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We report on the experimental realization and characterization of a qubit analog with semiconductor exciton-polaritons. In our system, a polaritonic condensate is confined by a spatially patterned pump laser in an annular trap that supports energy-degenerate vortex states of the polariton superfluid. Using temporal interference measurements, we observe coherent oscillations between a pair of counter-circulating vortex states coupled by elastic scattering of polaritons off the laser-imprinted potential. The qubit basis states correspond to the symmetric and antisymmetric superpositions of the two vortex states. By engineering the potential, we tune the coupling and coherent oscillations between the two circulating current states, control the energies of the qubit basis states, and initialize the qubit in the desired state. The dynamics of the system is accurately reproduced by our theoretical two-state model, and we discuss potential avenues to implement quantum gates and algorithms with polaritonic qubits analogous to quantum computation with standard qubits.

We monitor the orbital degree of freedom of exciton-polariton condensates confined within an optical trap and unveil the stochastic switching of persistent annular polariton currents under pulse-periodic excitation. Within an elliptical trap, the low-lying in energy polariton current states manifest as a two-petaled density distribution with a swirling phase. In the stochastic regime, the density distribution, averaged over multiple excitation pulses, becomes homogenized in the azimuthal direction. Meanwhile, the weighted phase, extracted from interference experiments, exhibits two compensatory jumps when varied around the center of the trap. Introducing a supplemental control optical pulse to break the reciprocity of the system enables the transition from a stochastic to a deterministic regime, allowing for controlled polariton circulation direction., (© 2024. The Author(s).)

Concentric ring exciton polariton condensates emerging under non-resonant laser pump in an annular trapping potential support persistent circular currents of polaritons. The trapping potential is formed by a cylindrical micropillar etched in a semiconductor microcavity with embedded quantum wells and a repulsive cloud of optically excited excitons under the pump spot. The symmetry of the potential is subject to external control via manipulation by its pump-induced component. In the manuscript, we demonstrate excitation of concentric ring polariton current states with predetermined vorticity which we trace using interferometry measurements with a spherical reference wave. We also observe the polariton condensate dynamically changing its vorticity during observation, which results in pairs of fork-like dislocations on the time-averaged interferogram coexisting with azimuthally homogeneous photoluminescence distribution in the micropillar., (© 2023. The Author(s).)

We resolve single-shot polariton condensate polarization dynamics, revealing a high degree of circular polarization persistent up to T=170  K. The statistical analysis of pulse-to-pulse polariton condensate polarization elucidates the stochastic nature of the polarization pinning process, which is strongly dependent on the pump laser intensity and polarization. Our experiments show that by spatial trapping and isolating condensates from their noisy environment it is possible to form strongly spin-polarized polariton condensates at high temperatures, offering a promising route to the realization of polariton spin lattices for quantum simulations.

Spatially confined, trapped polariton condensates have been shown to exhibit strong stochastic on-site spin polarization and in longer polariton condensate chains, distance controlled ferromagnetic and antiferromagnetic spin couplings. Until now, little is known, on how such polariton condensates spatially separated from their exciton reservoirs are trapped and formed. Here, we investigate the properties and formation dynamics of two main families of polariton condensates, those overlapping with the pump reservoir and those in confined geometries, under pulsed nonresonant excitation. The observed reduction in polariton condensation threshold and energy blueshift in trapped case is attributed to exciton reservoir-condensate spatial separation, whereas time-resolved photoluminescence measurements, reveal distinct relaxation and condensate formation dynamics with pair parametric scattering process being the dominant relaxation mechanism in trapped geometry., (© 2020 IOP Publishing Ltd.)

Exciton polaritons in high quality semiconductor microcavities can travel long macroscopic distances (>100  μm) due to their ultralight effective mass. The polaritons are repelled from optically pumped exciton reservoirs where they are formed; however, their spatial dynamics is not as expected for pointlike particles. Instead we show polaritons emitted into waveguides travel orthogonally to the repulsive potential gradient and can only be explained if they are emitted as macroscopic delocalized quantum particles, even before they form Bose condensates.

A primary limitation of the intensively researched polaritonic systems compared to their atomic counterparts for the study of strongly correlated phenomena and many-body physics is their relatively weak two-particle interactions compared to disorder. Here, we show how new opportunities to enhance such on-site interactions and nonlinearities arise by tuning the exciton-polariton dipole moment in electrically biased semiconductor microcavities incorporating wide quantum wells. The applied field results in a twofold enhancement of exciton-exciton interactions as well as more efficiently driving relaxation towards low energy polariton states, thus, reducing condensation threshold.

