Near-equilibrium thermalised quantum light
Lead Research Organisation:
Imperial College London
Department Name: Physics
Abstract
Almost 60 years have passed since the first laser, one of the most important inventions of the 20th century, yet new mechanisms enabling highly coherent, directional light sources are still being discovered. Very recently, Bose-Einstein Condensation (BEC) of light has enabled exploration of the links between quantum statistics, phase transitions and lasers, not only expanding our understanding, but inspiring light sources with new capabilities. Such sources will enable simulation of quantum processes, otherwise intractable using modern computers, and imaging and sensing beyond the quantum limit by exploiting their unique quantum coherence properties.
It's not a trivial statement that photons can be made to thermalise and undergo Bose-Einstein condensation (BEC) at room temperature. A fluorescent medium in an optical resonator is optically excited. The resonator has many optical modes, but one has a well-defined ground state. Photons emitted into the resonator modes undergo thermalisation by absorption and re-emission with the fluorescent medium. This is facilitated by the vibrational states of the medium, which relax rapidly, to maintain thermal equilibrium. Quantum statistics ensure that, with enough photons, BEC will occur, even at room temperature, resulting in a macroscopic population of the ground-state resonator mode. BEC is a universal process, so photon BEC can be compared to condensation in atomic systems, or exciton-polariton microcavities.
This project uses four ingredients to shift the science of photon-based BEC from fundamental to applied research: quantum correlations, semiconductor photon BEC, planar waveguide resonators, and theoretical underpinning. Those ingredients of this project are:
(A) Measurement and control of the quantum correlations among photons.
- While lasers have well-defined Poissonian number statistics, the number of statistics of BEC are greatly influenced by the the fluorescent medium. We will measure both intra- and inter-mode correlations. In contrast to lasers, we expect that media made of finite numbers of emitters will generate sub-Poissonian correlations, e.g. relative-number squeezing. Using pulsed pumping and time-resolved measurements of non-stationary statistics we will uncover how to characterise and exploit these highly non-classical states of light.
(B) Photon thermalisation and condensation in an inorganic semiconductor device.
- The media used for photon BEC so far have been liquid dyes. By using a very standard inorganic semiconductor (GaAs) in a very non-standard way as the thermalisation medium, we will make devices whose properties (emission spectrum, threshold pump power, correlations) can be tuned through well-established fabrication techniques, suitable for robust and commercially viable technology.
(C) New planar resonators for photon BEC control.
- Open microcavity resonators have proven suitable for photon BEC and are flexible in terms of the potential-energy landscape for photons. We will explore condensation of propagating photons using an in-plane distributed-resonator geometry, where time can be mapped to propagation dimension. Effectively, we will achieve sub-picosecond temporal control over BECs by spatially varying resonator designs.
(D) Theory of photon correlations.
- The whole project will include a strong theoretical analysis and modelling programme. The basic model to be used is based on quantum master equations, applicable to both dyes and semiconductors. It will be solved with powerful numerical techniques to predict quantum correlations for conditions that well describe the experiments.
Devices will be fabricated using existing collaborations by our project supporters with established methods. While this research is primarily curiosity-driven, it will uncover new quantum states of light, methods for characterising them, and routes to exploiting them, which will be useful for quantum sensing and simulation.
It's not a trivial statement that photons can be made to thermalise and undergo Bose-Einstein condensation (BEC) at room temperature. A fluorescent medium in an optical resonator is optically excited. The resonator has many optical modes, but one has a well-defined ground state. Photons emitted into the resonator modes undergo thermalisation by absorption and re-emission with the fluorescent medium. This is facilitated by the vibrational states of the medium, which relax rapidly, to maintain thermal equilibrium. Quantum statistics ensure that, with enough photons, BEC will occur, even at room temperature, resulting in a macroscopic population of the ground-state resonator mode. BEC is a universal process, so photon BEC can be compared to condensation in atomic systems, or exciton-polariton microcavities.
This project uses four ingredients to shift the science of photon-based BEC from fundamental to applied research: quantum correlations, semiconductor photon BEC, planar waveguide resonators, and theoretical underpinning. Those ingredients of this project are:
(A) Measurement and control of the quantum correlations among photons.
- While lasers have well-defined Poissonian number statistics, the number of statistics of BEC are greatly influenced by the the fluorescent medium. We will measure both intra- and inter-mode correlations. In contrast to lasers, we expect that media made of finite numbers of emitters will generate sub-Poissonian correlations, e.g. relative-number squeezing. Using pulsed pumping and time-resolved measurements of non-stationary statistics we will uncover how to characterise and exploit these highly non-classical states of light.
(B) Photon thermalisation and condensation in an inorganic semiconductor device.
