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Sequence of Operations Simplifies Photonic Quantum Computers

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Stanford University researchers have proposed a design for photonic quantum computer that uses readily available components and a laser to manipulate a single atom that can modify the state of the photons in the system via the phenomenon of quantum teleportation.

The researchers said their photonic design is simpler than today’s quantum computers that are difficult to scale up and that require temperatures colder than interstellar space to operate. Though scientists have previously created photonic quantum computers that operate at room temperature, which makes them a promising approach to quantum computing, it is challenging to construct large numbers of logic gates for photons that connect in a reliable fashion to enable complex calculations.

In the Stanford design, the atom can be reset and reused for many quantum gates. This eliminates the need to build multiple distinct physical gates, vastly reducing the complexity of building a quantum computer.

“Normally, if you wanted to build this type of quantum computer, you would have to take potentially thousands of quantum emitters, make them all perfectly indistinguishable, and then integrate them into a giant photonic circuit,” said Ben Bartlett, a Ph.D. candidate in applied physics and lead author of the paper. “Whereas with this design, we only need a handful of relatively simple components, and the size of the machine doesn’t increase with the size of the quantum program you want to run.”

The design requires only a fiber optic cable, a beamsplitter, a pair of optical switches, and an optical cavity. In addition to their commercial availability, the researchers said that each of these components are continually being refined since they are currently used in applications other than quantum computing. For example, telecommunications companies have been working to improve fiber optic cables and optical switches for years.

“What we are proposing here is building upon the effort and the investment that people have put in for improving these components,” said Shanhui Fan, the Joseph and Hon Mai Goodman Professor of the School of Engineering and senior author of the paper. “They are not new components specifically for quantum computation.”

The design itself consists of two sections: a storage ring and a scattering unit.

The storage ring, which functions like memory in a regular computer, is a fiber optic loop holding photons that travel around the ring. Analogous to bits that store information in a classical computer, each photon in the system represents a qubit. The photon’s direction of travel around the storage ring determines the value of the qubit, which can be 0 or 1.

Stanford graduate student Ben Bartlett and Shanhui Fan, professor of electrical engineering, have proposed a simpler design for photonic quantum computers using readily available components. Courtesy of Ben Bartlett / Rod Searcey.
Stanford graduate student Ben Bartlett and Shanhui Fan, professor of electrical engineering, have proposed a simpler design for photonic quantum computers using readily available components. Courtesy of Ben Bartlett/Rod Searcey.
Additionally, because photons can simultaneously exist in two states at once, an individual photon can flow in both directions at once, which represents a value that is a combination of 0 and 1 at the same time.

The researchers could manipulate a photon by directing it from the storage ring into the scattering unit, where it travels to a cavity containing a single atom. The photon interacts with the atom, causing the two to become entangled — a quantum phenomenon whereby two particles can influence one another even across great distances.

In the researchers’ quantum computer, the photon then returned to the storage ring. A laser alters the state of the atom. Because the atom and the photon are entangled, manipulating the atom influences the state of its paired photon.

“By measuring the state of the atom, you can teleport operations onto the photons,” Bartlett said. “So we only need the one controllable atomic qubit and we can use it as a proxy to indirectly manipulate all of the other photonic qubits.”

Because any quantum logic gate can be compiled into a sequence of operations performed on the atom, a user could in principle run any quantum program, of any size, using only one controllable atomic qubit. To run a program, the code is translated into a sequence of operations that direct the photons into the scattering unit and manipulate the atomic qubit.

Control of the way the atom and photons interact means the same device can run many different quantum programs.

“For many photonic quantum computers, the gates are physical structures that photons pass through, so if you want to change the program that’s running, it often involves physically reconfiguring the hardware,” Bartlett said. “Whereas in this case, you don’t need to change the hardware — you just need to give the machine a different set of instructions.”

The U.S. Department of Defense and the U.S. Air Force Office of Scientific Research funded the research, which was published in Optica (www.doi.org/10.1364/OPTICA.424258).

Photonics Spectra
Feb 2022
GLOSSARY
quantum
Smallest amount into which the energy of a wave can be divided. The quantum is proportional to the frequency of the wave. See photon.
quantum optics
The area of optics in which quantum theory is used to describe light in discrete units or "quanta" of energy known as photons. First observed by Albert Einstein's photoelectric effect, this particle description of light is the foundation for describing the transfer of energy (i.e. absorption and emission) in light matter interaction.
Research & TechnologyeducationAmericasStanfordquantumquantum opticsquantum computingquantum computerquantum devicesquantum metrologyinformation processingtelecomdatacomfiber opticsfiber optic componentscomponentslasersTechnology News

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