We experimentally investigate the feasibility of a bolometric device based on exciton-polaritons. Initial measurements presented in this work show that heating - via thermal expansion and bandgap renormalization - modifies the exciton-polariton propagation wavevector making exciton-polaritons propagation remarkably sensitive to thermal variations. By theoretical simulations we predict that using a single layer graphene absorbing layer, a THz bolometric sensor can be realized by a simple exciton-polariton ring interferometer device. The predicted sensitivity is comparable to presently existing THz bolometric devices with the convenience of being a device that inherently produces an optical signal output.

We realize bistability in the spinor of polariton condensates under nonresonant optical excitation and in the absence of biasing external fields. Numerical modeling of the system using the Ginzburg-Landau equation with an internal Josephson coupling between the two spin components of the condensate qualitatively describes the experimental observations. We demonstrate that polariton spin bistability strongly depends on the condensate's overlap with the exciton reservoir by tuning the excitation geometry and sample temperature. We obtain noncollapsing bistability hysteresis loops for a record range of sweep times, [10  μs, 1 s], offering a promising route to spin switches and spin memory elements.

Organic materials exhibit exceptional room temperature light emitting characteristics and enormous exciton oscillator strength, however, their low charge carrier mobility prevent their use in high-performance applications such as electrically pumped lasers. In this context, ultralow threshold polariton lasers, whose operation relies on Bose-Einstein condensation of polaritons - part-light part-matter quasiparticles, are highly advantageous since the requirement for high carrier injection no longer holds. Polariton lasers have been successfully implemented using inorganic materials owing to their excellent electrical properties, however, in most cases their relatively small exciton binding energies limit their operation temperature. It has been suggested that combining organic and inorganic semiconductors in a hybrid microcavity, exploiting resonant interactions between these materials would permit to dramatically enhance optical nonlinearities and operation temperature. Here, we obtain cavity mediated hybridization of GaAs and J-aggregate excitons in the strong coupling regime under electrical injection of carriers as well as polariton lasing up to 200 K under non-resonant optical pumping. Our demonstration paves the way towards realization of hybrid organic-inorganic microcavities which utilise the organic component for sustaining high temperature polariton condensation and efficient electrical injection through inorganic structure.

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.

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.

Tunable spin correlations are found to arise between two neighboring trapped exciton-polariton condensates which spin polarize spontaneously. We observe a crossover from an antiferromagnetic to a ferromagnetic pair state by reducing the coupling barrier in real time using control of the imprinted pattern of pump light. Fast optical switching of both condensates is then achieved by resonantly but weakly triggering only a single condensate. These effects can be explained as the competition between spin bifurcations and spin-preserving Josephson coupling between the two condensates, and open the way to polariton Bose-Hubbard ladders.

Light amplification by stimulated emission of radiation, well-known for revolutionising photonic science, has been realised primarily in fermionic systems including widely applied diode lasers. The prerequisite for fermionic lasing is the inversion of electronic population, which governs the lasing threshold. More recently, bosonic lasers have also been developed based on Bose-Einstein condensates of exciton-polaritons in semiconductor microcavities. These electrically neutral bosons coexist with charged electrons and holes. In the presence of magnetic fields, the charged particles are bound to their cyclotron orbits, while the neutral exciton-polaritons move freely. We demonstrate how magnetic fields affect dramatically the phase diagram of mixed Bose-Fermi systems, switching between fermionic lasing, incoherent emission and bosonic lasing regimes in planar and pillar microcavities with optical and electrical pumping. We collected and analyzed the data taken on pillar and planar microcavity structures at continuous wave and pulsed optical excitation as well as injecting electrons and holes electronically. Our results evidence the transition from a Bose gas to a Fermi liquid mediated by magnetic fields and light-matter coupling.

