Princeton University researchers have built a device in which a single electron can pass its quantum information to a particle of light. The particle of light, or photon, then acts as a messenger to carry the information to other electrons, creating connections that form the circuits of a quantum computer.

A fully packaged device for trapping and manipulating single electrons and photons. A series of on-chip electrodes (lower left and upper right) leads to the formation of a double quantum dot that confines a single electron below the surface of the chip. The photon, which is free to move within the full 7-millimeter span of the cavity, exchanges quantum information with the electron inside the double quantum dot. (Photo: Jason Petta research group, Princeton University Department of Physics)

Light can be used to link individual electrons that act as the bits, or smallest units of data, in a quantum computer. Quantum computers are advanced devices that, when realized, will be able to perform advanced calculations using tiny particles such as electrons that follow quantum rules rather than the physical laws of the everyday world.

Simple quantum computers have been made using trapped ions and superconductors, but technical challenges have slowed the development of silicon-based quantum devices. Silicon is inexpensive and is already widely used in smartphones and computers.

The researchers trapped both an electron and a photon in the device, then adjusted the energy of the electron in such a way that the quantum information could transfer to the photon. This coupling enables the photon to carry the information from one qubit to another located up to a centimeter away.

Quantum information is extremely fragile and can be lost entirely due to the slightest disturbance from the environment. Photons are more robust against disruption, and can potentially carry quantum information not just from qubit to qubit in a quantum computer circuit, but also between quantum chips via cables.

For these different types of particles to talk to each other, researchers built a semiconductor chip from layers of silicon and silicon-germanium. This structure trapped a single layer of electrons below the surface of the chip. Next, tiny wires, each just a fraction of the width of a human hair, were placed across the top of the device. These nanometer-sized wires allowed the researchers to deliver voltages that created an energy landscape capable of trapping a single electron, confining it in a region of the silicon called a double quantum dot.

The same wires were used to adjust the energy level of the trapped electron to match that of the photon, which is trapped in a superconducting cavity fabricated on top of the silicon wafer. Previously, semiconductor qubits could only be coupled to neighboring qubits. By using light to couple qubits, it may be feasible to pass information between qubits at opposite ends of a chip.

The electron's quantum information consists of nothing more than the location of the electron in one of two energy pockets in the double quantum dot. The electron can occupy one or the other pocket, or both simultaneously. By controlling the voltages applied to the device, the researchers can control which pocket the electron occupies.

The new circuit design brings the wires closer to the qubit and reduces interference from other sources of electromagnetic radiation. To reduce this noise, the researchers put in filters that remove extraneous signals from the wires that lead to the device. The metal wires also shield the qubit, making the qubits 100 to 1,000 times less noisy than the ones used in previous experiments.

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NASA Tech Briefs Magazine

This article first appeared in the March, 2017 issue of NASA Tech Briefs Magazine.

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