A group led by Lihong Wang, Bren Professor of Medical Engineering and Electrical Engineering, demonstrates how quantum entanglement may be used to make significant advances in microscopy in a publication published in Nature Communications. When two particles get entangled, the state of one particle is dependent on the state of the other particle even though the particles are not in close proximity to each other. Quantum entanglement is something that Albert Einstein famously called “spooky action at a distance” since it defies his theory of relativity.
Particles of any kind can be entangled, as predicted by quantum theory. Photons are the entangled particles in Wang’s innovative method of microscopy, which he calls quantum microscopy by coincidence (QMC). Biphotons are pairs of entangled photons that act in some respects like a single particle with twice the velocity of a single photon, which is crucial for Wang’s microscopy.
Particles with greater momenta have shorter wavelengths because, according to quantum physics, all particles are also waves, and the wavelength of a wave is inversely linked to the momentum of a particle. A biphoton’s wavelength is half that of a single photon because it has twice as much momentum.
For QMC to function, this is essential. Features of objects smaller than half the wavelength of light used by the microscope can only be imaged. By shortening the wavelength of the light, the microscope will be able to view even finer details.
There are other ways to shorten the wavelength of light utilized in a microscope besides using quantum entanglement. When compared to red light, green light has a shorter wavelength, and when compared to green light, purple light has a shorter wavelength. But there’s another strange fact about quantum physics: shorter wavelengths of light contain greater power. The problem is that the energy contained in light with a wavelength small enough to picture tiny things is so great that it can destroy the objects being scanned, especially living things like cells. That’s why being sunburned happens when you’re exposed to ultraviolet (UV) radiation, which has a relatively short wavelength.
QMC gets around this limitation by employing biphotons, which have the shorter wavelength of higher-energy photons and the lower energy of longer-wavelength photons.
“UV light is harmful to cells,” Wang explains. To paraphrase: “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.”
The optical device developed by Wang’s group transforms some of the photons traveling through a specific type of crystal into biphotons. Even with this unique crystal, the conversion only happens once per million photons. Each biphoton, which is composed of two individual photons, is separated and sent down different routes by means of mirrors, lenses, and prisms to ensure that only one of the paired photons penetrates the object being imaged. The signal photon is the one that makes it through the object, while the other, which does not, is known as the idler photon. These photons go via further optical components before landing on a detector linked to a computer, which uses the data from the photon to reconstruct an image of the cell. Despite the presence of the object and the fact that the two photons are traveling in opposite directions, they stay entangled as a biphoton acting at half the wavelength.
Although several groups have explored the possibility of biphoton imaging, Wang’s lab was the first to implement a practical system. We have created a faster and more accurate entanglement measurement method and what we believe to be a valid theory. We were able to resolve cells at the microscopic level.
An infinite number of photons can be theoretically entangled with one another, however doing so would only serve to increase the momentum and shorten the wavelength of the resulting multiphoton.
According to Wang, more study may make it possible to entangle even more photons, however, he does point out that doing so decreases the already low odds of a successful entanglement (which can be as low as one in a million).
“Quantum Microscopy of Cells at the Heisenberg Limit,” an article reporting the research, was published in Nature Communications on April 28. Xin Tong (MS ’21), a graduate student in medical engineering, Zhe He (PhD ’19), a research associate in medical engineering, and Lei Li (PhD ’19), an assistant professor in electrical and computer engineering at Rice University, are all co-authors.
The National Institutes of Health and the Chan Zuckerberg Initiative supported this study financially.