Quantum light for investigating complex molecules and materials

09:00-09:05 Welcome by the Royal Society and lead organiser

09:05-09:35 Novel nonlinear spectroscopy and imaging of molecules with quantum light

Professor Shaul Mukamel, University of California Irvine, USA

Abstract

Quantum light opens up new avenues for spectroscopy by utilizing parameters of the quantum state of light as control knobs and through the variation of photon statistics by coupling to matter. Nonlinear optical signals induced by quantized light fields and entangled photon pairs will be surveyed and classified. When a molecule interacts with an external field, the phase information is imprinted in the state of the field in a detectable way. Entangled-photon pairs are not subjected to the classical Fourier limitations on their joint temporal and spectral resolution. Combined time and frequency resolution not possible by classical light can therefore be achieved. A novel quantum diffraction-based imaging technique whereby one photon of an entangled pair is diffracted of a sample and detected in coincidence with its twin is presented. Imaging is possible with weak quantum fields, avoiding damage to delicate biological samples. Strong coupling of molecules to the quantum vacuum field of micro cavities that can be used to manipulate their photophysical and photochemical reaction pathways and polariton relaxation are demonstrated. For photosynthetic antennae. 

[1] “Quantum phase-sensitive diffraction and imaging using entangled photons”, Shahaf Asban, Konstantin E. Dorfman, and, Shaul Mukamel. PNAS (2019) 116, 11673-11678
[2] "Entangled two-photon absorption spectroscopy", Frank Schlawin, Konstantin E. Dorfman and S. Mukamel. Acc. Chem. Res., 51, 2207-2214 (2018)
[3] Konstantin E. Dorfman, Frank Schlawin, and Shaul Mukamel. "Nonlinear optical signals and spectroscopy with quantum light", Rev. Mod. Phys. 88, 045008 (2016)
[4] Suppression of Population transport and Control of Exciton Distributions by Entangled Photons ", F. Schlawin, K.E. Dorfman, B.P. Fingerhut, and S. Mukamel. Nature Communications, 4:1782: DOI: 10.1038/ncomms2802 (2013).

09:35-09:50 Discussion

09:50-10:20 Sub shot noise transmission spectroscopy and imaging with correlated photon pairs

Dr Jonathan Matthews, Department of Physics, University of Bristol, UK

Abstract

Optics is routinely used to measure physical parameters. Quantum mechanics can describe how each strategy is limited in precision, as quantified by the resources used to perform each type of optical measurement — the fundamental noise floor of optical measurements using a laser is the shot noise limit. Application of quantum states of light can reduce noise for optical measurement below this limit. In this presentation, I will discuss recent demonstrations using correlated photon pairs to reach and surpass this limit in practice for transmission estimation [1,2], and how it can be applied to transmission spectroscopy [3] and imaging [4]. We will discuss how resources are accounted in experiments, the physical principles used and we will cover steps being taken to take the proof of principle post-selected experiments into practical use outside of the quantum optics laboratory, including a recent work on passive noise suppression of pulsed fibre laser emission to the shot noise limit [5]. 

References 
[1] Sabines-Chesterking, J. et al. Phys. Rev. Applied, 8, 014016 (2017) 
[2] Moreau, P. et al. Sci. Reports. 7, 6256 (2017)
[3] Whittaker, R. et al. New J. Phys. 023013, 19, (2017) 
[4] J. Sabines-Chesterking, et al. Optics Exp. 21, 30810 (2019)
[5] E. Allen et al, pre-print: arXiv:1093.12598

10:20-10:35 Discussion

10:35-11:00 Coffee break

11:00-11:30 Quantum light spectroscopy in organic and biological molecules

Professor Ted Goodson, Department of Chemistry, University of Michigan, USA

Abstract

This talk will focus on the investigations of nonlinear optical effects utilizing quantum light.  Illustrations of absorption and fluorescence of the entangle two photon process in organic and biological systems will be presented.  Investigations of the entanglement characteristics and their effect on the nonlinear excitation process will be discussed.  The results and discussion will be directed toward further understanding of the nature of the interaction of entangled light with organic matter with a view of possible applications.

