Final Report Summary - POLAFLOW (Polariton condensates: from fundamental physics to quantum based devices)
The POLAFLOW consortium, coordinated by Lecce (PI D. Sanvitto), with partners in Madrid, Spain, later in Wolverhampton, UK (group led by F.P. Laussy) and Heraklion, Crete (group led by P. Savvidis) aims at investigating the fundamental dynamics of polariton quantum fluids and study their prospects for applications as all-optical devices, in particular as components for a quantum technology.
Polaritons are mixed states of light and matter, with a peculiar particle dispersion (markedly non-parabolic), large nonlinearities from their strong particle interactions (as compared to photons in nonlinear crystals) and high coherence (as compared to solid state qubits). Basically, they have the coherence of light and the interaction strength of matter. With these quasi-particles, it is possible to create so-called a “polariton condensate”, namely, an highly occupied single-state wavefunction, in loose analogy with the textbook Bose-Einstein condensate (BEC). The analogy breaks down from the out-of-equilibrium nature of polaritons, since they have a short lifetime (from few hundreds of femptoseconds to hundreds of picoseconds), which prevents their formation from the standard route of a phase-transition but requires an external pumping. Once formed and within their short lifespan, such condensates can nevertheless be manipulated and observed. In particular, polaritons can propagate, be routed and brought to interact and/or interfere so as to perform useful tasks, and it was the aim of POLAFLOW to investigate the possible applications of these versatile and wondrous objects.
In the project we have achieved more than was originally foreseen in the original proposal. In fact, we have even opened to a different area of research, focusing also on organic and hybrid materials and plasmonics. In particular, we have realised an AND and OR gates working with polariton flows and demonstrating in this way cascadability and gain, two essential elements needed to implement an all-optical logic based on polariton circuits. We have investigated frequency and time correlation in polariton condensates and demonstrated a new phenomenology of the second order correlation function (g2) which extends the results of time correlation to a much richer landscape: the one of energy correlations. In this way, we have generalised the well known Hanbury-Brown and Twiss effect. In addition to what was described in the original proposal and thanks to the development of a very fast detection technique, we could study time-resolved Rabi oscillations between the exciton and photon states of a collective polariton population. This has led to the observation of a rich dynamics of the oscillations which not only depends on the polariton lifetime and population occupancy of the two branches, but also–and surprisingly–on the incoherent injection of polaritons into the ground (lower) state of the system. Acting with multiple laser beam we demonstrated full control on the light-matter state of the polaritons, showing the possibility of producing coherent beam of light with an arbitrary fast rotation of their polarisation and realising full Poincaré beams in time (passing through all states of polarisations in a single pulse). Exploiting the same technique, we have also observed a unique self-focusing effect when polaritons are suddenly populated by a short-time laser pulse. This effect is theoretically under study and seems to be unique of the polariton particle, while completely absent in other quantum systems or even classical nonlinear media. Also in this direction, we have studied the effect of polariton interactions on quantum vortices injected in a polariton fluid. We have discovered an interesting phenomenology of attraction, scattering and rotation due to several force fields, sometime unknown from the standard atomic condensates. Furthermore, we have studied the combination of coherent polarisation fields and vorticity and how these evolve in time under the influence of a dense polariton population. We have observed X-waves and room-temperature superfluidity in an organic-based microcavity. Lately, we have demonstrated that a polariton condensate in a very high finesse microcavity undergoes a BKT phase transition, which was until now relegated only to condensates at equilibrium (for which the 2016 Nobel prize was awarded).
The project has also spurred a purely theoretical line to support the experiment but that sometimes went farther than was technologically possible to implement to date. In particular, a concept for a new type of quantum light has been proposed that consists of 100% emission of energy in packets of exactly N photons. Such a device was designed based on a particular case of strong light-matter coupling, between a single emitter (such as an excitonic transition in a quantum dot) and a small volume photonic mode. The theory shows that this makes it possible to obtain bundles of photons of any desired number by driving the system at adequate resonant frequencies. Further down this research line, the full photon-correlations from resonance fluorescence has been mapped in N-dimensional frequency hyperspace and a concept for a universal quantum emitter, able to generate any combination of pre-determined photon output, has been hinted at and is currently under active investigation. Using particular cases of this general idea, a concept of “Mollow spectroscopy” has been put forward that consists at studying the statistics of transmitted light from a quantum source that excites a target with a tunable range of photon statistics. Some particular cases of this general idea of a new kind of quantum spectroscopy have been successfully implemented in the laboratory with conventional two-photon sources generated by a nonlinear crystal. In this configuration, we have been able to positively answer two fundamental questions raised by POLAFLOW at the start of the project: are polaritons good carriers of quantum information and are interactions sufficiently strong to affect the quantum state of a single polariton? Such questions that have been both answered positively suggest that polaritons can be extremely useful for many applications in quantum optics, communication, metrology and even for photo lithography and medical applications.
