Theoretical Physicist Li Ge Uses Microlaser to Expand Dimensions of Qubits

A quantum bit, or qubit, is the basic unit of information in quantum computing. A qubit is made of a quantum system that contains subatomic particles such as electrons and photons.

In conventional computing, bits can be any two distinct states, expressed by zeros or ones. But in quantum computing, these states can exist at the same time. This phenomena is known as superposition, and it allows quantum computers to process large amounts of information and solve more complex problems than classical computers. However, current quantum communication technologies tend to have limited storage space and high sensitivity to interference.

Now, brand new research, published recently in Nature, explains how the dimensions of qubits can be further expanded on a chip-scale laser to increase information capacity. The study authors say this demonstration of computing power has the potential to inspire new, high-capacity quantum communication technologies for real-world applications.

The researchers set out with the goal to encode more information on a single qubit using photons of light. “In order to get higher information capacity, there are different approaches,” said study co-author Professor Li Ge (GC/College of Staten Island, Physics, Nanoscience), a theoretical physicist with a focus on optics and photonics.

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“One is just to say, ‘Hey, I have more of those copies of zeros and ones.’ That's the conventional thinking. Another is to say, ‘Why can't I just encode more information in a single bit?’ So, instead of superposition, and zero and one, maybe I can have superposition and 0, 1, 2, 3. That gives us more freedom to encode information, tackling problems on a larger scale.”

The researchers performed experiments with a hyperdimensional microlaser, a tiny semiconductor laser built on a 200-nanometer-thick wafer, coated with an alloy of gallium and indium arsenide. The device emitted near infrared light at a wavelength of 1,550 nanometers.

The physicists used two optical micro ring resonators — nano-sized lasers that shoot photons in a confined, ring-shaped structure — to manipulate light into higher dimensions of communication. In this case, a qubit, usually represented by two angles on a sphere, was expanded into a "qudit" with six angles of information on a higher-dimensional sphere. 

“Light, as we know, is a form of electromagnetic wave,” Ge said, which travels up and down, and left and right. Considered to be the zeros and ones of conventional quantum communication, these waves can be combined so the light travels clockwise and counterclockwise.

“But now, instead of this clockwise and counterclockwise motion of light, we can actually add more control, or more degrees of freedom to a single photon,” said Ge.

“That is what we're doing here. Instead of this clockwise and counterclockwise movement, which call ‘spin,’ we also encode what we call the ‘orbital angular momentum’ onto the photons themselves,” said Ge, referring to the momentum from the orbital motion of an object.

Ge says the research brings scientists a step closer to the next generation of high-capacity, noise-resilient computing technologies, including ground-to-satellite communications and optical fibers. The results from a follow-up study are expected to be published in the spring.

In 2010, Ge co-discovered coherent perfect absorbers, known as time-reversed lasers or anti-lasers, which absorb photons until they dissipate. The finding was covered by The New York Times, Scientific American, Nature, Nature Photonics and the BBC. Ge received a National Science Foundation Career award in 2019.

The current study was carried out in collaboration with the University of Pennsylvania and funded by the National Science Foundation.

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