensate. At a critical bifurcation density under nonresonant excitation, the whole condensate spontaneously magnetizes and randomly adopts one of two ellipticallypolarized (up to 95% circularly-polarized) states with opposite handedness of polarization. The magnetized condensate remains stable for many seconds at 5 K, but at higher temperatures it can flip from one magnetic orientation to another. We optically address these states and demonstrate the inversion of the magnetic state by resonantly injecting 100-fold weaker pulses of opposite spin. Theoretically, these phenomena can be well described as spontaneous symmetry breaking of the spin degree of freedom induced by different loss rates of the linear polarizations.Condensation of exciton-polaritons (polaritons) spontaneously breaks the global phase symmetry [1][2][3][4][5]. Owing to their easy optical interrogation, high-speed (ps) interactions, and macroscopic coherence (over hundreds of microns) [6], polariton condensates are excellent candidates to probe and exploit for sensing [7,8], spinoptronics [9][10][11], new optoelectronic devices [12][13][14], and quantum simulators [15]. The driven-dissipative multicomponent polariton system can undergo additional bifurcations and condense into states which are not eigenstates of the single-particle Hamiltonian, but many-body states with reduced symmetry [16,17]. Thus, we should expect that two-component exciton-polariton condensates can also show spontaneous symmetry breaking bifurcations in their polarization state. Spin studies of microcavity polaritons have been of great interest in recent years [18][19][20][21][22][23][24][25][26][27][28][29]. However, spontaneous symmetry-breaking bifurcation of spin has not been observed before.Here, we demonstrate spontaneous magnetization in an exciton-polariton condensate, as a direct result of bifurcations in the spin degree of freedom. Utilizing an optically trapped geometry, condensates spontaneously emerge in either of two discrete spin-polarized states that are stable for many seconds, > 10 10 longer than their formation time. These states emit highly circularly-polarized coherent light (up to 95%) and have opposite circular polarizations. The condensate stochastically condenses in a left-or right-circularly polarized state, with an occurrence likelihood that can be controlled by the ellipticity * ho278@cam.ac.uk † jjb12@cam.ac.uk of the nonresonant pump. The two spin-polarized states can be initialized and switched from one state to another with weak resonant optical pulses. Our system has potential applications in sensing, optical spin memories and spin switches, and it can be implemented for studying long-range spin interactions in polariton condensate lattices. This article is structured as follows: in Section I we review trapped polariton condensates and the current understanding of polarization in untrapped polariton condensates. In Section ...

Practical challenges to extrapolating Moore's law favour alternatives to electrons as information carriers. Two promising candidates are spin-based and all-optical architectures, the former offering lower energy consumption, the latter superior signal transfer down to the level of chip-interconnects. Polaritons-spinor quasi-particles composed of semiconductor excitons and microcavity photons-directly couple exciton spins and photon polarizations, combining the advantages of both approaches. However, their implementation for spintronics has been hindered because polariton spins can be manipulated only optically or by strong magnetic fields. Here we use an external electric field to directly control the spin of a polariton condensate, bias-tuning the emission polarization. The nonlinear spin dynamics offers an alternative route to switching, allowing us to realize an electrical spin-switch exhibiting ultralow switching energies below 0.5 fJ. Our results lay the foundation for development of devices based on the electro-optical control of coherent spin ensembles on a chip.

We demonstrate that multiply coupled spinor polariton condensates can be optically tuned through a sequence of spin-ordered phases by changing the coupling strength between nearest neighbors. For closed four-condensate chains these phases span from ferromagnetic (FM) to antiferromagnetic (AFM), separated by an unexpected crossover phase. This crossover phase is composed of alternating FM-AFM bonds. For larger eight-condensate chains, we show the critical role of spatial inhomogeneities and demonstrate a scheme to overcome them and prepare any desired spin state. Our observations thus demonstrate a fully controllable nonequilibrium spin lattice.

We investigate the many-body interactions of excited species in the quantum wells formed in DA 2 PbI 4 , and identify biexciton formation with a binding energy of 50 meV. Biexcitons (stable four-particle states formed from the interaction of two excitons) have been well studied in quantum well structures [11,12] and are of interest for lasing applications due to their intrinsic potential for reducing selfabsorption compared to conventional excitonic lasing schemes, for example in 3D or 2D-3D perovskites. As well as recent reports of wavelength-tunable amplified spontaneous emission (ASE) in 3D perovskites [13] and ASE in mixed 2D-3D perovskites [14] there have been previous reports of biexciton lasing at 16 K with high excitation densities. [15] Here we demonstrate biexciton lasing in cavities formed from this 2D perovskite above liquid nitrogen temperatures (125 K). We additionally observe emission from higher-lying energy states, which we attribute to the formation of more than one structural phase in the film. Films of DA 2 PbI 4 were spin-cast from solutions of dodecylammonium iodide and lead iodide onto glass as described previously. [16] The films are multicrystalline, and scanning electron microscope images suggest grain sizes in the range 1-10 µm (Figure S8, Supporting Information). We examine the multiexciton properties of DA 2 PbI 4 through a series of photon-intensity and temperature dependent photoluminescence (PL) measurements. We use the PL spectrum at low temperature for assignment of the species present. The film was excited at 3.10 eV with 200 fs laser pulses (1 kHz repetition rate), resulting in a peak carrier concentration of 9.6 × 10 19 cm −3. The time-integrated PL spectrum at 10 K is shown in Figure 1a. The Frenkel defects reported previously [16] are clearly seen around 2.0 eV (labelled F). There are four further emission peaks we have labelled XX (≈2.37 eV), X (≈2.42 eV-the main emission peak reported previously), X′X′ (≈2.53 eV) and X′ (≈2.58 eV). We identify X as the exciton emission of DA 2 PbI 4 and XX as the biexciton resulting from two X excitons coupling together, as will be shown below. The lifetimes of all species aside from the Frenkel defects were shorter than the 3 ns instrument response time of our intensified charge-coupled device (iCCD). The contribution of each species to the overall PL was determined by modeling the PL data as five independent Gaussian peaks for Layered Ruddlesden-Popper-type (2D) metal-halide perovskites exhibit markedly increased exciton binding energies, exceeding 150 meV, compared to their 3D counterparts. Many-body physics, enabled by Coulomb interactions, plays a strong role and raises the biexciton binding energy to 50 meV. Here, photoluminescence at a range of temperatures and carrier concentrations in thin films of the layered perovskite material (C 12 H 25 NH 3) 2 PbI 4 is reported. Biexcitons are directly observed up to a sample temperature of 225 K. An optical microcavity (comprising a distributed Bragg reflector and a metal mirror), with photo...

We demonstrate that the synchronization of a lattice of solid-state condensates when inter-site tunnelling is switched on, depends strongly on the weak local disorder. This finding is vital for implementation of condensate arrays as computation devices. The condensates here are nonlinear bosonic fluids of exciton-polaritons trapped in a weakly disordered Bose-Hubbard potential, where the nearest neighboring tunneling rate (Josephson coupling) can be dynamically tuned. The system can thus be tuned from a localized to a delocalized fluid as the number density, or the Josephson coupling between nearest neighbors increases. The localized fluid is observed as a lattice of unsynchronized condensates emitting at different energies set by the disorder potential. In the delocalized phase the condensates synchronize, and long-range order appears, evidenced by narrowing of momentum and energy distributions, new diffraction peaks in momentum space, and spatial coherence between condensates. Our work identifies similarities and differences of this nonequilibrium crossover to the traditional Bose-glass to superfluid transition in atomic condensates. * ho35@st-andrews.ac.uk