11:30-11:45 Discussion

11:45-12:15 Challenges in measurement of two-photon absorption cross-sections with nonclassical light

Dr Ralph Jimenez, JILA, National Institute of Standards and Technology, Boulder, USA

Abstract

We are investigating methods to advance the accuracy and reliability of molecular spectroscopy with nonclassical light. Molecular two-photon absorption (TPA) of correlated photon pairs produced by spontaneous parametric down-conversion (SPDC) is expected to occur in a linear excitation regime, and has been reported to occur at many orders of magnitude lower photon flux than classical TPA. A major challenge in observing nonclassical excitation of molecules is to discriminate TPA from one-photon losses. Standard transmittance measurements are insufficient because single-photon loss due to scattering, absorption, or misalignment all scale linearly with flux. In contrast, a distinction between TPA and single-photon loss in a sample can be made by measuring changes in the photon statistics of the transmitted beam. The second order intensity correlation function, g⁽²⁾, is insensitive to linear losses, yet changes in g⁽²⁾ are related to the strength of the two-photon interaction. We performed comprehensive transmittance experiments for Zinc tetraphenylporphyrin (ZnTPP) in solution using an 810 nm pulsed SPDC source. Prima facie, the transmittance changes are consistent with a cross section of ~10^-17 cm². However, we observed only a weak dependence on signal-idler photon time delay and a small change in g⁽²⁾, indicating that the cross section for correlated TPA by ZnTPP is much smaller than previously reported. 

12:15-12:30 Discussion

13:30-14:00 Manipulating the character and shape of ultrashort quantum light states

Dr Marco Bellini, National Institute of Optics (CNR-INO), Italy

Abstract

A fully controlled state and mode engineering of nonclassical light is one of the major challenges in the path of future quantum technologies.
In recent years, quantum state engineering has quickly evolved, with new tools and techniques, such as photon addition and subtraction, which have demonstrated their extreme versatility for performing operations normally unavailable in the realm of Gaussian quantum optics. While photon subtraction can enhance nonclassicality and entanglement in a quantum light state, photon addition has the unique capability of creating nonclassicality and entanglement from scratch, whatever the input. However, engineering quantum states in a single, well-defined, mode is rarely enough. In real experiments, states are often prepared in modes that do not coincide with those used for their processing and detection, or for the optimal coupling to matter systems. Gaining access to the rich mode structure of quantum light would also greatly increase the capacity of communicating, manipulating, and storing quantum information. For all these tasks, and for those related to possible spectroscopic applications of nonclassical light, it is of fundamental importance to gain a full control over the modes that host the quantum states. In this talk, I will briefly present some of our recent experimental results towards the controlled generation, manipulation, and detection of the quantum state and mode structure of ultrashort light wavepackets.

14:00-14:15 Discussion

14:15-14:45 Exploiting quantum correlations for spectroscopy, imaging and remote sensing

Professor John Rarity FRS, University of Bristol, UK

Abstract

In this talk I will review recent work on sensing technologies focussing on pair photon correlations applied to rangefinding and remote sensing. These experiments primarily exploit the time (energy) correlations between photons [1] and were recently popularised as quantum illumination [2]. I will review recent experimental results showing the possibility of 3D imaging and covert rangefinding using parametric pair photon sources. I will also review work towards practical photon limited gas sensing using near infra-red wavelengths [3] and extensions to longer wavelengths using non-linear interferometry [4] in bulk and chip scale experiments [5, 6]. 

[1] J.G. Rarity et al Applied Optics 29, 2939 (1990).

[2] S. Lloyd, Science 321, 1463 (2008).

[3] M Quatrevalet, JGR, et al, IEEE J. Selected Topics in Quantum Electronics 23, 157 (2017).

[4] T.J. Herzog, JGR et al Physical Review Letters, 72, 629 (1994).

[5] T. Ono, JGR et al, Optics letters 44 (5), 1277-1280 (2019). 

[6] L. Rosenfield JGR et al arXiv:1906.10158.

