Historically originated in the 1660s through the study of visible light dispersed by a prism according to its wavelength, spectroscopy has evolved into the prime matter investigation technique, sprouting an uncountable amount of experimental realizations of increasing complexity, based on non-linear couplings of an electromagnetic field with atomic and molecular systems. Conventional nonlinear spectroscopy uses classical light to detect matter properties through the variation of its response with frequencies and time delays among complex pulses sequences.
Quantum optics studies those properties of light which cannot be described by classical electromagnetic theory. Among them, the strong photon correlations enabled by quantum mechanics known as quantum entanglement, which have been proven to be critical in diverse areas, ranging from secure quantum communication, computing and metrology. These two disciplines are, to date, distinct branches of photonics, with different communities and objectives. Very recently, however, visionary theoretical work suggested implications of quantum optics for spectroscopy. Building on the complementary expertise of the team members in the fields of quantum optics, spectroscopy and biochemistry, here we propose the experimental realization of a spectroscopic set up, exquisitely conceived to exploit the properties of entangled photons to prepare and interrogate out-of-equilibrium states of matter, free from the ties imposed by the Heisenberg principle. Specifically, we will develop an efficient source of entangled photon pairs, which will be used to perform two-photon absorption with unprecedented energy selectivity and efficiency. Entangling the photons will be the key to disentangle system dynamics, which otherwise would be obscured by the constrained time-energy distributions of individual particles. This will enable conveying energy into molecular reaction centres throughout manifolds of complex pathways in an orchestrated way.
Experimental realizations of entangled light spectroscopy are still behind the few theoretical studies [17]. The reason is manifold: on the one hand a deep understanding of the proposed schemes is necessary, to be able to identify the critical details and implement experiments accordingly, leaving out unnecessary complications. On the other hand, it is imperative to master quantum photonic technologies, which are not relevant to traditional nonlinear time-resolved spectroscopies. If accepted, this proposal will boost the collaboration between two leading experimental groups in these complementary fields, cross fertilizing the different expertise from both, in order to guarantee a successful development of an entangled light spectroscopic setup. At the same time, it will allow the application to biosciences, superseding the current control capabilities of reaction pathways relevant for energy redistribution processes in photosynthesis. Understanding this process has inspired much work of an inter-disciplinary nature to uncover basic mechanisms at play, in view of technological application for alternative energy sources development [21-24]. Despite the efforts, bio-inspired artificial systems remain still less efficient and less stable than their natural counterparts. In particular, excitation energy transfer within the RC and the subsequent charge separation cascade convert the sunlight's energy into a chemical potential with near unit efficiency. The precise mechanisms regulating this process and its efficiency are currently under debate [25]. Critically, elucidating the specific way that rules the many-body physics of the bacterial RC could provide important insights for the construction of optoelectronic devices, which benefit from using the near-IR region of the solar spectrum. In order to realize efficient artificial photosystems, it is crucial to obtain a detailed picture of the involved electronic energy levels and relevant energy pathways in natural light harvesters. Taking advantage of the unique entangled photons correlations, the proposed spectroscopic protocol with quantum light offers a promising tool to reveal this structure with time and frequency resolution beyond those reachable by classical analogues.
This project is framed in a research program aimed at developing novel quantum technologies. The interest of European Union in this research field is expressed by the 1 billion Quantum Technology Flagship initiative (https://ec.europa.eu/digital-single-market/en/news/european-commission-w...), which is planned to start in 2018. One of the main objectives of the Flagship is the development of novel quantum sensing technologies, to exploit the potential of quantum physics to improve sensitivity for imaging, metrology and spectroscopy. Thus, this research project is expected to contribute in this direction by applying quantum resources towards improve the resolution achievable in spectroscopic techniques. This will significantly increase the capability of Sapienza University of Rome to obtain additional funding with the European Flagship.
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