Entangled photons pave the way for quantum microscopy

Scientists present a new concept called quantum microscopy by coincidence (QMC) which is the key to improving the resolution, speed, and contrast-to-noise ratios.
Tejasri Gururaj
The quantum microscopy apparatus.
The quantum microscopy apparatus.

Lance Hayashida/Caltech 

A biphoton state is a quantum state representing two entangled photons. This means that the state of one depends on the state of the other, no matter how far away they are. Biphotons have been used extensively for various quantum imaging techniques, such as quantum holography. However, the method has several problems, including low speeds and spatial resolutions, which have hindered its progress.

Now, a group of scientists from Caltech have developed a quantum microscope capable of seeing quantum behavior at twice the resolution of the images. The study, published in Nature Communications, was led by Lihong V. Wang from Caltech and can potentially revolutionize the field of quantum imaging.

The team presents a new concept called quantum microscopy by coincidence (QMC) which is the key to improving the resolution, speed, and contrast-to-noise ratios.

Quantum microscopy by coincidence (QMC)

A microscope works by magnifying objects that are too small to be visible to the naked eye. However, there is a catch. Microscopes only work when the object's minimum size is half of the wavelength of the light used by the microscope. In order to observe smaller objects, a microscope must use light with a shorter wavelength, like ultraviolet light.

However, using shorter wavelengths of light can burn or damage living cells which need to be observed using a microscope. This is where QMC solves the problem. 

Entangled photons pave the way for quantum microscopy
UV light has a shorter wavelength than visible light

QMC uses biphotons (two photons) which have twice the momentum of a photon. As you may recall from physics classes in school, the momentum is inversely related to the wavelength. Therefore, a biphoton has half the wavelength of an individual photon, giving us twice the resolution!

The physical setup and the future of quantum imaging

The setup consists of a laser that produces the biphoton. The two photons are split up such that one photon passes through the object being observed, and the other one follows a parallel path avoiding the object entirely. At the end of the path, the photons reach the detector where the image is built.

To demonstrate the capabilities of their setup, the team used it to produce imaging of cancer cells. The quantum microscope was able to distinguish between the cellular structures, while a classical microscope was unable to do so. Additionally, the method did not destroy the cells, which is another important achievement of their study.

This research could have huge implications for the field of medical imaging where cells have to be observed at the microscopic level and a higher resolution is needed to identify problems. 

Future research could focus on using multiple photons to further increase the resolution while minimizing the noise that arises from the photons' interaction with the environment.

Study abstract:

Entangled biphoton sources exhibit nonclassical characteristics and have been applied to imaging techniques such as ghost imaging, quantum holography, and quantum optical coherence tomography. The development of wide-field quantum imaging to date has been hindered by low spatial resolutions, speeds, and contrast-to-noise ratios (CNRs). Here, we present quantum microscopy by coincidence (QMC) with balanced pathlengths, which enables super-resolution imaging at the Heisenberg limit with substantially higher speeds and CNRs than existing wide-field quantum imaging methods. QMC benefits from a configuration with balanced pathlengths, where a pair of entangled photons traversing symmetric paths with balanced optical pathlengths in two arms behave like a single photon with half the wavelength, leading to a two-fold resolution improvement. Concurrently, QMC resists stray light up to 155 times stronger than classical signals. The low intensity and entanglement features of biphotons in QMC promise nondestructive bioimaging. QMC advances quantum imaging to the microscopic level with significant improvements in speed and CNR toward the bioimaging of cancer cells. We experimentally and theoretically prove that the configuration with balanced pathlengths illuminates an avenue for quantum-enhanced coincidence imaging at the Heisenberg limit.

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