Periodic Reporting for period 2 - PhoQuS (Photons for Quantum Simulation)
Periodo di rendicontazione: 2020-04-01 al 2022-03-31
Quantum simulation aims at reproducing and predicting experimentally the behaviour of complex quantum systems that are difficult to model analytically or numerically. It is a very active field for which several platforms, such as ultracold-atoms, trapped ions or superconducting circuits have been proposed. The targeted breakthrough of this project is the development of a novel platform for analogue quantum simulations based on the so-called quantum fluids of light.
The fluids of light are realised in different photonic systems with suitable non-linearities, allowing to engineer an effective photon-photon interaction. Our main objectives were to fully understand the superfluid and quantum turbulent regimes for quantum fluids of light and to achieve simulations of systems of very different nature, ranging from condensed matter to astrophysics.
The PhoQuS approach merges the theoretical and experimental techniques of controlled manipulation of quantum systems (photons) with many-body and quantum field physics. The project is based on fundamental physics and have an impact on fundamental science (quantum optics, solid state physics, cosmology, quantum gravity) and quantum technologies (quantum simulations).
The fluids of light are realised in different photonic systems with suitable non-linearities, allowing to engineer an effective photon-photon interaction. Our main objectives were to fully understand the superfluid and quantum turbulent regimes for quantum fluids of light and to achieve simulations of systems of very different nature, ranging from condensed matter to astrophysics.
The PhoQuS approach merges the theoretical and experimental techniques of controlled manipulation of quantum systems (photons) with many-body and quantum field physics. The project is based on fundamental physics and have an impact on fundamental science (quantum optics, solid state physics, cosmology, quantum gravity) and quantum technologies (quantum simulations).
In a first phase, we addressed the full characterisation of quantum fluids of light properties, including the observation of superfluid light and of turbulent regimes with the creation of topological excitations. In a second phase, we exploited the established playground to address the simulation of Black Holes in various geometries, to study phase transitions in driven-dissipative systems and to assess the potential of the photonic platforms to simulate strongly correlated systems.
1. Experimental observation and theoretical description of superfluid light
The characterisation of the superfluid behaviour of quantum fluids of light and the observation of quantum turbulence in several systems allowed measuring with an unprecedented resolution the spectrum of the collective excitations of driven dissipative quantum fluid of light. It allowed also to demonstrate that the stochastic evolution of the phase of a driven dissipative condensate follows the Kardar-Parisi-Zhang universlity class. This is an important breakthrough in the field as it demonstrates for the first time that quantum fluids of light can go beyond mean field physics and simulate systems whose evolution is governed by purely quantum effects such as quantum phase fluctuations in this specific case.
2. Simulation of unreachable systems, from condensed matter to astrophysics
The analogue quantum simulation of astrophysical and condensed matter systems was accompanied with studies of phase transitions unique to driven dissipative platforms. Experiments were prepared via theoretical studies, including numerical simulations of stochastic phenomena so as to enable the observation of predicted astrophysical effects (eg, Hawking radiation or superradiance). These also resulted in the discovery of new phenomena related to black hole horizons: analogue models, like black holes, have quasi-normal modes and calculations showed for the first time that these are excited by the zero-point fluctuations of the electromagnetic field. The general derivation of the result suggests that the same behaviour should be valid also for real black holes, due to the quantum fluctuation of the gravitational field. These setups also allowed demonstrating of single photon nonlinearity in polaritons, pushing quantum fluids of light deeply in the quantum regime and enabling the realization of fully programmable quantum logic gates in a fully scalable system.
3. Observation of turbulent regime with the generation of various topological excitations
On the one hand, numerical simulations led to the discovery of novel applications of photon fluids for the realisation of large-scale simulations of condensed matter models, and for universal computing applications. In particular, calculations of light propagation guided the implementation of a record size, fully scalable Ising machine. On the other hand, a new regime of propagation was discovered for polaritons, which stems from their dynamics out of equilibrium. This led to the development of all-optical methods to impring topological excitations of the fluid (soliton and vortex pairs) and to the observation of their controlled propagation over macroscopic distances. This control was also put to use to solve an optical maze, thus demonstrating the taming of the so-called snake instabilities, which inherently limit the excitation and propagation of topological excitations in quantum fluids.