14:45-15:00 Discussion

15:00-15:30 Tea break

15:30-16:00 Infrared metrology with visible light

Dr Leonid Krivitskiy, Agency for Science, Technology and Research (A*STAR), Singapore

Abstract

Infrared (IR) optical range is important for material characterization and sensing. Also, imaging in the IR range yields superior image contrast due to a significant reduction of scattering losses. Thus IR metrology is widely used in petrochemical, pharma, biomedical, homeland security, and other areas.
Even though there are well-developed conventional methods for IR metrology, the remaining challenges are associated with high cost, low efficiency and regulatory requirements for IR light sources and detectors. To mitigate these issues the team is developing new quantum-enabled techniques which allow them to retrieve properties of materials in the IR range from the measurements of visible range photons. The approach is based on the nonlinear interference of frequency correlated photons produced via spontaneous parametric down conversion (SPDC) [1, 2]. Within this process, one of the photons is generated in the visible range, and its correlated counterpart in the IR range is used to probe the properties of the medium. The visibility and phase of the observed fringes depend on the properties of the IR photon, which interacts with the sample. This allows the team to infer the properties of the sample in the IR range from the measurements of visible range photons. In a series of experiments, they demonstrate the IR spectroscopy [1-3], tunable optical coherence tomography (OCT) [4], and polarimetry [5]. In all these demonstrations the IR properties (absorption spectra, refractive index, 3D images, and polarization) are inferred from the measurements of the interference pattern in the visible range thus making IR measurements more affordable.

[1] D. Kalashnikov, A. Paterova, S. Kulik, L. Krivitsky, Nature Photonics 10, 98 (2016).

[2] A. Paterova, S. Lung, D. Kalashnikov, L. Krivitsky, Scientific reports 7, 42608 (2017).

[3] A. Paterova, H. Yang, Ch. An, D. Kalashnikov, L. Krivitsky, New Journal of Physics 20, 043015 (2018).

[4] A. Paterova, H. Yang, Ch. An, D. Kalashnikov, L. Krivitsky, Quantum Science & Technology 3, 025008 (2018).

[5] A. Paterova, H. Yang, C. An, D. Kalashnikov, L. Krivitsky, Opt. Express 27, 2589-2603 (2019).

16:00-16:15 Discussion

16:15-16:45 Single photon spectroscopy of atoms and solids

Dr Alex S Clark, Imperial College London, UK

Abstract

The absorption and reflection of light allows us to determine the properties of the world around us. Alex S Clark will present two recent experiments using single photons to perform absorption spectroscopy. The first uses photons generated by four-wave mixing in optical fiber. Two pump photons (~750nm) are annihilated to simultaneously generate time-correlated pairs of photons at widely spaced wavelengths (~650nm signal and ~900nm idler). The researchers tune the pump wavelength to scan idler photons across the absorption band-edge of gallium arsenide placed in the idler beam path. The transmission of the idler photons alone does not reveal the absorption due to overlapping Raman noise, but when correlated with detected signal photons, the absorption is revealed, a form of ‘ghost-spectroscopy’.  The second experiment uses a bright pump laser to saturate rubidium atoms in a vapour cell, while a counter-propagating and cross-polarized single photon level probe reveals narrow transmission peaks corresponding to the hyperfine excited state energy levels of the atoms. The pump and probe are independently tunable, meaning atoms moving in all directions can be addressed by compensating the counter-propagating optical frequencies for the relevant Doppler shifts. This experiment forms a basis for building an atomic quantum memory.

16:45-17:00 Discussion

17:00-18:15 Poster session

09:00-09:30 TBC

Abstract

09:30-09:45 Discussion

09:45-10:15 Quantum Spectroscopy with entangled photon absorption and emission

Dr Frank Schlawin, Department of Physics, University of Oxford, UK

Abstract

Quantum spectroscopy aims to exploit quantum properties of radiation. The use of entangled photons promises nonlinear spectroscopies at low photon fluxes and the circumvention of certain classical Fourier uncertainties that restrict the simultaneous time and frequency resolution. Much less attention has been paid to the spectroscopic information contained in the light emitted from a molecular sample. In this talk, we propose photon correlation measurements as a new spectroscopic tool. We will analyse the information contained in photon correlation measurements of a tractable model system, where we show that these can signal the presence of quantum coherence. Deviations from the counting statistics of independent emitters constitute a direct fingerprint of quantum coherence in the steady state. In contrast to typical approaches of two-dimensional spectroscopy, which rely in the use of fast coherent pulses, the proposed method can be employed in stationary setups in which the system is driven by thermal light, enabling measurements under the natural conditions of solar light-harvesting. Furthermore, we show that, even when coherence is washed out in the stationary state, coherent dynamics can still be evidenced by frequency-resolved counting statistics.

