Chasing ever-higher qubit counts in near-term quantum computers continually demands recent feats of engineering.
Among the many troublesome hurdles of this scaling-up race is refining how qubits are measured. Devices called parametric amplifiers are traditionally used to do these measurements. But because the name suggests, the device amplifies weak signals picked up from the qubits to conduct the readout, which causes unwanted noise and may result in decoherence of the qubits if not protected by additional large components. More importantly, the bulky size of the amplification chain becomes technically difficult to work around as qubit counts increase in size-limited fridges.
Cue the Aalto University research group Quantum Computing and Devices (QCD). They’ve a hefty track record of showing how thermal bolometers may be used as ultrasensitive detectors, and so they just demonstrated in an April 10 Nature Electronics paper that bolometer measurements may be accurate enough for single-shot qubit readout.
A brand new approach to measuring
To the chagrin of many physicists, the Heisenberg uncertainty principle determines that one cannot concurrently know a signal’s position and momentum, or voltage and current, with accuracy. So it goes with qubit measurements conducted with parametric voltage-current amplifiers. But bolometric energy sensing is a fundamentally different form of measurement — serving as a method of evading Heisenberg’s infamous rule. Since a bolometer measures power, or photon number, it just isn’t sure so as to add quantum noise stemming from the Heisenberg uncertainty principle in the best way that parametric amplifiers are.
Unlike amplifiers, bolometers very subtly sense microwave photons emitted from the qubit via a minimally invasive detection interface. This manner factor is roughly 100 times smaller than its amplifier counterpart, making it extremely attractive as a measurement device.
‘When considering of a quantum-supreme future, it is straightforward to assume high qubit counts within the 1000’s and even thousands and thousands might be commonplace. A careful evaluation of the footprint of every component is completely crucial for this massive scale-up. We’ve shown within the Nature Electronics paper that our nanobolometers could seriously be regarded as a substitute for conventional amplifiers. In our very first experiments, we found these bolometers accurate enough for single-shot readout, freed from added quantum noise, and so they devour 10,000 times less power than the everyday amplifiers — all in a tiny bolometer, the temperature-sensitive a part of which might fit within a single bacterium,’ says Aalto University Professor Mikko Möttönen, who heads the QCD research group.
Single-shot fidelity is a crucial metric physicists use to find out how accurately a tool can detect a qubit’s state in only one measurement versus a mean of multiple measurements. Within the case of the QCD group’s experiments, they were capable of obtain a single-shot fidelity of 61.8% with a readout duration of roughly 14 microseconds. When correcting for the qubit’s energy rest time, the fidelity jumps as much as 92.7%.
‘With minor modifications, we could expect to see bolometers approaching the specified 99.9% single-shot fidelity in 200 nanoseconds. For instance, we are able to swap the bolometer material from metal to graphene, which has a lower heat capability and may detect very small changes in its energy quickly. And by removing other unnecessary components between the bolometer and the chip itself, we are able to not only make even greater improvements on the readout fidelity, but we are able to achieve a smaller and simpler measurement device that makes scaling-up to higher qubit counts more feasible,’ says András Gunyhó, the primary writer on the paper and a doctoral researcher within the QCD group.
Prior to demonstrating the high single-shot readout fidelity of bolometers of their most up-to-date paper, the QCD research group first showed that bolometers may be used for ultrasensitive, real-time microwave measurements in 2019. They then published in 2020 a paper in Nature showing how bolometers product of graphene can shorten readout times to well below a microsecond.
The work was carried out within the Research Council of Finland Centre of Excellence for Quantum Technology (QTF) using OtaNano research infrastructure in collaboration with VTT Technical Research Centre of Finland and IQM Quantum Computers. It was primarily funded by the European Research Council Advanced Grant ConceptQ and the Future Makers Program of the Jane and Aatos Erkko Foundation and the Technology Industries of Finland Centennial Foundation.
