Going Beyond Regular Limits of Optical Imaging By Using Quantum Information

Researchers are going beyond normal limits of optical resolution by using the best of classical and quantum information.

[Above]Results of the joint deconvolution algorithm fusing both classical and quantum information for 2 photon absorption. Insets (A) and (D) are the highest resolution images using classical information (k = 1, q = 4) for a mean photon count of = 500 (A) and = 5 million (D). Insets (B) and (E) combine both classical and quantum image orders (k = 1,2,3) also for mean photon counts of = 500 (B) and = 5 million(E). Inset (C) and (F) show the radial average ofthe Fourier transform of the reconstructions compared to the actual image used in the simulation for mean photon count levels = 500 (C) and = 5 million (F). Scale bar is 5λ.

A revolution is under way in optical microscopy where the quantum properties of light are exploited to extract additional information from quantum correlations that are absent in the classical interpretation. Such quantum information brings new possibilities but also its own set of limitations. Here, we develop a broader computational imaging approach to fuse quantum and classical information to provide a general solution that jointly exploits both forms of information for super-resolution microscopy.

Over the past few decades, numerous super-resolution microscopy methods have emerged that circumvent the optical diffraction barrier by bringing new information into the measurements. This new content is injected by exploiting photophysical properties such as structured illumination, localization, saturated absorption, or coherent nonlinear scattering. These super-resolution imaging techniques treat the total light signal collected during the image exposure time classically, and thus, information content is consequently restricted.

Additional imaging information can be accessed on the basis of fluctuations of the optical signals emitted by the object or, alternatively, imparted onto the illumination (excitation) beam. Correlations in the emitted light from an object can be exploited for scalable enhancements in imaging resolution—provided that those temporal fluctuations can be measured. Typically camera integration times (greater than milliseconds) are sufficiently long such that fluctuations of the emitted light intensity are essentially averaged to undetectable levels under many circumstances. The temporal fluctuations of photons detected in an optical signal depend on the nature of the emitters, the emitter environment, the integration time, and the detector configuration.

[The new work] focus attention on self-luminescent emitters, such as fluorescent molecules, quantum dots, or color centers, such as nitrogen vacancies in nanodiamonds. In the case of luminescent objects, the collected light lacks spatial coherence, which means that the light emitted at distinct spatial locations add together through an incoherent intensity sum upon detection. Single quantum emitters exhibit emission intensity fluctuations on 2 relevant time scales, with classical correlations encompassing times greater than microseconds and quantum correlations occurring on submicrosecond times. Intensity fluctuations can happen when the emitter is removed from the population, either temporarily as occurs with quantum dot blinking or molecular photoswitching, permanently because of photobleaching, or during the waiting period for re-excitation of the emitter to the excited state after photon emission.

Results

They exploits 2 co-operative mechanisms for improving the imaging resolution: anti-bunching and nonlinear generation of addition spatial frequency harmonics. These 2 resolution enhancement pathways result in broadened spatial frequency support. A common feature of super-resolution microscopies is that the higher-spatial-frequency information is obtained at the price of reduced SNR in that information. The fusion of both the multiple pathways for high-spatial-frequency information enables a substantially higher SNR in the images than would be possible by directly using a single super-resolution image. Another advantage brought by this approach is improved imaging speed.