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IMPROVING QUANTUM MICROSCOPY VIA RAMAN PHOTON PAIRS 1

Marlan O. Scully

Published Jan 1, 2005• 1 citation
OpenAlex

Abstract

In ordinary (classical) microscopy and lithography, the limit of resolution is governed by the wavelength of light used to il-luminate the sample. However, by using the correlated photon pairs produced in Raman quantum erasure, the resolution of two-photon quantummicroscopy is substantially improved. To that end, we here derive the two-photon quantum state gener-ated by sequential Raman emissions and investigate potential applications. The photon-photon correlation function for Ra-man pairs is of interest in itself and has features in common with photon antibunching. 1Asim Barut enjoyed and brought joy to all of Physics. He was interested in Concepts of Physics, Vol. II (2005) 261

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In ordinary (classical) microscopy and lithography, the limit of resolution is governed by the wavelength of light used to il-luminate the sample. However, by using the correlated photon pairs produced in Raman quantum erasure, the resolution of two-photon quantummicroscopy is substantially improved. To that end, we here derive the two-photon quantum state gener-ated by sequential Raman emissions and investigate potential applications. The photon-photon correlation function for Ra-man pairs is of interest in itself and has features in common with photon antibunching. 1Asim Barut enjoyed and brought joy to all of Physics. He was interested in Concepts of Physics, Vol. II (2005) 261
We show that by using the strongly correlated photon pairs generated in a Raman quantum erasure scheme (Scully M and Drühl K 1982 Phys. Rev. A 25 2208), it is possible to exceed the Rayleigh resolution limit of classical microscopy. The complete analysis of the underlying physics is given here. Further discussion of the physics and potential applications are presented in a companion paper (Scully M O 2004 Improving quantum microscopy and lithography via Raman photon pairs: I. Biological applications, submitted).
High Resolution Image Download MS PowerPoint Slide The resolution of a classical imaging system is limited by diffraction. This limit can be overcome, for example, by implementing various forms of localization microscopy in which the center of a fluorescence distribution is estimated to an accuracy scaling with the square root of the number of detected photons, N . In quantum imaging the object can be illuminated using correlated photon-pairs, leading early work to suggest that a 2 improvement could be obtained as a result of averaging the position of N = 2 events. However, similar to quantum lithography, which relies upon quantum illumination using entangled photon-pairs and two-photon absorption, the minimum resolvable feature size is reduced by a factor of 2, not just 2 . Quantum imaging schemes can also lead to a factor of 2 improvement. By using a similar source of correlated photon-pairs to illuminate an object, a single-photon sensitive camera to detect the photon-pairs, and an image processing algorithm to record and sum the bisector positions of the transmitted photon-pairs, we realize a similar factor of ×2 improvement in image resolution, surpassing that of most earlier quantum imaging work.
Higher-order correlations of the radiation field improve resolution in stellar interferometers, as in the Hanbury-Brown–Twiss effect. It is also possible to improve microscopic resolution beyond the Rayleigh limit by using quantum light fields composed of entangled photons. Focusing on two photons, we distinguish two types of entanglement: frequency entanglement, where the photons in different paths are correlated in frequency, and path entanglement, where the correlation between paths is in photon number. Two paradigms of quantum microscopy are discussed: spectral microscopy, where path- and frequency-entangled photons produced in cascade decay of two atoms make possible sub-natural linewidth resolution of atomic levels, and spatial microscopy, where path-entangled photons emitted by an atomic array produce sub-wavelength diffraction resolution as compared to an equivalent classical grating. These scenarios require two-photon correlation or coincidence measurements. The connection between the two paradigms, and the two types of entanglement, highlights the link between the temporal and spatial aspects of quantum interferometry.
Novel imaging techniques utilizing nondegenerate, correlated photon pairs sparked intense interest during the last couple of years among scientists of the quantum optics community and beyond. It is a key property of such “ghost imaging” or “quantum interference” methods that they use those photons of the correlated pairs for imaging that never interacted with the sample, allowing detection in a spectral range different from that of the illumination of the object. Extensive applications of these techniques in spectroscopy and microscopy are envisioned, however, their limited spatial resolution to date has not yet supported real-life microscopic investigations of tiny biological objects. Here we report a modification of the method based on quantum interference by using a seeding laser and confocal scanning, that allows the improvement of the resolution of imaging with undetected photons by more than an order of magnitude, and we also present examples of application in the microscopy of biological samples.
Technological advancements in the creation, manipulation and detection of quantum states of light have motivated the application of such states to overcome classical limits in sensing and imaging. In particular, there has been a surge of recent interest in super-resolution imaging based on principles of quantum optics. However, the application of such schemes for practical imaging of biological samples is demanding in terms of signal-to-noise ratio, speed of acquisition and robustness with respect to sample labeling. Here, we re-introduce the concept of quantum image scanning microscopy (Q-ISM), a super-resolution method that enhances the classical image scanning microscopy (ISM) method by measuring photon correlations. Q-ISM was already utilized to achieve super-resolved images of a biological sample labeled with fluorescent nanoscrystals whose contrast is based entirely on a quantum optical phenomenon, photon antibunching. We present here an experimental demonstration of the method and discuss with further details its prospects for application in life science microscopy.
State-of-the-art microscopes use intense lasers that can severely disturb biological processes, function and viability. This introduces hard limits on performance that only quantum photon correlations can overcome. In this talk I will report recent work from my laboratory which demonstrates this absolute quantum advantage [1]. We show, specifically, that quantum correlations enable signal-to-noise beyond the photodamage-free capacity of conventional microscopy. Broadly, this represents the first demonstration that quantum correlations can allow sensing beyond the limits introduced by optical intrusion upon the measurement process. We achieve this in a coherent Raman microscope, which we use to image molecular bonds within a cell with both quantum-enhanced contrast and sub-wavelength resolution. This allows the observation of nanoscale biological structures that would otherwise not be resolved. Coherent Raman microscopes allow highly selective biomolecular finger-printing in unlabelled specimens, but photodamage is a major roadblock for many applications. By showing that this roadblock can be overcome, our work provides a path towards order-of-magnitude improvements in both sensitivity and imaging speed.