They discovered a new technique they say will be capable of controlling millions of spin qubits—the basic units of information in a silicon quantum processor.
Until now, quantum computer engineers and scientists have worked with a proof-of-concept model of quantum processors by demonstrating the control of only a handful of qubits.
“Up until this point, controlling electron spin qubits relied on us delivering microwave magnetic fields by putting a current through a wire right beside the qubit,” Dr. Pla says.
“This poses some real challenges if we want to scale up to the millions of qubits that a quantum computer will need to solve globally significant problems, such as the design of new vaccines.
“First off, the magnetic fields drop off really quickly with distance, so we can only control those qubits closest to the wire. That means we would need to add more and more wires as we brought in more and more qubits, which would take up a lot of real estate on the chip.”
And since the chip must operate at freezing cold temperatures, below -270°C, Dr. Pla says introducing more wires would generate way too much heat in the chip, interfering with the reliability of the qubits.
Rather than having thousands of control wires on the same thumbnail-sized silicon chip that also needs to contain millions of qubits, the team looked at the feasibility of generating a magnetic field from above the chip that could manipulate all of the qubits simultaneously.
Dr. Pla and the team introduced a new component directly above the silicon chip—a crystal prism called a dielectric resonator. When microwaves are directed into the resonator, it focuses the wavelength of the microwaves down to a much smaller size.
“The dielectric resonator shrinks the wavelength down below one millimeter, so we now have a very efficient conversion of microwave power into the magnetic field that controls the spins of all the qubits.
“There are two key innovations here. The first is that we don’t have to put in a lot of power to get a strong driving field for the qubits, which crucially means we don’t generate much heat. The second is that the field is very uniform across the chip, so that millions of qubits all experience the same level of control.”