Researchers propose simpler design for quantum computers

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Today’s quantum computers are complex to build, difficult to expand, and require temperatures colder than interstellar space to operate. These challenges led researchers to explore the possibility of building quantum computers that work using photons – particles of light. Photons can easily transmit information from one place to another, and optical quantum computers can operate at room temperature, so this approach is promising. However, although people have succeeded in creating single quantum “logical gates” for photons, it is difficult to create a large number of gates and reliably link them to perform complex calculations.

Now, Stanford University researchers have proposed a simpler design for optical quantum computers using readily available components, according to a paper published November 29 in optics. Their proposed design uses a laser to manipulate a single atom that can modify the state of photons via a phenomenon called “quantum teleportation.” The atom can be reset and reused for many quantum gates, eliminating the need to build multiple distinct physical gates, greatly reducing the complexity of building a quantum computer.

“Normally, if you wanted to build this kind of quantum computer, you would have to take thousands of quantum emitters, make them completely indistinguishable, and then combine them into a giant photonic circuit,” said Ben Bartlett, Ph.D. Dr.. Candidate in Applied Physics and lead author of the paper. “Whereas with this design we only need a few relatively simple components, and the device does not increase in size with the size of the quantum program you want to run.”

This remarkably simple design requires only a few pieces of equipment: a fiber optic cable, a beam spreader, a pair of optical switches, and an optical cavity.

Fortunately, these ingredients already exist and are commercially available. They are also constantly being improved as they are currently used in applications other than quantum computing. For example, telecom companies have been improving fiber optic cables and optical switches for years.

“What we are proposing here is to build on the efforts and investments that people have made to improve these components,” said Shanhui Fan, Joseph and Hon Mae Goodman Professor in the School of Engineering and senior author on the paper. “It’s not specifically new components of quantum computation.”

new design

The scholars’ design consists of two main parts: a storage ring and a scattering unit. The storage ring, which functions similarly to memory in a regular computer, is a ring of optical fibers that contains multiple photons that travel around the ring. Similar to the bits that store information in a traditional computer, in this system, each photon represents a quantum bit, or “qubit”. The direction the photon travels around the storage ring determines the qubit value, which can be like a bit, 0 or 1. Additionally, because photons can exist simultaneously in two states at once, a single photon can flow in both directions at once. One, which represents a value consisting of a combination of 0 and 1 at the same time.

The researchers can manipulate the photon by directing it from the storage ring to the scattering unit, where it travels into a cavity containing a single atom. The photon then interacts with the atom, causing the two to “entangle”, a quantum phenomenon in which two particles can influence each other even over great distances. Then the photon returns to the storage ring, and the laser changes the state of the atom. Because the atom and the photon are entangled, manipulating the atom also affects the state of the photon it is associated with.

“By measuring the state of the atom, you can transfer the remote processes to the photons,” Bartlett said. “So we only need one controllable atomic qubit that we can use as an alternative to indirectly manipulating all the other optical qubits.”

Since any quantum logic gate can be grouped into a series of operations on the atom, you can, in principle, run any quantum program of any size with a single controllable atomic qubit. To run a program, the code is translated into a series of operations that direct the photons to the scattering unit and manipulate the atomic qubits. Since you can control the way the atom and photons interact, the same device can run many different quantum programs.

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


A new kind of quantum computer


more information:
Ben Bartlett et al., Deterministic photonic quantum computation in an artificial time dimension, optics (2021). DOI: 10.1364 / OPTICA.424258

Presented by Stanford University

the quote: Researchers propose simpler design for quantum computers (2021, November 29) Retrieved on November 29, 2021 from https://phys.org/news/2021-11-simpler-quantum.html

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