New silicon structure opens the gate to quantum computers

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Researchers at Princeton University have constructed silicon hardware that can control quantum behaviour between two electrons with extremely high precision.

The team constructed a gate that controls interactions between the electrons in a way that allows them to act as the quantum bits of information, or qubits, necessary for quantum computing. The demonstration of this two-qubit gate is being seen as an early step in building a more complex quantum computing device from silicon.

"The creation of this high-fidelity two-qubit gate opens the door to larger scale experiments,” said Jason Petta, a professor of physics at Princeton University.

Silicon-based devices could prove less expensive and easier to manufacture than other technologies for achieving a quantum computer and although other quantum devices containing 50 or more qubits have been developed, they have required exotic materials such as superconductors or charged atoms held in place by lasers.

Quantum computers are expected to be able to factor extremely large numbers and could help researchers understand the physical properties of extremely small particles such as atoms and molecules, leading to advances in areas such as materials science and drug discovery.

Building a quantum computer requires the creation of qubits and their coupling to each other with high fidelity. Silicon-based quantum devices use a quantum property of electrons called "spin" to encode information. The spin can point either up or down in a manner analogous to the north and south poles of a magnet. In contrast, conventional computers work by manipulating the electron's negative charge.

Achieving a high-performance, spin-based quantum device has been hampered by the fragility of spin states, but by building the silicon quantum devices in Princeton's Quantum Device Nanofabrication Laboratory, the researchers were able to keep the spins coherent for relatively long periods of time.

The two-qubit gate was constructed by layering tiny aluminum wires onto a highly ordered silicon crystal. The wires deliver voltages that trap two single electrons, separated by an energy barrier, in a well-like structure called a double quantum dot.

By temporarily lowering the energy barrier, the researchers allow the electrons to share quantum information, creating the quantum state called entanglement. These entangled electrons are now ready for use as qubits, which are like conventional computer bits but while a conventional bit can represent a zero or a 1, each qubit can be simultaneously a zero and a 1, greatly expanding the number of possible permutations that can be compared instantaneously.

According to David Zajac, a graduate student in physics at Princeton and first-author on the study, "This is the first demonstration of entanglement between two electron spins in silicon, a material known for providing one of the cleanest environments for electron spin states."

The researchers demonstrated that they can use the first qubit to control the second qubit, signifying that the structure functioned as a controlled NOT (CNOT) gate, which is the quantum version of a commonly used computer circuit component. The behaviour of the first qubit is controlled by applying a magnetic field. The gate produces a result based on the state of the first qubit: If the first spin is pointed up, then the second qubit's spin will flip, but if the first spin is down, the second one will not flip.

"The gate is basically saying it is only going to do something to one particle if the other particle is in a certain configuration," Petta said. "What happens to one particle depends on the other particle."

The researchers showed that they can maintain the electron spins in their quantum states with a fidelity exceeding 99 percent and that the gate works reliably to flip the spin of the second qubit about 75 percent of the time.

The technology has the potential to scale to more qubits with even lower error rates, according to the researchers.