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March 05, 2014

QUANTUM MICROSCOPES – LIGHT YEARS AHEAD

At Microscope.com we often say that the technology of light microscopes has not changed much over the past 500 years. Strictly speaking, this is no longer true. Scientists at Hokkaido University in Japan have found a way to use ordinary light to detect objects smaller than the limit of traditional light microscopes. Up until now, light microscopes have been limited by the Rayleigh diffraction limit, which states that light cannot be used to resolve a structure smaller than its own wavelength. Since the shortest wavelength of visible light is a few hundred nanometers, scientists have turned to X-Rays and electron microscopes in order to resolve smaller elements.

However, for some time scientists have suspected that a weird effect of quantum mechanics known as entanglement might overcome the Rayleigh limit and the Hokkaido team have done just that. Einstein referred to Entanglement as “spooky action at a distance”. It involves two photons in opposite polarization states that become entangled so that even when separated by infinitely large distances (think light years), changes to one are reflected in the other. Using such entangled photons, the microscope visualizes much smaller structures than could be achieved with ordinary light.

In this case, the scientists generated entangled photons by converting a laser beam into pairs of photons that were in opposite polarization states at the same time (superposed particles). The physicists used special nonlinear crystals to achieve this superposition and then focused the entangled photons on two adjacent spots on a flat glass plate with a Q-shaped pattern made in relief on the plate’s surface. This pattern is only 17 nanometers higher than the rest of the plate and almost impossible to see with a standard optical microscope. However using the entangled pairs, the lettering was completely visible and 1.35 times sharper (signal-to-noise ratio) than the standard quantum limit.

entangled-microscope

Inevitably, obtaining an image was not as simple as using an eyepiece. In this case, the team generated the image electronically by measuring the difference in optical path length between the two beams, a difference that is caused by the marginally thicker glass where the letter rises up from the surrounding surface. The scientists could not measure this directly, so they used the interference pattern of both beams as they passed through the glass. Since each of the entangled protons provides information about the other, the process is more efficient and results in a sharper image.

The major drawback to the process is that it took almost a full day for the microscope to generate actual images so a key improvement for commercial development is the ability to speed image development. However, the team has proven that the refractive index of light microscopes can be enhanced which could have a major beneficial impact on Life Sciences and other disciplines such as cryptography. Currently, in order to observe transparent organisms in such detail, X-Rays or electron microscopes are required. Both are expensive while X-Rays cause damage to living cells. Since this entangled photons approach uses simple lasers with infrared rays, it offers both an inexpensive and harmless solution to future generations.

[Research paper: 10.1038/ncomms3426 – “An entanglement-enhanced microscope”; open-access PDF]