‘Hot’ dots for quantum computing

Two separate teams of researchers have shown they can build quantum logic circuits using silicon quantum dots that operate at a significantly higher temperature than existing ones, potentially opening the door to less expensive, more complex machines.

The most successful quantum computers to date are based on superconductors operating at extremely low temperatures, just above absolute zero. Heat typically makes quantum information less stable. Scientists have used silicon for quantum computing before. But previous devices operated at just 100 mK, and it’s hard to see a way to maintain such a temperature with the millions of qubits and their associated electronics that a quantum computer would need. But the teams showed they could manipulate and read the states of silicon quantum dots at much more technologically feasible, though still very low, temperatures.

Both teams use silicon to create quantum dots roughly 20 nm in size. Quantum dots are nanoparticles so small that their electronic properties are governed by quantum mechanics. Pairs of quantum dots can serve as a single basic element in a quantum logic device, called a quantum bit or qubit. Silicon qubits can be made to hold just one or a few electrons. Measuring the spin of those electrons determines whether the bit is a 1 or a 0. The spins are flipped using a burst of microwaves. Many elements have an inherent nuclear spin, but the researchers didn’t want that to interfere with their ability to read and manipulate the spin of the electron, so they made their quantum dots from silicon-28, a naturally spin-free isotope.

Both groups got around this by figuring out a way to read their silicon qubits’ states at higher temperatures. These silicon qubits must still operate at extremely cold temperatures, just above 1 kelvin (-272.15 °C). But that’s an order of magnitude greater than the temperature at which earlier qubits work, and that increase makes a world of difference, says Andrew Dzurak, a professor of nanoelectronics at the University of New South Wales, Australia, who led one of the teams (Nature 2020, DOI: 10.1038/s41586-020-2171-6). Cooling to 100 mK requires dilution refrigerators, which use a mix of helium-3 and helium-4 isotopes to step down temperatures through a series of stages, and can cost $1 million. To get to the 1.5 K, his team demonstrated a simpler, less-expensive cooling system that uses only helium-3.

The compatibility of these new devices with simpler cooling systems also opens up the possibility of quantum integrated circuits analogous to conventional integrated circuits, says Menno Veldhorst, who led the other research team, based at QuTech, a collaboration between TU Delft and TNO, a research organization in the Netherlands. Conventional computers are made up of billions of transistors just nanometers in size integrated into highly complex circuits that can be manufactured on tremendous scales. But the qubits in today’s quantum computers are each addressed by their own individual wiring systems, a somewhat bulky set-up, which makes them difficult to build and operate.

Control circuits for superconducting qubits are kept in a warmer stage of the refrigerator and connected to individual qubits, which are micron-scale, by cables. Both heat from the circuitry and the bulk of the cable limit the number of qubits such a setup can hold.

Because the silicon quantum dots operate at higher temperatures—Veldhorst’s at 1.1 K—and are at the same size scale as transistors on the control chips, they can be placed together with no cables, and might even be fabricated on the same chip using the same technology used to build computer chips (Nature 2020, DOI: 10.1038/s41586-020-2170-7). “They are made in the same way transistors are,” Veldhorst says. “This may be the fastest route to having many qubits.”

Quantum computing promises machines capable of operations no classical computer could perform, such as modeling the physics of molecules or breaking today’s strongest encryption. But to do that they’ll need to contain millions, perhaps billions, of qubits. In October, Google unveiled a quantum computer that it claimed achieved quantum supremacy, the first demonstration that a quantum computer could perform a calculation no classical computer could. That system used 53 superconducting qubits, and Google engineers acknowledged that that system probably cannot scale up to the size needed for practical use.

Veldhorst performed operations using two qubit logic gates, which is necessary to generate entanglement between electrons, one of the quantum mechanical properties that give quantum computing its power. While Dzurak’s paper only includes one gate, he says the group has since shown two-gate operation. “The great thing about the two papers is they show just how reproducible and reliable this technology is,” Dzurak says.

The research is “rather elegant,” says HongWen Jiang, a physicist at the University of California, Los Angeles. “The work represents a technological breakthrough for semiconductor-based quantum computing,” he says.