Polaritons are mixed states of light and matter, with a peculiar particle dispersion (markedly non-parabolic), large nonlinearities from their strong particle interactions (as compared to photons in nonlinear crystals) and high coherence (as compared to solid state qubits). Basically, they have the coherence of light and the interaction strength of matter. With these quasi-particles, it is possible to create so-called a “polariton condensate”, namely, an highly occupied single-state wavefunction, in loose analogy with the textbook Bose-Einstein condensate (BEC). The analogy breaks down from the out-of-equilibrium nature of polaritons, since they have a short lifetime (from few hundreds of femptoseconds to hundreds of picoseconds), which prevents their formation from the standard route of a phase-transition but requires an external pumping. Once formed and within their short lifespan, such condensates can nevertheless be manipulated and observed. In particular, polaritons can propagate, be routed and brought to interact and/or interfere so as to perform useful tasks, and it was the aim of POLAFLOW to investigate the possible applications of these versatile and wondrous objects.
In the project we have achieved more than was originally foreseen in the original proposal. In fact, we have even opened to a different area of research, focusing also on organic and hybrid materials and plasmonics. In particular, we have realised an AND and OR gates working with polariton flows and demonstrating in this way cascadability and gain, two essential elements needed to implement an all-optical logic based on polariton circuits. We have investigated frequency and time correlation in polariton condensates and demonstrated a new phenomenology of the second order correlation function (g2) which extends the results of time correlation to a much richer landscape: the one of energy correlations. In this way, we have generalised the well known Hanbury-Brown and Twiss effect. In addition to what was described in the original proposal and thanks to the development of a very fast detection technique, we could study time-resolved Rabi oscillations between the exciton and photon states of a collective polariton population. This has led to the observation of a rich dynamics of the oscillations which not only depends on the polariton lifetime and population occupancy of the two branches, but also–and surprisingly–on the incoherent injection of polaritons into the ground (lower) state of the system. Acting with multiple laser beam we demonstrated full control on the light-matter state of the polaritons, showing the possibility of producing coherent beam of light with an arbitrary fast rotation of their polarisation and realising full Poincaré beams in time (passing through all states of polarisations in a single pulse). Exploiting the same technique, we have also observed a unique self-focusing effect when polaritons are suddenly populated by a short-time laser pulse. This effect is theoretically under study and seems to be unique of the polariton particle, while completely absent in other quantum systems or even classical nonlinear media. Also in this direction, we have studied the effect of polariton interactions on quantum vortices injected in a polariton fluid. We have discovered an interesting phenomenology of attraction, scattering and rotation due to several force fields, sometime unknown from the standard atomic condensates. Furthermore, we have studied the combination of coherent polarisation fields and vorticity and how these evolve in time under the influence of a dense polariton population. We have observed X-waves and room-temperature superfluidity in an organic-based microcavity. Lately, we have demonstrated that a polariton condensate in a very high finesse microcavity undergoes a BKT phase transition, which was until now relegated only to condensates at equilibrium (for which the 2016 Nobel prize was awarded).
The project has also spurred a purely theoretical line to support the experiment but that sometimes went farther than was technologically possible to implement to date. In particular, a concept for a new type of quantum light has been proposed that consists of 100% emission of energy in packets of exactly N photons. Such a device was designed based on a particular case of strong light-matter coupling, between a single emitter (such as an excitonic transition in a quantum dot) and a small volume photonic mode. The theory shows that this makes it possible to obtain bundles of photons of any desired number by driving the system at adequate resonant frequencies. Further down this research line, the full photon-correlations from resonance fluorescence has been mapped in N-dimensional frequency hyperspace and a concept for a universal quantum emitter, able to generate any combination of pre-determined photon output, has been hinted at and is currently under active investigation. Using particular cases of this general idea, a concept of “Mollow spectroscopy” has been put forward that consists at studying the statistics of transmitted light from a quantum source that excites a target with a tunable range of photon statistics. Some particular cases of this general idea of a new kind of quantum spectroscopy have been successfully implemented in the laboratory with conventional two-photon sources generated by a nonlinear crystal. In this configuration, we have been able to positively answer two fundamental questions raised by POLAFLOW at the start of the project: are polaritons good carriers of quantum information and are interactions sufficiently strong to affect the quantum state of a single polariton? Such questions that have been both answered positively suggest that polaritons can be extremely useful for many applications in quantum optics, communication, metrology and even for photo lithography and medical applications.