Polariton Fluids: Quantum Hydrodynamics & Devices

• Polariton fluids of light are coherent ensembles of exciton–polariton quasiparticles in microcavities, enabling quantum hydrodynamics studies in driven, dissipative systems.
• They exhibit superfluid phenomena such as vortex nucleation and soliton formation, with precise optical control over density, phase, and flow velocity.
• Recent advances leverage these systems for integrated polaritonic devices, quantum simulators, and exploration of non-Hermitian topology and analog gravity.

A polariton fluid of light is a macroscopically coherent, strongly interacting quasi-bosonic ensemble of exciton–polariton quasiparticles in a driven, dissipative microcavity setting. Formed by the strong coupling of quantum-well excitons and microcavity photons, polariton fluids inherit an extremely light effective mass from their photonic component ($m^*\sim10^{-5}m_e$) and sizable repulsive interactions from their excitonic fraction ($g\sim10^{-3}$ meV·$\mu$m$^2$). Under coherent or nonresonant optical pumping, the system achieves a driven–dissipative Bose–Einstein–like condensate, exhibiting quantum hydrodynamic phenomena analogous to matter-wave superfluids—namely, superfluid flow, quantized vortex nucleation, soliton formation, and non-linear pattern dynamics (Sanvitto et al., 2011, Claude et al., 2022, Stepanov et al., 2018). Importantly, all aspects of the fluid—including density, phase, flow velocity, and potential landscape—can be precisely controlled and measured by optical means, enabling a reconfigurable platform for both fundamental quantum fluid physics and integrated polaritonic devices.

1. Formation and Microscopic Description of Polariton Fluids

Exciton–polaritons are hybrid quasiparticles formed via strong coupling between quantum-well excitons and confined photon modes in semiconductor planar microcavities or, increasingly, in all-optical waveguides and perovskite whispering-gallery microresonators (Sanvitto et al., 2011, Montagnac et al., 18 Aug 2025). The lower polariton (LP) branch possesses a highly photonic character, with effective masses as low as $m^*\sim10^{-5}m_e$, and the interaction constant $g$ is set by the excitonic fraction. Under sufficient continuous or pulsed optical pumping, a large fraction of polaritons occupy the same macroscopic quantum state, forming a non-equilibrium Bose–Einstein condensate.

The mesoscopic dynamics of the condensate field $\Psi(\mathbf{r},t)$ are governed by a generalized driven–dissipative Gross–Pitaevskii equation: $$i\hbar\,\frac{\partial \Psi}{\partial t} = -\frac{\hbar^2}{2m^*}\nabla^2\Psi + V(\mathbf{r})\Psi + g\,|\Psi|^2\Psi - i\hbar\frac{\gamma}{2}\Psi + F_\mathrm{pump}(\mathbf{r},t)$$ where $V(\mathbf{r})$ is an external potential (which can be optically imprinted), $\gamma$ is the polariton decay rate, and $F_\mathrm{pump}$ represents the coherent optical drive (Sanvitto et al., 2011, Wang et al., 2021). The interplay of nonlinear interactions, gain, loss, and arbitrary potential engineering enables the realization of quantum hydrodynamics in regimes unachievable in atomic BECs.

2. Quantum Hydrodynamics and Collective Excitations

Polariton fluids manifest collective excitation spectra described by Bogoliubov theory. For a uniform condensate, the dispersion relation of elementary excitations includes two branches: $$\omega_B(k) = \pm \sqrt{E_k \left(E_k + 2g n\right)}, \quad E_k = \frac{\hbar^2 k^2}{2m^*}, \quad n=|\Psi|^2$$ At long wavelengths ($k\xi \ll 1$), one obtains a linear phonon regime with a sound speed $c_s = \sqrt{g n/m^*}$, while at high $k$ the dispersion becomes parabolic (Ballarini et al., 2019, Claude et al., 2022). The driven–dissipative regime modifies these features, introducing spectral linewidths and reservoir-induced renormalization of the sound velocity and blueshifts (Stepanov et al., 2018, Claude et al., 2022).

Crucially, the presence of excitonic reservoirs and nonresonant background populations leads to a two-component fluid: only the coherent condensate fraction contributes to collective excitation dynamics, while the reservoir provides static blueshifts and modifies critical hydrodynamic velocities (Stepanov et al., 2018). Precise high-resolution probe spectroscopy and energy-resolved measurements have directly validated these predictions and quantified the condensate–reservoir composition (Claude et al., 2022).

3. Superfluidity, Vortices, and Solitonic Phenomena

Polariton superfluidity is evidenced by the existence of a critical velocity $v_L$ (Landau velocity), below which flow past defects is dissipationless [$c_s = \sqrt{g n/m^*}$]. When the flow velocity $v_f$ exceeds $c_s$ (Mach number $\mathrm{Ma} = v_f/c_s > 1$), the system undergoes hydrodynamic instability and nucleates topological excitations such as quantized vortex–antivortex pairs. Optical control of both flow and obstacle geometry permits deterministic positioning and tracing of vortex nucleation and propagation trajectories, as well as the permanent trapping and storage of vortices via dynamically shaped “dark” masks (Sanvitto et al., 2011).