10:15-10:30 Discussion

10:30-11:00 Coffee break

11:00-11:30 Nonlinear multi wave mixing spectroscopy with quantum light

Professor Konstantin Dorfman, State Key Laboratory of Precision Spectroscopy, East China Normal University, China

Abstract

The progress in quantum optics utilizes a unique photon state configuration for engineering of the ultimate light-matter interactions with relatively simple material systems (qubits). It results in a broad range of applications including radiation sources, quantum communication, information, computing and nanotechnology. The development of the ultrafast multidimensional nonlinear spectroscopy that has been enabled by progress in ultrafast optical technology provides a unique tool for probing complex molecules, semiconductors, nanomaterials by classical light fields. In particular squeezed light generated in four-wave mixing experiments provide to be an exciting multiphoton analogue of the single photon sources that have robust quantum correlations. In this talk I reviewed our recent results in the field of nonlinear spectroscopy with quantum light. In particular, I show how quantum squeezing generation in the four-wave mixing can be used as a multidimensional spectroscopy tool for probing Brillouin and Raman scattering. We further show how various interferometers such as cascading and SU(1,1) commonly used in quantum information theory provide an additional degree of control of spectroscopic signals with enhanced resolution. Our theory, which contains both microscopic treatment of the squeezing generation as well as noise treatment, is in a good agreement with the recent experiments in hot rubidium vapor.

11:30-11:45 Discussion

11:45-12:15 Understanding the quantum efficiency of light harvesting photosynthetic systems one photon at a time

Professor Birgitta Whaley, Department of Chemistry, University of California Berkely, USA

Abstract

Photosynthetic light harvesting of plants and bacteria in vivo is characterized by the remarkable capability of producing electron-hole pairs from absorbed photons with near unit quantum efficiency under weak illumination conditions.  This quantum efficiency is usually measured from bulk kinetic studies.  While ultrafast spectroscopic studies have revealed much insight into the microscopic dynamics of excitonic energy transport that follows the initial process of photoexcitation, the microscopic dynamics and energetics of the initial absorption of photons from the sun’s radiation field are still poorly understood.  Yet these are essential components for a complete and consistent microscopic understanding of the dynamical processes underlying the quantum efficiency for conversion of the energy of a single photon to an electron-hole pair. I shall present a theoretical analysis of scattering/absorption of states of light with exactly one photon from a realistic light harvesting system. The analysis leads to calculation of transmission, fluorescence, and absorption probabilities for comparison with experiments using quantum light sources.  Our approach yields the relations between output and input photon fluxes, in addition to probabilities of excitation and excitonic energy transport in the light harvesting complex, including the effects of phonons in the latter, i.e., the vibrations of the chromophores and their protein environment.   While the basic theory employs a density matrix approach, producing average excitation probabilities for different excitonic states as well as photon fluxes for transmission and fluorescence, I shall also present a theory for calculation of individual quantum trajectories based on the detection and counting of fluorescent photons.  

12:15-12:30 Discussion

13:30-14:00 Cavity-engineering of molecular and materials properties from first principles QEDFT

Professor Angel Rubio, Max Plank Institute for Structure and Dynamics of Matter at Hamburg, Germany

Abstract

Computer simulations that predict the light-induced change in the physical and chemical properties of complex systems usually ignore the quantum nature of light. Here we will show the effects of quantum-photons can be  included in the newly developed quantum electrodynamics density-functional formalism (QEDFT). We provide an overview of how well-established concepts in the fields of quantum chemistry and material sciences have to be adapted when the quantum nature of light becomes important and present the  novel framework we have developed of quantum electrodynamics density-functional formalism (QEDFT). We illustrate the method to molecular complexes identifying fundamental changes in the Born-Oppenheimer surfaces, conical intersections (chemical reactivity and energy transfer and  spectroscopy. We also show how periodic driving of many-body interacting 2D systems allow to design Floquet states of matter with tunable electronic properties on ultrafast time scales (and cavity induced-topology). This work paves the road for the development of what can be coined as QED-materials and QED-chemistry.

