Newswise — Using a “spooky” phenomenon of quantum physics, Caltech researchers have discovered a way to double the resolution of light microscopes.
The journal Nature Communications has published a paper by a team led by Lihong Wang, Bren Professor of Medical Engineering and Electrical Engineering, demonstrating a significant advancement in microscopy through the use of quantum entanglement. Quantum entanglement is a phenomenon where two particles are linked, causing the state of one particle to affect the state of the other, regardless of their proximity. This concept, referred to as "spooky action at a distance" by Albert Einstein, was unexplainable by his theory of relativity.
Quantum theory suggests that any type of particle can be entangled, and in the case of Lihong Wang's new microscopy technique, known as quantum microscopy by coincidence (QMC), the entangled particles are photons. When two photons are entangled, they form a biphoton, which behaves similarly to a single particle with twice the momentum of a single photon. This is an essential feature of Wang's microscopy technique.
In accordance with quantum mechanics, particles can be described as waves, and their wavelength is inversely proportional to their momentum. Particles with larger momentum have shorter wavelengths. As a biphoton has twice the momentum of a photon, its wavelength is half the size of the individual photons.
Indeed, the shorter the wavelength of the light used by a microscope, the higher its resolution. This is because a microscope can only distinguish details in an object that are separated by a distance greater than the wavelength of the light used. In QMC, the use of entangled photons with half the wavelength of individual photons allows for imaging of much smaller features than traditional microscopy, resulting in higher resolution images.
Although quantum entanglement is one way to reduce the wavelength of light in microscopy, it is not the only method. For instance, green light has a shorter wavelength than red light, while purple light has a shorter wavelength than green light. However, as per another peculiarity of quantum physics, light with shorter wavelengths contains more energy. Consequently, when the light's wavelength becomes small enough to observe tiny objects, it carries so much energy that it can harm the imaged items, including living cells. This is why exposure to ultraviolet (UV) light, which has a very short wavelength, can result in sunburn.
QMC gets around this limit by using biphotons that carry the lower energy of longer-wavelength photons while having the shorter wavelength of higher-energy photons.
“Cells don’t like UV light,” Wang says. “But if we can use 400-nanometer light to image the cell and achieve the effect of 200-nm light, which is UV, the cells will be happy, and we’re getting the resolution of UV.”
Wang's team constructed an optical apparatus that emits laser light into a specialized crystal which transforms some of the photons into biphotons. Despite the rarity of the conversion, with only one in a million photons being affected, the biphotons consisting of two discrete photons were split and directed through different pathways using a combination of mirrors, lenses, and prisms. One photon, the signal photon, passed through the object being imaged, while the other, the idler photon, did not. The photons continued to move through the optical system until they arrived at a detector that was connected to a computer. The computer used the signal photon's information to create an image of the object. Despite being separated and traveling through different paths, the paired photons remained entangled as a biphoton, behaving at half the wavelength while still carrying out the imaging process.
Wang’s team was not the first to work on biphoton imaging, but they were the first to successfully create a viable system using the concept. They developed a rigorous theory and a faster and more accurate entanglement-measurement method, which allowed them to achieve microscopic resolution and image cells.
While there is no theoretical limit to the number of photons that can be entangled with each other, each additional photon would further increase the momentum of the resulting multiphoton while further decreasing its wavelength.
Wang says future research could enable entanglement of even more photons, although he notes that each extra photon further reduces the probability of a successful entanglement, which, as mentioned above, is already as low as a one-in-a-million chance.
The paper detailing the breakthrough in microscopy, titled "Quantum Microscopy of Cells at the Heisenberg Limit," was published in the April 28 issue of Nature Communications. The paper was co-authored by Zhe He and Yide Zhang, both postdoctoral scholar research associates in medical engineering, medical engineering graduate student Xin Tong (MS '21), and Lei Li (PhD '19), formerly a medical engineering postdoctoral scholar and currently an assistant professor of electrical and computer engineering at Rice University.
Funding for the research was provided by the Chan Zuckerberg Initiative and the National Institutes of Health.
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