Beyond vortices, polariton fluids exhibit rich soliton dynamics, including dark solitons with healing length $\xi = \hbar / \sqrt{2m^* g n}$ and Josephson vortices in phase-twisted junction configurations (Walker et al., 2017, Caputo et al., 2018). Disorder pinning and structural inhomogeneities can stabilize multiple vortex–antivortex pairs in perovskite microcavities at room temperature (Montagnac et al., 18 Aug 2025).

4. All-Optical Reservoir Engineering, Beam Steering, and Circuitry

Optical manipulation of polariton reservoir landscapes creates spatially varying complex potentials for the condensate, enabling quantum analogs of refractive index engineering, microlenses, and on-chip planar resonators. Pump intensity profiles are “written” into two-dimensional arrays using spatial light modulators, producing lens- and lattice-shaped reservoir fields that guide, focus, and trap polariton flows. The effective refractive index $n'$ and focal length $f$ of such lenses are directly governed by reservoir blueshift terms and gain (Wang et al., 2021): $$V_\mathrm{eff}(\mathbf{r}) = G N(\mathbf{r}) + G (\eta P(\mathbf{r})/\Gamma) + i (\hbar \xi/2) N(\mathbf{r})$$

$$n' \simeq \sqrt{1 - V_r/(\hbar \omega_s)} \quad \text{and} \quad f = R/(1 - n)$$

These approaches enable all-optical reconfiguration of polariton circuits for logic, signal routing, clock generation, and topological excitation control (Wang et al., 2021, Alyatkin et al., 2020). Loss and dephasing effects are minimized by spatially separating condensates from their reservoirs.

5. Topological, Non-Hermitian, and Many-Body Lattice Phenomena

By projecting arrays of nonresonant pump spots in tailored geometric patterns, polariton fluids realize lattice models with complex (non-Hermitian) gain/loss profiles and tunable topological band structures. Experiments have implemented barrier lattices, Lieb and edge-centered arrays, observing nonequilibrium transitions between gain-guided and trapped condensation regimes, multimode competition, and energy tomography of Bloch bands (Alyatkin et al., 2020). Complex band features, including Dirac crossings and exceptional points, are accessible owing to the simultaneous nonlinearity and gain/loss engineering. These platforms constitute scalable quantum simulators for exploring non-Hermitian topology, phase transitions, and strongly correlated polaritonic phases.

6. Analog Gravity, Curved Spacetime Simulation, and Quantum Field Signatures

Precise optical control of polariton density and phase profiles, combined with their quantum hydrodynamics, enables the simulation of quantum field theory on tailored curved spacetimes (Giacobino et al., 16 Dec 2025, Falque et al., 2023). Spatial variation of flow velocity and sound speed creates acoustic horizons (surfaces where $|\mathbf{v}| = c_s$), mimicking black-hole event horizons and ergoregions. Spectroscopic mapping of excitations across these horizons reveals the emergence of negative-energy Bogoliubov modes (downstream of the horizon) and validates the mapping to a massive Klein–Gordon equation in a curved metric.

This setting permits the investigation of analog Hawking radiation, spontaneous and stimulated pair creation, and amplification analogous to Penrose and Zeld'ovich effects (Jacquet et al., 2020). Detection schemes including pump–probe spectroscopy, balanced correlators, and homodyne measurements can access mode amplification, quadrature squeezing, and entanglement. The platform serves as a testbed for analog gravity phenomena—dynamical instabilities, quasi-normal modes, and rotational superradiance—at sub-picosecond timescales and micron spatial resolution (Giacobino et al., 16 Dec 2025, Falque et al., 2023).

7. Outlook and Applications in Quantum Photonic Technologies

Polariton fluids of light represent a uniquely reconfigurable and optically accessible quantum fluid platform, integrating quantum hydrodynamics, many-body physics, and photonic circuit design. Demonstrated applications include ultrafast all-optical switches, logic elements, planar interferometers, and long-lived coherent states at room temperature in perovskite and Bloch surface wave microcavities (Montagnac et al., 18 Aug 2025, Lerario et al., 2014). Recent advances in polaromechanical metamaterials enable GHz phonon-mediated coupling and synchronization effects, opening new avenues for hybrid quantum devices and hypersound transduction (Chafatinos et al., 2021).

Theoretical developments reveal tunable superfluid critical velocities exceeding Landau bounds via optically induced hybridization, non-equilibrium impurity dynamics, and negative drag phenomena (2002.01435, Vashisht et al., 2021). Future directions include the exploitation of transient and rotational horizon structures, entanglement and quantum information transfer in curved spacetime analogs, and the exploration of topological and non-Hermitian phase transitions in engineered polaritonic lattices.

In summary, polariton fluids of light provide experimentally controlled access to quantum fluid effects, topological excitations, non-equilibrium phase transitions, and analog quantum field theory phenomena. Optical programmability, scalability, and direct access to both density and phase variables position this platform at the interface of quantum simulation, photonic circuit engineering, and fundamental studies of quantum matter.