1. Experimental observation and theoretical description of superfluid light
The characterisation of the superfluid behaviour of quantum fluids of light and the observation of quantum turbulence in several systems allowed measuring with an unprecedented resolution the spectrum of the collective excitations of driven dissipative quantum fluid of light. It allowed also to demonstrate that the stochastic evolution of the phase of a driven dissipative condensate follows the Kardar-Parisi-Zhang universlity class. This is an important breakthrough in the field as it demonstrates for the first time that quantum fluids of light can go beyond mean field physics and simulate systems whose evolution is governed by purely quantum effects such as quantum phase fluctuations in this specific case.
2. Simulation of unreachable systems, from condensed matter to astrophysics
The analogue quantum simulation of astrophysical and condensed matter systems was accompanied with studies of phase transitions unique to driven dissipative platforms. Experiments were prepared via theoretical studies, including numerical simulations of stochastic phenomena so as to enable the observation of predicted astrophysical effects (eg, Hawking radiation or superradiance). These also resulted in the discovery of new phenomena related to black hole horizons: analogue models, like black holes, have quasi-normal modes and calculations showed for the first time that these are excited by the zero-point fluctuations of the electromagnetic field. The general derivation of the result suggests that the same behaviour should be valid also for real black holes, due to the quantum fluctuation of the gravitational field. These setups also allowed demonstrating of single photon nonlinearity in polaritons, pushing quantum fluids of light deeply in the quantum regime and enabling the realization of fully programmable quantum logic gates in a fully scalable system.
3. Observation of turbulent regime with the generation of various topological excitations
On the one hand, numerical simulations led to the discovery of novel applications of photon fluids for the realisation of large-scale simulations of condensed matter models, and for universal computing applications. In particular, calculations of light propagation guided the implementation of a record size, fully scalable Ising machine. On the other hand, a new regime of propagation was discovered for polaritons, which stems from their dynamics out of equilibrium. This led to the development of all-optical methods to impring topological excitations of the fluid (soliton and vortex pairs) and to the observation of their controlled propagation over macroscopic distances. This control was also put to use to solve an optical maze, thus demonstrating the taming of the so-called snake instabilities, which inherently limit the excitation and propagation of topological excitations in quantum fluids.
Concerning future activities, the main challenging objectives are to address fully quantum features of the simulated systems. Typically, this will consist in measuring quantum correlations.
Firstly, the all-optical control on our quantum fluid out of equilibrium enables novel system designs for next generation experiments in which quantum correlations will be measurable (e.g. between the Hawking radiation escaping the acoustic horizon and its partner falling inside it). This is key to understanding quantum gravity as, in our system, Hawking radiation bears the imprint of the spacetime structure very near the horizon.
Secondly, we have demonstrated single photon nonlinearity in polaritons. This is the key to developing fully programmable quantum logic gates on the single photon level as well as to the analogue quantum simulation of strongly correlated systems. Although the achieved nonlinearity strength could yet be insufficient in the current platform, polariton lattices composed of several coupled micropillars (each behaving as a highly non linear device close to the quantum regime) have been realized in the framework of PhoQuS and constitute a very promising platform in this direction.
Finally, an unexpected route forward that opened during the project is the possibility to exploit photon fluids for applications in computing and machine learning, as classical and quantum simulators.
Firstly, the all-optical control on our quantum fluid out of equilibrium enables novel system designs for next generation experiments in which quantum correlations will be measurable (e.g. between the Hawking radiation escaping the acoustic horizon and its partner falling inside it). This is key to understanding quantum gravity as, in our system, Hawking radiation bears the imprint of the spacetime structure very near the horizon.
Secondly, we have demonstrated single photon nonlinearity in polaritons. This is the key to developing fully programmable quantum logic gates on the single photon level as well as to the analogue quantum simulation of strongly correlated systems. Although the achieved nonlinearity strength could yet be insufficient in the current platform, polariton lattices composed of several coupled micropillars (each behaving as a highly non linear device close to the quantum regime) have been realized in the framework of PhoQuS and constitute a very promising platform in this direction.
Finally, an unexpected route forward that opened during the project is the possibility to exploit photon fluids for applications in computing and machine learning, as classical and quantum simulators.