Friday, October 7, 2022

Silicon qubits for quantum computing

In a race to build quantum computing hardware, silicon begins to shine.

Silicon spin qubits satisfy the necessary criteria for quantum information processing. However, a demonstration of high-fidelity state preparation and readout combined with high-fidelity single- and two-qubit gates has been lacking. Now, scientists from Princeton University are taking a step towards using silicon-based technologies in quantum computing.

Using a two-qubit silicon quantum device, scientists obtained an unprecedented level of fidelity at above 99 percent. This is the highest fidelity achieved for a two-qubit gate in a semiconductor and is on par with the best results achieved by competing technologies.

Scientists were also able to capture two electrons and force them to interact. The spin state of each electron can be used as a qubit, and the interaction between the electrons can entangle these qubits.

This operation is crucial for quantum computation, and scientists performed this operation at a fidelity level exceeding 99.8 percent.

Adam Mills, a graduate student in the Department of Physics at Princeton University, said, “Silicon spin qubits are gaining momentum [in the field]. It’s looking like a big year for silicon overall.”

“In a qubit, you can encode zeros and ones, but you can also have superpositions of these zeros and ones. This means that each qubit can be simultaneously a zero and a one. This concept, called superposition, is a fundamental quality of quantum mechanics and allows qubits to do operations that seem amazing and otherworldly. In practical terms, it allows the quantum computer a greater advantage over conventional computers in, for example, factoring very large numbers or isolating the most optimal solution to a problem.”

The spin in spin qubits is a quantum property that acts as a tiny magnetic dipole that can be used to encode information. Quantum mechanically, the electron’s spin can align with the magnetic field generated in the lab, be oriented anti-parallel to the area (spin-down), or be in a quantum superposition of spin-up and spin-down.

Mills said, “In general, silicon spin qubits have advantages over other qubit types. The idea is that every system will have to scale up to many qubits. And right now, the other qubit systems have real physical limitations to scalability. Size could be a real problem with these systems. There’s only so much space you can cram these things into.”

Unlike conventional superconducting qubit that is 300 microns across, this two-qubit silicon quantum device is just about 100 nanometers across.

Jason Petta, the Eugene Higgins Professor of Physics at Princeton, said, “The other advantage of silicon spin qubits is that conventional electronics today are based on silicon technology. Our feeling is that if you want to make a million or ten million qubits that are required to do something practical, that’s only going to happen in a solid-state system that can be scaled using the standard semiconductor fabrication industry.”

“One of the bottlenecks for the technology of spin qubits is that the two-qubit gate fidelity up until recently has not been that high. It’s been well below 90 percent in most experiments.”

For the experiment, scientists first need to capture a single electron, get it into a specific region of space and then make it dance. To do so, they constructed a cage. This took the form of a wafer-thin semiconductor made primarily out of silicon. The team patterned little electrodes to the top of this, which created the electrostatic potential used to corral the electron. Two of these cages, each separated by a barrier, or gate, constituted the double quantum dot.

By adjusting the voltage on these gates, scientists momentarily pushed the electrons together and made them interact. They dubbed this as a two-qubit gate.

Due to the interaction, each spin qubit evolves according to the state of its neighboring spin qubit, hence causing entanglement in quantum systems.

Petta said that “the results of this experiment place this technology — silicon spin qubits — on an equal footing with the best results achieved by the other major competing technologies. This technology is on a strongly increasing slope, and I think it’s just a matter of time before it overtakes the superconducting systems.”

“Another important aspect of this paper is that it’s not just a demonstration of a high fidelity two-qubit gate, but this device does it all. This is the first demonstration of a semiconductor spin qubit system where we have integrated the entire system’s performance — the state preparation, the readout, the single-qubit control, the two-qubit control — all with performance metrics that exceed the threshold you need to make a larger-scale system work.”

Journal Reference:

  1. Adam Mills, Charles Guinn, Michael Gullans et al. Two-qubit silicon quantum processor with operation fidelity exceeding 99%. DOI: 10.1126/sciadv.abn5130
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