Optical microscopy is a powerful tool that has been widely used in science for the last five centuries or so, but it’s not without its limitations; one way to overcome these restrictions is to employ quantum optical principles.
Light microscopy is one of the earliest uses of optics. A standard optical microscope uses visible light and a system of lenses to magnify small objects and create an image, which can be photographed using simple cameras. However, the technique is limited by the diffraction limit of spatial resolution; Enrst Abbe found that an infinitely small point object becomes a blurry circle in an image.
Point Spread Function
This phenomenon is known as the point spread function (PSF) of a system and as a result, objects can not be resolved closer than about half of the visible light’s wavelength. This diffraction limit was mathematically calculated in the 19th Century, long before quantum mechanics, and assumed that light can be described using classical waves.
Although a popular tool – especially in biology – many subcellular features can’t be imaged. Low levels of light are required when illuminating a biological sample so as not to damage the cells, but the resulting images can be grainy and fuzzy – just like unclear photos taken in poorly lit spaces; this is caused by the short noise effect and limits the amount of information you can take from the image.
So, while a useful tool, optical microscopy suffers from poor sensitivity and resolution. Both of these can be overcome by employing quantum light and quantum correlation measurements.
Quantum optics is based on the notion that light is made out of photons rather than classical waves, proved with a simple experiment using a Hanbury-Brown and Twiss setup. If a stream of single photons is split between two sensitive detectors, only one will register a detection each time, which would not be expected if light could be described using only classical waves.
The detectors are anti-correlated – meaning that they are not comparable, as one quantity increases, the other decreases. This phenomenon is known as photon antibunching, a hallmark of the non-classicality of light, and can be used in super resolution microscopy.
Novel Quantum Microscopy Tool
Quantum microscopy has become a novel tool in allowing microscopic properties and quantum particles to be measured and directly observed, and has enabled scientists to obtain sub-short-noise imaging sensitivities and super resolution beyond the diffraction limit.
The first microscope to make use of the quantum concept was the scanning tunnelling microscope, which paved the way for the photoionization microscope and the quantum entanglement microscope.
Quantum microscopy uses entangled photons such as those that show quantum correlations. Performing an action on one of the two entangled photons can affect the other even at a distance. This ability to give information about each other means quantum microscopy can yield higher-quality images than standard microscopy.
The quantum nature of light can used as a source of extra information in order to break the diffraction limit in super resolution microscopy, which combines several types of microscopy to overcome the diffraction limit imposed by Abbe’s analysis.
The resolution of current optical microscopes is limited at half the wavelength of a photon, the Rayleigh limit, and breaking this barrier is difficult. Many techniques, including Photoactivated Localisation Microscopy (PALM) and Stochastic Optical Reconstruction Microscopy (STORM rely on a fluorescent scene fluctuating with time, thereby breaking the statement of a stationary image), but also necessitate the additional step of labeling objects of interest with a fluorescent dye.
All Solid-State Super Twinning Photon Microscope
A new project called the All Solid-State Super Twinning Photon Microscope or SUPERTWIN is also aiming to overcome the limits of classical optics by employing quantum photonics. This project, currently in progress, will develop a prototype microscope exploiting entangled photons, and could represent a radical new line of technology for super resolution imaging.
References and Further Reading