- The media used for photon BEC so far have been liquid dyes. By using a very standard inorganic semiconductor (GaAs) in a very non-standard way as the thermalisation medium, we will make devices whose properties (emission spectrum, threshold pump power, correlations) can be tuned through well-established fabrication techniques, suitable for robust and commercially viable technology.
(C) New planar resonators for photon BEC control.
- Open microcavity resonators have proven suitable for photon BEC and are flexible in terms of the potential-energy landscape for photons. We will explore condensation of propagating photons using an in-plane distributed-resonator geometry, where time can be mapped to propagation dimension. Effectively, we will achieve sub-picosecond temporal control over BECs by spatially varying resonator designs.
(D) Theory of photon correlations.
- The whole project will include a strong theoretical analysis and modelling programme. The basic model to be used is based on quantum master equations, applicable to both dyes and semiconductors. It will be solved with powerful numerical techniques to predict quantum correlations for conditions that well describe the experiments.
Devices will be fabricated using existing collaborations by our project supporters with established methods. While this research is primarily curiosity-driven, it will uncover new quantum states of light, methods for characterising them, and routes to exploiting them, which will be useful for quantum sensing and simulation.
Planned Impact
The proposed research is largely curiosity-driven, with an eye being kept on possible applications. As such, the main impact of the work will be within academia. The academic impact will be enhanced by organising a workshop (for which funds have been requested). The workshop will bring together scientists from a variety of academic communities, potentially including quantum optics, solid-state and laser physicists, materials scientists and photonic engineers.
Some of the techniques, especially the in-plane resonators and solid-state device advances, can be expected to be picked up by research groups, both within academia and in industrial research and development.
The training of PhD students and research staff working on the project is taken very seriously. The investigators are strongly linked to two Centres for Doctoral Training. Staff and students will be encouraged to participate in conferences and workshops, and make use of careers services such as Imperial College's Postdoc and Fellows Development Centre.
The investigators and research staff will have the opportunity to link up with Imperial College's excellent outreach programmes to discuss this work and its implications with the general public. PhD students recruited to work on the project via Centres for Doctoral Training will participate in events with public audiences as part of their training. This work will be made public, both through outreach and open access publications.
Some of the techniques, especially the in-plane resonators and solid-state device advances, can be expected to be picked up by research groups, both within academia and in industrial research and development.
The training of PhD students and research staff working on the project is taken very seriously. The investigators are strongly linked to two Centres for Doctoral Training. Staff and students will be encouraged to participate in conferences and workshops, and make use of careers services such as Imperial College's Postdoc and Fellows Development Centre.
The investigators and research staff will have the opportunity to link up with Imperial College's excellent outreach programmes to discuss this work and its implications with the general public. PhD students recruited to work on the project via Centres for Doctoral Training will participate in events with public audiences as part of their training. This work will be made public, both through outreach and open access publications.
Publications
Hunter-Gordon M
(2020)
Quantum simulation of the dephasing Anderson model
in Physical Review A
Walker BT
(2019)
Noncritical Slowing Down of Photonic Condensation.
in Physical review letters
Rodrigues JD
(2021)
Learning the Fuzzy Phases of Small Photonic Condensates.
in Physical review letters
Holmes Z
(2020)
Enhanced Energy Transfer to an Optomechanical Piston from Indistinguishable Photons.
in Physical review letters
Description | We have performed both theoretical and experimental studies on how light can undergo phase transitions by interacting with matter in open optical microcavities - two high-quality mirrors held about 1 micron apart. In the theory work we discovered that close to these phase transitions critical slowing down occurs (which is expected but still interesting) but also in a specific regime we call "decondensation" there is slowing down of dynamics even far from the phase transition (which is novel and unexpected). Part of this theory work involved developing a new hierarchical approximation technique for solving an important class of equations of motion. We have used these technical advances to help us study transport and localization of light. In the experiments, we have recently observed time-resolved statistics (averages and second-order correlations) of photon condensates after a short pulse of excitation. As a result, we have inferred that there is strong timing jitter in the formation of a condensate showing convincingly that Bose-Einstein condensation is a stochastic not deterministic process. We have mapped the system response over a larger parameter space, and showed how the condensation and lasing phase transitions can be understood with the assistance of machine learning through a new concept we call "fuzzy phases". We have worked with our partners in Sheffield to fabricate samples of semiconductors suitable for use in open microcavities for studies of thermalisation and non-linear optics. We have also made observations with collaborators in Nice on thermal-photon emission spectra from standard devices (VCSELs, with monolithic not open microcavities), which show how ubiquitous photon Bose-Einstein condensation may be. With our collaborators in Oxford, we have fabricated extraordinary mirror substrate shapes for further statistical mechanics studies of light in dye-filled microcavities. Our work with Sheffield has now led to the demonstration of Photon BEC in semiconductor quantum well micro cavities. These a robust and highly commercialised materials. This means that this research can move to exploitation phase, which is our next step of research. We have now submitted this work for publication. This work opens new fundamental research directions including the coherence properties of semiconductor lasers and the potential to harness superfluid properties of light. Since our demonstration of Condensation of light in a semiconductor system, the goals of our project have now been completely realised. This demonstration has opened new avenues of research, which we now aim to exploit by applying for further funding. |
Exploitation Route | The proposed research is largely curiosity-driven, with an eye being kept on possible applications. As such, the main impact of the work will be within academia. The academic impact will be enhanced by organising a workshop towards the end of the grant, bringing together scientists from a variety of academic communities, potentially including quantum optics, solid-state and laser physicists, materials scientists and photonic engineers. Cross-fertilization of ideas across these themes will also be used to attract companies who might be interesting in exploiting what we learn in this grant. This project is strongly supported by two UK partner institutions. At Oxford, Jason Smith's team are world leading at fabricating open microcavities, using them for a variety of applications (including a new spinout company, High Q Instruments). In particular, we are keen to work with Oxford High Q on their sensing projects, which utilise related concepts. The Sheffield group (E. Clarke, but also M. Skolnick, D. Lidzey, D. Krizanovskii) run the national III-V semiconductor facility and also have interests in both semiconductor and organic exciton-polariton condensates in microcavities. The training of PhD students and research staff working on the project is taken very seriously. Staff and students will be encouraged to participate in conferences and workshops, and make use of careers services such as Imperial College's Postdoc and Fellows Development Centre. The investigators and research staff will have the opportunity to link up with Imperial College's excellent outreach programmes to discuss this work and its implications with the general public. The software underlying the simulations is available as an open-source repository https://github.com/photonbec/PyPBEC . We have demonstrated Photon BEC in semiconductor quantum well samples. These devices are robust and potentially scalable for commercial exploitation. This project will now move towards applications of Photon BEC. This will be for ultra-stable lasers, computation using photon condensation and correlated light sources. |
Sectors | Aerospace Defence and Marine Digital/Communication/Information Technologies (including Software) Environment Healthcare Pharmaceuticals and Medical Biotechnology Other |
Description | Photons for Quantum Simulation |
Amount | € 299,975,750 (EUR) |
Funding ID | 820392 |
Organisation | European Commission |
Sector | Public |
Country | European Union (EU) |
Start | 09/2018 |
End | 03/2022 |
Title | Software model of photon condensation in dye filled micro cavities |
Description | The software underlying the simulations is available as an open-source repository https://github.com/photonbec/PyPBEC . |
Type Of Material | Computer model/algorithm |
Year Produced | 2020 |
Provided To Others? | Yes |
Impact | Too early to say as this has only recently been uploaded to GitHUB software repository. |
URL | https://github.com/photonbec/PyPBEC |
Description | Ed Clarke University of Sheffield |
Organisation | University of Sheffield |
Country | United Kingdom |
Sector | Academic/University |
PI Contribution | Design and Characterization of plasmonic lasers based on III-V semiconductor materials |
Collaborator Contribution | Growth of III-V materials for plasmonic lasers and some device processing including electron beam lithography, photo-lithography, etching. |
Impact | Still in progress. We intend to demonstrate the metal-based plasmonic lasers with GaAs as a laser gain medium. |
Start Year | 2011 |
Description | Semiconductor sample fabrication |
Organisation | University of Sheffield |
Country | United Kingdom |
Sector | Academic/University |
PI Contribution | We have specified the samples to be fabricated. In future, we will characterise these samples and make use of them for photon thermalisation in open optical microcavities. |
Collaborator Contribution | They have fabricated samples of semiconductors (specifically GaAs, both bulk and quantum wells, on a distributed Bragg mirror on a GaAs substrate). |
Impact | Samples have been fabricated. |
Start Year | 2019 |
Title | PyPBEC |
Description | Software to simulate fluorescent media in multimode optical resonators, most specifically targetted at photon thermalisation and condensation in microcavities. The software has been used extensively within our group, and we have made it open source on GitHub. The software runs in python, and is not a stand-alone tool, but rather it is aimed at scientists with some programming experience. |
Type Of Technology | Software |
Year Produced | 2020 |
Open Source License? | Yes |
Impact | The physical model implemented by the software is applicable to many research topics, and potentially to undergraduate teaching projects too, so we anticipate it being used and contributed to by several research groups. There is an article from our team, which has passed peer review and will be published soon (pre-print https://arxiv.org/abs/2010.08325) which builds on the software. |
URL | https://github.com/photonbec/PyPBEC |