Researchers Finland say they are the first to determine the state of a superconducting qubit using a bolometer – a device that measures radiant heat. While the fidelity and speed of the readout fall far short of state-of-the-art conventional methods for quantum computing, the technique has the potential to be more scalable than current methods and could be immune to some types of noise.
Quantum computers use quantum bits (qubits) to store and process information. At the end of a calculation the quantum states of qubits must be read to extract the result. Some of the most advanced quantum computers to date – including those developed by Google and IBM – use qubits made from superconducting electronic circuits that are operated at very low temperatures.
Reading these qubits is currently a complex and difficult process, explains Mikko Möttönen of Aalto University and Finland’s VTT Technical Research Centre: “If you tried to measure the voltage directly it would be very challenging, because the voltage is tiny. Instead a microwave resonator is coupled to the qubit, and depending on the state of the qubit the frequency of the resonator gets shifted slightly.”
This process involves injecting microwaves into the measurement circuit, and then reading the state as a change in the phase of the current–voltage oscillations of the microwaves caused by their interaction with the qubit.
This is not ideal, as Möttönen explains: “The signal that you can put into this measurement circuit is very, very weak, so if you were to take it to room temperature without amplifying you would measure nothing,” he says. This is important, because the results of a quantum calculation must be ultimately be relayed to electronics operating at room temperature.
Uncertainty principle
“So on the way up to room temperature there will be several stages of amplifiers, and each of these amplifiers must add some noise. It can be done quite well, but it’s not at the same level of accuracy as quantum logic at the moment.” Another problem is more fundamental: measuring the state of the qubit involves measuring voltage and current, and Heisenberg’s uncertainty principle limits the precision at which these can both be known simultaneously.
Measuring the power emitted by the qubit with a bolometer circumvents both these problems. The measurement can be made in the refrigerator, so repeated amplification is unnecessary. Moreover, as there is no need for complete knowledge of the phase of a microwave in order to read off the energy level of the qubit, Heisenberg’s uncertainty principle does not limit the measurement’s accuracy in this way.
The researchers connected their bolometer to a standard superconducting qubit chip with a coupled microwave resonator. However, they made a slight tweak to the input. In a standard measurement scheme, the input microwave frequency is intermediate between the two resonant frequencies. “In that case, there’s no information in the amplitude of the oscillation because you’re in the middle of the resonances, so you always excite the same amplitude,” says Möttönen.
Slightly higher power
Instead, the researchers drove the resonator at the ground state frequency of the qubit. “Now we just channel the signal to the absorber of the bolometer,” says Möttönen. If the qubit is in the ground state, the bolometer detects slightly higher power because of resonance. If the qubit is in the excited state, then the signal in the bolometer is lower – and this difference is used to determine the state of qubit.
New bolometer could lead to better cryogenic quantum technologies
For this technique to work at very high fidelity, a very fast and very sensitive bolometer is needed to measure the quantum state before it decays. In 2020, the Finnish researchers unveiled a bolometer that used graphene as its absorber – a fast and sensitive design that was intended for use in quantum computing. Unfortunately, this bolometer degraded over time and the team instead used an older bolometer design involving interfaces between superconductors and normal metals.
Möttönen says that the researchers had initially not expected the older design to be effective for reading out the states of individual qubits. He also expects that the read-out fidelity could be boosted using improved graphene bolometers. “I’m hoping to get the new graphene bolometers out of the oven soon,” he says.
David Pahl at the Massachusetts Institute of Technology believes that the work is very preliminary, but potentially very important. He says that the two most important performance metrics for a scheme to read out quantum states are the fidelity and the speed: “The state of the art speed that we’ve seen in the past year is 0.1 μs and 99.5% fidelity…[Möttönen and colleagues] showed 14 μs and 61.7%,” he says.
Pahl points out that the bolometer is much more compact than amplifier-based systems. Amplifiers require bulky isolators, whereas bolometers could potentially be integrated on a chip, he says. He also points out that Heisenberg’s uncertainty principle does impose some theoretical limits on the sensitivity of a bolometer, but says that today’s devices are far from those limits. He looks forward to seeing the results with graphene bolometers.
The research is published in Nature Electronics.
