(Nanowerk News) Using a “creepy” phenomenon of quantum physics, Caltech researchers have found a way to double the resolution of a light microscope.
In a paper that appears in the journal Nature Communications (“Cell quantum microscopy at the Heisenberg limit”), a team led by Lihong Wang, Bren Professor of Medical Engineering and Electrical Engineering, demonstrated achieving a leap forward in microscopy through what is known as quantum entanglement.
Quantum entanglement is a phenomenon in which two particles are linked in such a way that the state of one particle is bonded to the state of the other particle regardless of whether the particles are in close proximity to one another. Albert Einstein famously called quantum entanglement “spooky action at a distance” because it couldn’t be explained by his theory of relativity.
According to quantum theory, all kinds of particles can be entangled. In the case of Wang’s new microscopy technique, dubbed quantum coincidence microscopy (QMC), the entangled particles are photons. Collectively, the two entangled photons are known as biphotons, and, important to Wang’s microscope, they behave in some ways as single particles having twice the momentum of a single photon.
Because quantum mechanics says that all particles are also waves, and that the wavelength of a wave is inversely proportional to the momentum of the particle, particles with more momentum have shorter wavelengths. So, because a biphoton has twice the momentum of a photon, its wavelength is half the length of the individual photon.
This is key to how QMC works. Microscopes can only describe the features of objects whose minimum size is half the wavelength of light used by the microscope. Reducing the wavelength of that light means the microscope can see smaller objects, which results in increased resolution.
Quantum entanglement isn’t the only way to reduce the wavelength of light used in microscopes. Green light has a shorter wavelength than red light, for example, and purple light has a shorter wavelength than green light. But due to another quirk of quantum physics, light with shorter wavelengths carries more energy. So once you get down to light with a wavelength that is small enough to image small objects, the light carries so much energy that it will damage the object that is being imaged, especially living things like cells. This is why ultraviolet (UV) rays, which have very short wavelengths, give you sunburns.
QMC overcomes this limit by using biphotons which carry a lower energy than the longer wavelength photons while having a shorter wavelength than the higher energy photons.
“Cells don’t like UV light,” Wang said. “But if we can use 400 nanometer light to image cells and achieve the effect of 200 nm light, which is UV, the cell will be excited, and we get UV resolution.”
To achieve that, Wang’s team built optical equipment that shines laser light into a special type of crystal that converts some of the photons that pass through it into biphotons. Even using this particular crystal, the conversion is extremely rare and occurs in about one in a million photons. Using a series of mirrors, lenses, and prisms, each biphoton—which is actually two discrete photons—is separated and passed along two paths, so that one of the photons passes through the object being imaged and the other does not. Photons that pass through the object are called signal photons, and those that do not are called idle photons. These photons then continue through more optics until they reach a detector connected to a computer which builds an image of the cell based on the information carried by the signal photons. Remarkably, the paired photons remain entangled as biphotons that behave at half-wavelength even though there are separate objects and paths.
Wang’s lab isn’t the first to work on this kind of biphoton imaging, but it’s the first to build a viable system using the concept. “We developed what we believe to be a rigorous theory as well as a faster and more accurate method of measuring attachment. We achieved microscopic resolution and imaged cells.”
While there is no theoretical limit to the number of photons that can be entangled with each other, each additional photon further increases the resulting multiphoton momentum while further reducing its wavelength.
Wang said that future research could allow for the entanglement of more photons, although he noted that each additional photon further reduces the probability of successful entanglement, which, as noted above, is already as low as a one-in-a-million chance.