A Superconducting Device May Twist Heisenberg’s Uncertainty Principle
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An instrument from the 1800s recently got a quantum makeover to measure qubit performance.
All quantum systems throw off noise when measured, making them very difficult to compare.
A nanobolometer and a vacuum environment could cut that noise to near 0.
In new research, scientists from Aalto University in Finland say they’ve skirted the Heisenberg Uncertainty Principle that underpins—or undermines—every experiment in quantum mechanics. The secret is an instrument called a bolometer, and using it could help scientists who continue to work on qubit-based quantum computers.
Aalto’s Quantum Computing Group (QCG) previously made news in 2019 by establishing that bolometers could be used to measure performance in quantum computing. The group’s new research further shows that these “nano bolometers” also avoid the noise introduced into quantum measurements by other methods.
But what is a bolometer? And what is this noise?
On the particle level, the unit of quantum computing is the qubit. It mimics the traditional electricity-based on/off switch of our computer bits, but shows quantum behaviors like superposition. Measuring traditional computing is simple, because electricity through semiconductors, resistors, and conductive wires is quite subdued. In other words, there's less noise—what you’re measuring isn’t moving around and making your data less reliable and consistent.
Measuring the performance of qubits, on the other hand, is difficult even when pursued in the simplest ways. That’s because of the Heisenberg Uncertainty Principle, which tells us that observing a quantum system inherently makes it behave differently. In this case, that new behavior creates noise.
Reducing noise is a major project in quantum computing—largely because, so far, these systems rely heavily on abstract and theoretical discussions in lieu of measurements that scientists can compare to one another. Even the amount of noise is hard to predict. So, to combat it, researchers have tried different things, like “squeezing” the noise all the way into one variable so the other stays more true, using a parametric amplifier.
In their new peer-reviewed paper in Nature Electronics, the researchers from Aalto University explain the limits of that approach. “Parametric amplifiers,” they wrote, “can offer high gain and low noise, but introduce challenges in terms of scaling to large numbers of qubits.” And these amplifiers still perpetuate the noise from the Heisenberg Uncertainty Principle itself, which has been considered an immovable object in the fight against noise—until this team decided to try a bolometer.
A bolometer is an instrument that uses a resistor in order to snare and accumulate heat. It was invented by onetime Smithsonian secretary Samuel Langley—an early contemporary of the Wright brothers who helped lay the groundwork for successful airplanes. And it’s simple enough: a temperature-controlled chamber on one side is sealed with a cover made of a resistor, like metal, that changes in response to temperature. The difference between the two sides can be measured. Today, scientists don’t really even use metal. Instead, they use superconducting materials that are cooled to near absolute zero, so that even the tiniest change can be measured.
Bolometers, in this case nanobolometers, “have been shown to be fast and sensitive enough for the readout of superconducting qubits, reaching thermal time constants in the range of hundreds of nanoseconds and energy resolution of a few typical microwave photons,” the researchers explain in their paper. And because nanobolometers both aren’t amplifying anything and are operating in a vacuum, they avoid added Heisenberg noise altogether.
By tuning them carefully, the researchers reduced noise until their measurements were as noiseless as possible. This process works for systems with many qubits, which is essential as quantum computers “scale up” from literally a handful to enough to constitute a usable computer. And nanobolometers, they concluded, are “relatively simple to fabricate and operate.” All this, and they didn’t even use the coolest or newest bolometers, leaving room for the next research group (or QCG’s own next cohort) to iterate with better, faster, stronger nanobolometers.
“For example, we can swap the bolometer material from metal to graphene, which has a lower heat capacity and can detect very small changes in its energy quickly,” researcher András Gunyhó said in a statement. “And by removing other unnecessary components, we can achieve a smaller and simpler measurement device that makes scaling-up to higher qubit counts more feasible.”
Gunyhó says the right advanced materials could jump from this team’s 92.7 percent fidelity measurement to 99.9 percent—arguably once a pipe dream, but maybe within our reach at last.
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