Another record on the road to fully operational and capable quantum computers has been broken: full control of a 6-qubit quantum processor in silicon.
The researchers call it “an important step” for the technology.
Qubits (or quantum bits) are the quantum equivalents of classical computing bits, only they can potentially process much more information. Thanks to quantum physics, they can be in two states at once, instead of just 1 or 0.
The difficulty is getting many qubits to behave as we need them to, which is why this jump to six is important. Being able to run them on silicon, the same material used in today’s electronic devices, makes the technology potentially more viable.
“The challenge of quantum computing today consists of two parts,” says quantum computing researcher Stephan Philips of Delft University of Technology in the Netherlands. “To develop qubits of sufficiently good quality and to develop an architecture that allows building large qubit systems.”
“Our work fits into both categories. And since the overall goal of building a quantum computer is an enormous endeavor, I think it’s fair to say that we’ve made a contribution in the right direction.”
Qubits are made of individual electrons fixed in a row, 90 nanometers apart (a human hair is about 75,000 nanometers in diameter). This line of “quantum dots” is placed in silicon, using a structure similar to the transistors used in standard processors.
The six-qubit quantum processor. The qubits are created by adjusting the voltage on the chip’s red, blue, and green wires. SD1 and SD2 are extremely sensitive electric field sensors that can detect the charge of a single electron. These sensors, along with advanced control schemes, allowed the researchers to place individual electrons in locations labeled 1-6, which were then operated as qubits. (Philips et al., Nature, 2022)
By making careful improvements to the way electrons were prepared, managed and monitored, the team was able to successfully control their spin, the quantum mechanical property that enables the qubit state.
The researchers were also able to create logic gates and entangle systems of two or three electrons, on demand, with low error rates.
The researchers used microwave radiation, magnetic fields and electric potentials to control and read the electron spin, operating them as qubits and making them interact with each other as needed.
“In this research, we push the envelope of the number of qubits in silicon and achieve high initialization fidelities, high readout fidelities, high single-qubit gate fidelities, and high two-qubit state fidelities,” also says the electrical engineer Lieven Vandersypen, from the Delft University of Technology.
“What really stands out is that we demonstrate all these features together in a single experiment in a record number of qubits.”
So far, only 3-qubit processors have been successfully built in silicon and controlled to the required level of quality, so we’re talking about a big step forward in terms of what’s possible in this kind of qubit
There are different ways to build qubits, including in superconductors where many more qubits have been made to work together, and scientists are still figuring out which method might be the best way forward.
The advantage of silicon is that the manufacturing and supply chains are already in place, meaning the transition from a science lab to a real machine should be easier. Work continues to further increase the qubit register.
“With careful engineering, it is possible to increase the number of silicon spin qubits while maintaining the same precision as for individual qubits,” says electrical engineer Mateusz Madzik of Delft University of Technology.
“The key building block developed in this research could be used to add even more qubits in future iterations of the study.”
The research has been published in Nature.