JCP special topic on quantum light in physical chemistry and molecular spectroscopy

It has long been recognized by the quantum optics community that quantum states of light can provide benefits for sensing and metrology. In recent years, the chemistry community has explored the possibility of applying the resources of quantum light toward molecular spectroscopy and imaging, and in fabricating chemical systems that can generate quantum light sources. In this context, quantum light refers to states of light that can produce chemically relevant information that cannot be obtained by using any conventional light sources such as lasers and thermal or thermal-like (e.g., LEDs) sources. Examples of quantum light include single photon states, entangled two-photon or multi-photon states, and squeezed states (i.e., a state having unequal quantum uncertainties in the real and imaginary parts of the electric field). The quantum coherence and/or entanglement in such states is known to provide a quantum advantage in certain applications such as sensing of biological samples using a quantum diamond microscope.¹ However, the full range of possible quantum-enabled applications in areas related to physical chemistry, molecular spectroscopy, and biological imaging is not yet known.

This JCP special topic issue presents current research in the applications of quantum light in chemical physics, physical chemistry, and closely related fields. The topics covered include sources and detectors of quantum light and novel proposals of techniques offering unique advantages of quantum light in these areas. Following brief summaries of the papers, which fall into five broad categories, we offer our perspective on the future of this research area, reflecting some of its more controversial aspects.

Our contribution (Raymer et al.²) to this special issue is a tutorial on the basic theory of quantum optics and its application to one- and two-photon excitation of molecular systems with quantum light. Asban et al.³ offer a perspective on the application of optical interferometric techniques when combined with quantum light for molecular spectroscopy. Gonoskov and Gra$f̈$fe⁴ formulate a new method to treat theoretically the interaction of complex light–matter systems, including the possibility of light-mode entanglement and nonclassical light generation during the interaction. Carnio et al.⁵ study the ability of optimized entangled two-photon fields to optically excite specific target transitions that are embedded in a multi-level quantum system.

Schlawin et al.⁶ turn the previous topic around and show how, in principle, general quantum states of multimode light can be characterized using the nonlinear optical frequency conversion process of a Raman transition in a molecule.

Lerch and Stefanov⁷ experimentally study a scheme for tailoring the spectral-temporal entanglement of photon pairs produced by parametric down conversion, and they use the system to compare sum-frequency generation in a nonlinear crystal with classical pulses or with entangled photon pairs.

Szoke et al.⁸ experimentally study the design and characterization of a high-flux source of spectral-temporal-entangled photon pairs based on parametric down conversion for studying the feasibility of nonlinear spectroscopies using such entangled light. Avanaki and Schatz⁹ theoretically study the properties of quantum dot cascade emitters as a source of entangled photon pairs for on-demand heralded single-photons. Sharifi et al.¹⁰ use “double nanohole” apertures prepared lithographically in gold films to optically trap erbium-doped nanocrystals to enhance the emission rates of these single photon emitters at telecom wavelengths.

Soukup et al.¹¹ theoretically examine differences between diffraction of atoms in atomic beams from optical gratings in the cases that the light is treated as a classical wave and when the light is treated as a weak quantum mechanical coherent state. Chernyak and Mukamel¹² explore the fact that the electric and the magnetic field observables at two different points satisfy an uncertainty relation, implying possible consequences for spectroscopy of nanostructures. Santra and Malinovsky¹³ propose protocols that use auxiliary entangled states shared between two parties to enhance the success probability of photonic entanglement transformations.

To put the field into some perspective, there have been many intriguing theoretical proposals for using quantum light to enhance molecular spectroscopy and imaging. However, as pointed out, for example, by Lerch and Stefanov,⁷ and supported by the theory of Raymer et al.,² the main barrier to implementing such measurement schemes with entangled photons is the very low nonlinear cross section for two-photon absorption and other multi-photon processes in molecules. Because the field-matter coupling is so small, new experimental techniques will be needed to make the sought-after quantum advantages realizable. This could be done, in principle, by cavity or nanophotonic enhancement of the quantum-field strength or by increasing the number of molecules that interact with the quantum field. On the other hand, increasing the flux of photons that interact with the molecule in a specific optical mode will not likely reveal uniquely quantum-light-enhanced-effects because, in the limit of large photon numbers, quantum states of light become practically indistinguishable from classical states of light, as shown, for example, in Ref. 14.

We thank the other Guest Editors, P. James Schuck and Carlos Silva Acuna, and the JCP Editors, Jennifer Ogilvie, Qiang Shi, and Xiaoyang Zhu. In addition, we thank the JCP staff, Judith Thomas, Jenny Stein, and Olivia Zarzycki, who helped to make this special issue possible.

The authors have no conflicts to disclose.