14:00-14:15 Discussion

14:15-14:45 Interacting a handful of molecules and photons: nonlinear optics and polaritonic states

Professor Vahid Sandoghdar, Max Planck Institute for the Science of Light, Germany

Abstract

The interaction of light and matter at the nanometer scale lies at the heart of quantum optics because it concerns elementary processes such as absorption or emission of a photon by an atom. Over the past decade, we have shown that direct coupling of a photon to a single two-level atom should be possible via tight focusing. However, because transitions in quantum emitters are typically not closed, laboratory demonstrations of this idea fall short of the theoretical prediction. I shall report on recent achievements, where the branching ratio of a single organic molecule is improved by a substantial Purcell effect when coupled to a microcavity. Furthermore, we will discuss extension of the coherent linear and nonlinear experiments that become accessible in this regime to molecules coupled to subwavelength waveguides. Indeed, many interesting proposals in quantum optics and light-matter interaction rely on having multiple quantum emitters well-coupled to a single mode of light. We show that one-dimensional (1D) photonic channels including microresonator elements on a chip are ideal for coupling a well-defined number of identical emitters and indistinguishable photons. These developments make organic molecules viable candidates for the realization of novel superposition states of light and matter as well as integration in chip-based quantum optical circuits.

14:45-15:00 Discussion

15:00-15:30 Tea break

15:30-16:00 The interface between chemistry and QED: cavity control of chemical transformations

Professor Prineha Narang, John A Paulson School of Engineering and Applied Sciences, Harvard University, USA

Abstract

At the interface of chemistry and quantum optics, recent interest in strong light-matter coupling has led to a rapid convergence of these two  historically very different fields. Various experiments have explored the new regime of polaritonic chemistry, where matter and  the electromagnetic degrees of freedom become equally important. In chemistry, the idea of altering molecular processes by strongly coupling to  the vacuum fluctuations of the electromagnetic field has been suggested and demonstrated. Yet, the theoretical fundamentals of how the chemical landscape and induced dynamical processes are changed under strong light-matter coupling has remained elusive. In this context, I will discuss a new formalism at the intersection of cavity quantum-electrodynamics and electronic structure methods, quantum-electrodynamical density functional theory (QEDFT), to treat electrons, photons and phonons on the same quantized footing. So far, QEDFT studies have usually considered lossless cavities. Realistic cavities, however, exhibit finite photon lifetimes that lead to incoherent phenomena that may dominate the dynamics of the light-matter excitations. Therefore we have extended the QEDFT description to explicitly consider lossy (and very lossy) cavity modes. This allows us to perform first-principles studies of light-matter interactions ranging from the weak-coupling regime to the strong-coupling regime where hybrid light-matter polariton states are formed. We show that by correctly tuning the coupling it is possible to achieve cavity-mediated energy transfer between electronic excited states of the molecules. Further, I will introduce a new method to calculate polaritonic excited-state potential-energy surfaces for strongly coupled light-matter systems, we show how this coupling can be exploited to alter photochemical reaction pathways by influencing avoided crossings. Finally, using this ab initio understanding and new theoretical and computational methods, I will discuss a general methodology for optical control of chemical dynamics.

16:00-16:15 Discussion

16:15-16:45 Spectroscopy at the atomic scale by using x-ray spontaneous parametric down conversion

Professor Sharon Shwartz, Bar Ilan University, Israel

Abstract

Nonlinear interactions between x-rays and long wavelength radiation can be used as a powerful atomic scale probe for light-matter interactions and for properties of valence electrons [1-3]. The x-rays provide the high resolution whereas the optical/UV photons interact strongly with the valence electrons and provide the spectroscopic information. This probe can provide novel microscopic data in solids that are inaccessible by any existing method, hence to advance the understanding of many phenomena. Spontaneous parametric down conversion is a second order nonlinear optical effect, which is driven by the vacuum fluctuations. As such, the process requires only one input x-ray beam and it can be used as a probe at wavelengths where the material is opaque. The simple setup, the broad spectral range it spans, and the ability to reveal spectral information combined with structural information makes the effect of spontaneous parametric down conversion from x-ray into long wavelength radiation very unique.In my talk I will describe the recent progress in the study of x-ray into long wavelength radiation spontaneous parametric down conversion. I will discuss our new insight on the effect, the challenges, and the expected future developments. I will elaborate on our recent observation of the very strong nonlinearity in non-centrosymmetric crystals [4] and discuss the implications of this observation.

1. T. E. Glover et al. Nature 488, 603 (2012)

2. K. Tamasaku et al. Nat. Phys. 7, 705 (2011)

3. A . Schori et al. Phys. Rev. Lett. 119, 253902 (2017)

4. S. Sofer et al. arXiv:1904.13146

16:45-17:00 Discussion

17:00-17:15 Final remarks