Strongly coupled optical microcavities containing different exciton states permit the creation of hybrid-polariton modes that can be described in terms of a linear admixture of cavity-photon and the constituent excitons. Such hybrid states have been predicted to have optical properties that are different from their constituent parts, making them a test bed for the exploration of light-matter coupling. Here, we use strong coupling in an optical microcavity to mix the electronic transitions of two J-aggregated molecular dyes and use both non-resonant photoluminescence emission and photoluminescence excitation spectroscopy to show that hybrid-polariton states act as an efficient and ultrafast energy-transfer pathway between the two exciton states. We argue that this type of structure may act as a model system to study energy-transfer processes in biological light-harvesting complexes.

Polariton condensates are macroscopic quantum states formed by half-matter half-light quasiparticles, thus connecting the phenomena of atomic Bose-Einstein condensation, superfluidity, and photon lasing. Here we report the spontaneous formation of such condensates in programmable potential landscapes generated by two concentric circles of light. The imposed geometry supports the emergence of annular states that extend up to 100 μm, yet are fully coherent and exhibit a spatial structure that remains stable for minutes at a time. These states exhibit a petal-like intensity distribution arising due to the interaction of two superfluids counterpropagating in the circular waveguide defined by the optical potential. In stark contrast to annular modes in conventional lasing systems, the resulting standing wave patterns exhibit only minimal overlap with the pump laser itself. We theoretically describe the system using a complex Ginzburg-Landau equation, which indicates why the condensate wants to rotate. Experimentally, we demonstrate the ability to precisely control the structure of the petal condensates both by carefully modifying the excitation geometry as well as perturbing the system on ultrafast timescales to reveal unexpected superfluid dynamics.

We report observation of oscillations in the dynamics of a microcavity polariton condensate formed under pulsed nonresonant excitation. While oscillations in a condensate have always been attributed to Josephson mechanisms due to a chemical potential unbalance, here we show that under some localization conditions of the condensate, they may arise from relaxation oscillations, a pervasive classical dynamics that repeatedly provokes the sudden decay of a reservoir, shutting off relaxation as the reservoir is replenished. Using nonresonant excitation, it is thus possible to obtain condensate injection pulses with a record frequency of 0.1 THz.

Semiconductor microcavities are used to support freely flowing polariton quantum liquids allowing the direct observation and optical manipulation of macroscopic quantum states. Incoherent optical excitation at a point produces radially expanding condensate clouds within the planar geometry. By using arbitrary configurations of multiple pump spots, we discover a geometrically controlled phase transition, switching from the coherent phase-locking of multiple condensates to the formation of a single trapped condensate. The condensation threshold becomes strongly dependent on the programmed superfluid geometry and sensitive to cooperative interactions between condensates. We directly image persistently circulating superfluid and show how flows of light-matter quasiparticles are dominated by the quantum pressure in such configurable laser-written potential landscapes.

We report on the experimental observation of the nonlinear analogue of the optical spin Hall effect under highly nonresonant circularly polarized excitation of an exciton-polariton condensate in a GaAs/AlGaAs microcavity. The circularly polarized polariton condensates propagate over macroscopic distances, while the collective condensate spins coherently precess around an effective magnetic field in the sample plane performing up to four complete revolutions.

Tunneling of electrons through a potential barrier is fundamental to chemical reactions, electronic transport in semiconductors and superconductors, magnetism, and devices such as terahertz oscillators. Whereas tunneling is typically controlled by electric fields, a completely different approach is to bind electrons into bosonic quasiparticles with a photonic component. Quasiparticles made of such light-matter microcavity polaritons have recently been demonstrated to Bose-condense into superfluids, whereas spatially separated Coulomb-bound electrons and holes possess strong dipole interactions. We use tunneling polaritons to connect these two realms, producing bosonic quasiparticles with static dipole moments. Our resulting three-state system yields dark polaritons analogous to those in atomic systems or optical waveguides, thereby offering new possibilities for electromagnetically induced transparency, room-temperature condensation, and adiabatic photon-to-electron transfer.

Macroscopic quantum states can be easily created and manipulated within semiconductor microcavity chips using exciton-photon quasiparticles called polaritons. Besides being a new platform for technology, polaritons have proven to be ideal systems to study out-of-equilibrium condensates. Here we harness the photonic component of such a semiconductor quantum fluid to measure its coherent wavefunction on macroscopic scales. Polaritons originating from separated and independent incoherently pumped spots are shown to phase-lock only in high-quality microcavities, producing up to 100 vortices and antivortices that extend over tens of microns across the sample and remain locked for many minutes. The resultant regular vortex lattices are highly sensitive to the optically imposed geometry, with modulational instabilities present only in square and not triangular lattices. Such systems describe the optical equivalents to one- and two-dimensional spin systems with (anti)-ferromagnetic interactions controlled by their symmetry, which can be reconfigured on the fly, paving the way to widespread applications in the control of quantum fluidic circuits.

