Simple quantum computer uses commercially available components

Quantum computers are complex to build, difficult to scale up and harder yet to operate and maintain, requiring temperatures near absolute zero. These challenges have led researchers to explore the possibility of building photonic quantum computers: room-temperature quantum computers that use particles of light to carry information. While scientists have successfully created individual quantum logic gates for photons, it is challenging to construct larger numbers of gates with reliable connections; this is necessary to perform useful calculations.

The Stanford University team’s design uses a laser to manipulate a single atom that, in turn, can modify the state of the photons via quantum teleportation. The atom can be 'reset' and reused for many quantum gates, eliminating the need for many distinct physical gates and hugely reducing the complexity of building a quantum computer.

“Normally, if you wanted to build this type of quantum computer, you’d have to take potentially thousands of quantum emitters, make them all perfectly indistinguishable and then integrate them into a giant photonic circuit,” said Dr Ben Bartlett, a PhD candidate in applied physics and lead author of the Optica study.

“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 simple quantum computer requires only a few pieces of equipment that are commercially available: a fibre-optic cable, a beam splitter, a pair of optical switches and an optical cavity. Senior author Professor Shanhui Fan explained: “They are not new components specifically for quantum computation.”

The design consists of two main sections: a storage ring and a scattering unit. The storage ring, which corresponds to memory, is a fibre-optic loop holding multiple photons that travel around the ring. Each photon represents a qubit, with its direction of travel around the ring determining its value (0, 1, and combinations of the two). A photon can be manipulated by directing it from the storage ring into 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; this means that when the photon returns to the storage ring, when the state of the atom is altered with a laser, the state of the paired photon is also altered.

“By measuring the state of the atom, you can teleport operations onto the photons,” said Bartlett. “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.”

Any quantum logic gate can be compiled into a sequence of operations performed on the atom, so it is possible, in principle, to run any quantum program of any size using just 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. Because it is possible to control how the atom and photons interact, 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,” said Bartlett. “Whereas in this case, you don’t need to change the hardware; you just need to give the machine a different set of instructions.”