Quantum entanglement of photons doubles microscope resolution
Using a "weird" phenomenon of quantum physics, Caltech researchers have discovered a way to double the resolution of light microscopes.
In a paper published in the journal Nature Communications, a team led by Lihong Wang, the Bren Professor of Medical Engineering and Electrical Engineering, demonstrates a leap forward in microscopy through so-called quantum entanglement. Quantum entanglement is a phenomenon in which two particles are linked so that the state of one correlates with the state of the other, regardless of whether the particles are near each other. Albert Einstein called quantum entanglement "spooky action at a distance" because it couldn't be explained by his theory of relativity.
According to quantum theory, any type of particle can be entangled. In Wang's new microscopy technique, called coincidence quantum microscopy (QMC), the entangled particles are photons. Collectively, two entangled photons are called two-photons, and importantly for Wang's microscope, they behave in some ways as a single particle with twice the momentum of a single photon.
Since quantum mechanics says that all particles are also waves, and the wavelength of a wave is inversely proportional to the momentum of the particle, the particle with momentum has a smaller wavelength. Therefore, since a two-photon has twice the momentum of a photon, it has half the wavelength of a single photon.
This is key to the way QMC works. Microscopes can only image features on objects whose smallest size is half the wavelength of light used by the microscope. Reducing the wavelength of this light means the microscope can see smaller things, improving resolution.
Quantum entanglement isn't the only way to reduce the wavelength of light used in microscopes. For example, green light has a shorter wavelength than red light, and violet 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're exposed to light with a wavelength small enough to image tiny things, the light carries so much energy that it can damage the object being imaged, especially living things like cells. This is why very short wavelength ultraviolet (UV) rays can give you a sunburn.
This limitation is circumvented by using two-photons, which carry the lower energy of the longer-wavelength photon, while simultaneously having the shorter wavelength of the higher-energy photon.
"Cells don't like UV light," Wang said. "However, if we can image cells using 400-nanometer light and achieve the effect of 200-nanometer light, which is ultraviolet light, the cells are happy and we get ultraviolet resolution.
To achieve this, Wang's team built an optical device that shines laser light into a special crystal that converts some of the photons that pass through it into two-photons. Even with this particular crystal, this switch is extremely rare, occurring on the order of one in a million photons. Using a series of mirrors, lenses and prisms, each two-photon - effectively consisting of two discrete photons - is split and shuttled along two paths, so one of the paired photons passes through the object being imaged and the other does not. .
The photons that pass through the object are called signal photons, and the photons that do not pass through the object are called idle photons. Those photons then continue through more optics until they reach a detector connected to a computer that builds an image of the cell based on the information carried by the signal photons. Surprisingly, despite the presence of the object and its separate paths, the paired photons remained entangled as two-photons, which behaved at half the wavelength.
The lab isn't the first to investigate this kind of two-photon imaging, but it is the first to use the concept to create a working system. "We developed what we thought was a rigorous theory and faster, more accurate measurements of entanglement. We achieved microscopic resolution and cellular imaging.
While there is theoretically no limit to the number of photons that can be entangled with each other, each additional photon further increases the momentum of the resulting multiphoton while further reducing its wavelength.
Future research could entangle more photons, though he notes that each additional photon further reduces the probability of successful entanglement, which is already as low as one in a million, as mentioned above.
