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Quantum Computing Advances With Demo of Spin–Photon Interface in Silicon
Feb 2018
PRINCETON, N.J., Feb. 15, 2018 — Researchers have experimentally demonstrated a way to use light to connect silicon quantum bits (qubits) of information that are not immediately adjacent to each other. Their work could advance scientific efforts to make quantum computing devices from silicon.

A team led by researchers at Princeton University, in collaboration with colleagues at the University of Konstanz and the Joint Quantum Institute, created qubits from single electrons trapped in silicon chambers. The team demonstrated a way to transfer quantum information encoded in an electron spin to a photon.

Silicon qubits, spin-photon interface, Princeton University et al.

Researchers successfully coupled a single electron's spin, represented by the dot on the left, to light, represented as a wave passing over the electron, which is trapped in a double-welled silicon chamber (a quantum dot). The goal is to use light to carry quantum information to other locations on a quantum computing chip. Courtesy of Emily Edwards, University of Maryland.

The team showed strong coupling between a single spin in silicon and a single microwave-frequency photon, with spin–photon coupling rates of more than 10 MHz. The mechanism that enabled the coherent spin-photon interactions was based on spin-charge hybridization in the presence of a magnetic-field gradient. In addition to spin-photon coupling, the team demonstrated coherent control and dispersive readout of a single spin.

The researchers had previously demonstrated the successful coupling of two neighboring electron spins separated by only 100 nm. But coupling spin to light, in order to enable long-distance spin-spin coupling, remained a challenge.

In the current study, the team solved the problem of long-distance communication by coupling the qubit's information to a photon trapped above the qubit in the chamber. The photon’s wave-like nature allowed it to oscillate above the qubit.

The researchers at Princeton University included (left to right): Jason Petta, professor of physics, David Zajac, graduate student, Xiao Mi, graduate student and Stefan Putz, postdoctoral researcher. Silicon qubits, spin-photon interface in silicon.
The researchers at Princeton University included (left to right): Jason Petta, professor of physics, David Zajac, graduate student, Xiao Mi, graduate student and Stefan Putz, postdoctoral researcher. Courtesy of Denise Applewhite, Princeton University.

Researchers linked the information about the spin’s direction to the photon so that the photon could pick up a message such as “spin points up” from the qubit.

“The strong coupling of a single spin to a single photon is an extraordinarily difficult task akin to a perfectly choreographed dance,” researcher Xiao Mi said. “The interaction among the participants — spin, charge and photon — needs to be precisely engineered and protected from environmental noise, which has not been possible until now.”

To engineer the delicate coupling, the team tapped into the electromagnetic wave properties of light and coupled the photon’s electric field to the electron’s spin state. This work was built on the team’s previous findings, demonstrating coupling between a single electron charge and a single particle of light.

To coax the qubit to transmit its spin state to the photon, researchers placed the electron spin in a large magnetic field gradient, so that the electron spin had a different orientation depending on which side of the chamber it occupied. The magnetic field gradient, combined with the charge coupling previously demonstrated by the group, coupled the qubit’s spin direction to the photon’s electric field.

Ideally, as a next step the photon would then deliver the message to another qubit located within the chamber. Another possibility is that the photon’s message could be carried through wires to a device that read out the message. The researchers are working on these next steps in the process.

Silicon spin qubits are more resilient than competing qubit technologies to outside disturbances such as heat and vibrations.

The simple act of reading out the results of a quantum calculation can destroy the quantum state, a phenomenon known as “quantum demolition.” The researchers theorize that the current approach may avoid this problem because it uses light to probe the state of the quantum system. Light is already used as a messenger to bring cable and internet signals into homes via fiber optic cables, and it is also being used to connect superconducting qubit systems, but this is one of the first applications of light in silicon spin qubits.

Several steps are still needed before making a silicon-based quantum computer, professor Jason Petta said. Every day computers process billions of bits, and although qubits are more computationally powerful, most experts agree that 50 or more qubits are needed for quantum computers to achieve supremacy over their classical counterparts.

“This is a breakout year for silicon spin qubits," said Petta. “This work expands our efforts in a whole new direction, because it takes you out of living in a two-dimensional landscape, where you can only do nearest-neighbor coupling, and into a world of all-to-all connectivity. That creates flexibility in how we make our devices.”

The research was published in Nature (doi:10.1038/nature25769).

Smallest amount into which the energy of a wave can be divided. The quantum is proportional to the frequency of the wave. See photon.
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