A simpler design for quantum computers

Quantum computers today are complicated to build, difficult to expand, and require temperatures cooler than interstellar space to function. These challenges have led researchers to explore the possibility of building quantum computers that operate using photons, particles of light. Photons can easily carry information from one place to another, and photonic quantum computers can operate at room temperature, so this approach holds promise. However, although people have successfully created individual quantum “logic gates” for photons, it is difficult to build a large number of gates and connect them reliably to perform complex calculations.

Now researchers at Stanford University have come up with a simpler design for photonic quantum computers using readily available components, according to a document Posted in optical. Their proposed design uses a laser to manipulate a single atom which, in turn, can alter 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 separate 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 potentially take thousands of quantum emitters, make them all perfectly indistinguishable, and then fit them into a giant photonic circuit,” said PhD student Ben Bartlett. in applied physics and main author of the article. “While with this design, we only need a handful of relatively simple components, and the size of the machine does not increase with the size of the quantum program you want to run. “

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

Fortunately, these components already exist and are even commercially available. They are also continually being refined as they are currently used in applications other than quantum computing. For example, telecommunications companies have been striving for years to improve fiber optic cables and optical switches.

“What we are proposing here builds on the efforts and investments that people have made to improve these components,” said Shanhui Fan, Professor Joseph and Hon Mai Goodman of the School of Engineering and lead author of the article. “These are not new components specifically for quantum computing.”

An original design

The scientists’ design consists of two main sections: a storage ring and a scattering unit. The storage ring, which functions similarly to ordinary computer memory, is a loop of fiber optics containing several photons that travel around the ring. Analogously to bits that store information in a conventional computer, in this system, each photon represents a quantum bit, or “qubit”. The direction of movement of the photon around the storage ring determines the value of the qubit, which, like a bit, can be 0 or 1. In addition, since photons can exist simultaneously in two states at a time, an individual photon can flow in both directions at the same time. , which represents a value that is a combination of 0 and 1 at the same time.

Researchers can manipulate a photon by directing it from the storage ring to the scattering unit, where it travels to a cavity containing a single atom. The photon then interacts with the atom, causing the two to become entangled, a quantum phenomenon by which two particles can influence each other even over great distances. Then the photon returns to the storage ring and a laser changes the state of the atom. Because the atom and the photon are entangled, the manipulation of the atom also influences the state of its paired photon.

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

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

“For many photonic quantum computers, gates are physical structures that photons pass through, so if you want to change the current program, that 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. “

– This press release was originally published on the Stanford University website

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