Using a “scary” technique from quantum physics, Caltech researchers have discovered a way to double the resolution of optical microscopes. In an article in the journal Nature Communications, a group led by Lihong Wang, Bren Professor of Medical and Electrical Engineering, shows the quantum improvement in microscopy achieved through the so-called quantum entanglement.
Quantum entanglement is what causes two parts to be connected in a way that connects the state of one with the state of the other, whether the objects are close or not. Albert Einstein called quantum entanglement “anomalous behavior at a distance” because it cannot be explained by the theory of relativity.
According to quantum theory, any kind of particle can be connected. In Wang’s new microscopy technique, called quantum coincidence microscopy (QMC), the particles that collide are photons. Collectively, two photons combined are called a biphoton and, which is important for Wang’s microscopy, they behave in some respects as a single particle that is twice the size of a single photon.
Since quantum mechanics says that all particles are also waves, and that the wavelength of the wave is proportional to the energy of the particle, particles with larger periods have longer wavelengths. Therefore, since a biphoton has twice the energy of a photon, its wavelength is half that of an individual photon.
This is the key to how QMC works. Microscopes can image features that are at least half the wavelength of light used by the microscope. This reduction in the wavelength of light means the microscope can see even smaller objects, which results in better resolution.
Quantum entanglement isn’t the only way to reduce the wavelength of light used in a microscope. Green light has a shorter wavelength than red light, for example, and violet light has a shorter wavelength than green light. But due to another reason of quantum physics, light with shorter wavelengths carries more energy.
Therefore, as soon as you expose yourself to light with a wavelength short enough to photograph small objects, the light carries enough energy to damage the objects being photographed, especially living things. like cells. This is why ultraviolet (UV) light, which has a shorter wavelength, gives you burns.
QMC overcomes this limitation by using biphotons that carry low energy wavelength photons and short wavelength high energy photons.
“Cells don’t like UV light,” Wang says. “But if we can use 400 nanometer light to image the cell and get the effect of 200 nm light, which is UV, the cell will be happy and we get the resolution of UV.”
To achieve this, Wang’s team built an optical device that shines laser light into a special type of crystal that converts some of the photons that pass through it into biphotons. Even with this special crystal, the conversion is rare and occurs in about one million photons.
Using a system of mirrors, lenses and prisms, each biphoton – consisting of two different photons – is split and transmitted in two directions, so that one of the combined photons travels from that image and the other is not. A photon that passes through is called a signal photon, and one that does not pass through it is called an idle photon.
These images continue through other optical devices until they reach a detector connected to a computer that creates an image of the cell based on the information carried by the signal photons. Surprisingly, the combined photons remain combined as a biphoton that behaves in half of the wavelength despite the presence of the object and their different paths.
Wang’s laboratory is not the first to work on these two types of photography, but it is the first to develop a suitable method using this concept. “We have developed what we believe to be a robust concept and a fast and accurate method for measuring binding. We have achieved microscopic resolution and image cells.”
Although there is no rule regarding the number of photons that can be combined with each other, each additional photon will increase the number of the resulting multiphoton and decrease its wavelength.
Wang says that future research may enable the fusion of more photons, although he says that each photon that makes it easier reduces the chance of fusion. success, which, as mentioned above, is already like a million chances.
The paper describing the work, “Quantum Microscopy of Cells at the Heisenberg Limit,” published on April 28 issue of Nature Communications.
Source: California Institute of Technology