Periodic incorporation of quantum wells inside a one-dimensional Bragg structure is shown to enhance coherent coupling of excitons to the electromagnetic Bloch waves. We demonstrate strong coupling of quantum well excitons to photonic crystal Bragg modes at the edge of the photonic band gap, which gives rise to mixed Bragg polariton eigenstates. The resulting Bragg polariton branches are in good agreement with the theory and allow demonstration of Bragg polariton parametric amplification.

The increasing ability to control light-matter interactions at the nanometre scale has improved the performance of semiconductor lasers in the past decade. The ultimate optimization is realized in semiconductor microcavities, in which strong coupling between quantum-well excitons and cavity photons gives rise to hybrid half-light/half-matter polariton quasiparticles. The unique properties of polaritons-such as stimulated scattering, parametric amplification, lasing, condensation and superfluidity-are believed to provide the basis for a new generation of polariton emitters and semiconductor lasers. Until now, polariton lasing and nonlinearities have only been demonstrated in optical experiments, which have shown the potential to reduce lasing thresholds by two orders of magnitude compared to conventional semiconductor lasers. Here we report an experimental realization of an electrically pumped semiconductor polariton light-emitting device, which emits directly from polariton states at a temperature of 235 K. Polariton electroluminescence data reveal characteristic anticrossing between exciton and cavity modes, a clear signature of the strong coupling regime. These findings represent a substantial step towards the realization of ultra-efficient polaritonic devices with unprecedented characteristics.

Terahertz absorption in waveguides loaded with InAs/AlSb super-superlattice mesas reveals a frequency dependent crossover from loss to gain that is related to the Stark ladder produced by an applied dc electric field. Electric field domains appear to be suppressed in the super-superlattice composed of many very short segments of superlattice, interrupted by heavily doped InAs regions. Resonant crossover is indicated by an increase in terahertz transmission as the Stark splitting or Bloch frequency determined by the applied dc electric field exceeds the measurement frequency.

We propose a terahertz (far-infrared) circular dichroism-based life-detection technology that may provide a universal and unequivocal spectroscopic signature of living systems regardless of their genesis. We argue that, irrespective of the specifics of their chemistry, all life forms will employ well-structured, chiral, stereochemically pure macromolecules (>500 atoms) as the catalysts with which they perform their metabolic and replicative functions. We also argue that nearly all such macromolecules will absorb strongly at terahertz frequencies and exhibit significant circular dichroism, and that this circular dichroism unambiguously distinguishes biological from abiological materials. Lastly, we describe several approaches to the fabrication of a terahertz circular dichroism spectrometer and provide preliminary experimental indications of their feasibility. Because terahertz circular dichroism signals arise from the molecular machinery necessary to carry out life's metabolic and genetic processes, this life-detection method differs fundamentally from more well-established approaches based on the detection of isotopic fractionation, "signature" carbon compounds, disequilibria, or other by-products of metabolism. Moreover, terahertz circular dichroism spectroscopy detects this machinery in a manner that makes few, if any, assumptions as to its chemical nature or the processes that it performs.

A massive redistribution of the polariton occupancy to two specific wave vectors, zero and approximately 3.9x10(4) cm(-1), is observed under conditions of continuous wave excitation of a semiconductor microcavity. The "condensation" of the polaritons to the two specific states arises from stimulated scattering at final state occupancies of order unity. The stimulation phenomena, arising due to the bosonic character of the polariton quasiparticles, occur for conditions of resonant excitation of the lower polariton branch. High energy nonresonant excitation, as in most previous work, instead leads to conventional lasing in the vertical cavity structure.

We experimentally demonstrate resonant coupling between photons and excitons in microcavities which can efficiently generate enormous single-pass optical gains approaching 100. This new parametric phenomenon appears as a sharp angular resonance of the incoming pump beam, at which the moving excitonic polaritons undergo very large changes in momentum. Ultrafast stimulated scattering is clearly identified from the exponential dependence on pump intensity. This device utilizes boson amplification induced by stimulated energy relaxation.