You can use currently available components to build a relatively simple quantum computer design that uses a single atom to manipulate photons.

Today's quantum computers are complex to build, difficult to scale up, and require colder temperatures than interstellar space to operate. These challenges have prompted researchers to explore the possibility of building quantum computers that use photons (light particles) to work. Photons can easily transmit information from one place to another, and photon quantum computers can operate at room temperature, so this method is very promising. However, although people have successfully created individual quantum "logic gates" for photons, it is still challenging to construct a large number of gates and connect them in a reliable way to perform complex calculations.

Stanford University graduate student Ben Bartlett and electrical engineering professor Shanhui Fan proposed a simpler photonic quantum computer design using off-the-shelf components. (Image source: Courtesy of Ben Bartlett / Rod Searcey)

Now, based on a paper published on Optica on November 29, Stanford University researchers have proposed a simpler photonic quantum computer design that uses off-the-shelf components. The design they proposed uses a laser to manipulate a single atom, which in turn can change the state of photons through a phenomenon called "quantum teleportation." Atoms can be reset and reused in many quantum gates, without the need to build multiple different physical gates, which greatly reduces the complexity of building a quantum computer.

"Normally, if you want to build this type of quantum computer, you have to use potentially thousands of quantum emitters, make them completely indistinguishable, and then integrate them into a huge photonic circuit," the doctoral candidate Ben Bartlett said. Lead author in applied physics and papers. "With this design, we only need some relatively simple components, and the size of the machine will not increase with the size of the quantum program you want to run."

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

Fortunately, these components already exist and are even available on the market. They are also constantly improving because they are currently used in applications other than quantum computing. For example, telecommunications companies have been working on improving fiber optic cables and optical switches for many years.

"The recommendations we make here are based on the efforts and investments people have made to improve these components," said Prof. Joseph and Hanmai Goodman from the School of Engineering and Shanhui Fan, the senior author of the paper. "They are not new components specifically for quantum computing."

The scientist's design consists of two main parts: the storage ring and the scattering unit. The function of the storage ring is similar to the memory in an ordinary computer. It is an optical fiber loop that can accommodate multiple photons traveling around the ring. Similar to the bits of information stored in classical computers, in this system, each photon represents a qubit, or "qubit." The direction the photon travels around the storage ring determines the value of the qubit, just like a bit, it can be 0 or 1. In addition, since photons can exist in two states at the same time, a single photon can flow in two directions at the same time, which means that it is a combination of 0 and 1 at the same time.

Researchers can manipulate the photon by directing the photon from the storage ring into the scattering unit, where it travels to a cavity containing a single atom. Then the photon interacts with the atom, causing the two to "entangle" together. This is a quantum phenomenon in which two particles can influence each other even at a long distance. Then, the photon returns to the storage ring, and the laser changes the state of the atom. Because atoms and photons are entangled, manipulating atoms will also affect the state of their paired photons.

The animation of the photon quantum computer proposed by the researchers. On the left is the storage ring, which contains multiple back-propagating photons. On the right is the scattering unit, which is used to manipulate photon qubits. The sphere at the top is called the "Bloch sphere", which depicts the mathematical state of the atom and one of the photons. Because atoms and photons are entangled, manipulating atoms can also affect the state of photons. (Image source: Ben Bartlett)

"By measuring the state of the atom, you can transmit operations to photons," Bartlett said. "So we only need a 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 series of operations performed on atoms, in principle you can use only one controllable atomic qubit to run quantum programs of any size. To run the program, the code is translated into a series of operations that direct photons to the scattering unit and manipulate atomic qubits. Because you can control the way atoms and photons interact, the same device can run many different quantum programs.

"For many photonic quantum computers, a gate is a physical structure through which photons pass, so if you want to change a running program, it usually involves physically reconfiguring the hardware," Bartlett said. "In this case, you don't need to change the hardware-you just need to give the machine a different set of instructions."

Avik Dutt, a postdoctoral scholar at Stanford University, is also a co-author of this paper. Fan is a professor of electrical engineering, a member of Stanford Bio-X, and an affiliate of the Precourt Institute of Energy.

This research was funded by the U.S. Department of Defense and the U.S. Air Force Office of Scientific Research.

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Taylor Kubota, Stanford News Service: (650) 724-7707, tkubota@stanford.edu

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