Quantum computing chips could use heat to eliminate errors
Chalmers University of Technology, Lovisa Håkansson
A tiny cooling device can automatically reset malfunctioning components of a quantum computer. Its performance suggests that manipulating heat could also enable other autonomous quantum devices.
Quantum computers aren’t yet fully practical because they make too many errors. In fact, if qubits – key components of this type of computer – accidentally heat up and become too energetic, they can end up in an erroneous state before the calculation even begins. One way to “reset” the qubits to their correct states is to cool them down.
Simone Gasparinetti at Chalmers University of Technology in Sweden and his colleagues have delegated this task to an autonomous quantum “refrigerator” for the first time.
The researchers built two qubits and one “qutrit”, which can store more complex information than a qubit, from tiny superconducting circuits. The qutrit and one of the qubits formed a fridge for the second target qubit, which could eventually be used for computation.
The researchers carefully engineered the interactions between the three components to ensure that when the target qubit had too much energy, which caused errors, heat automatically flowed out of it and into the two other elements. This lowered the target qubit’s temperature and reset it. Because this process was autonomous, the qubit-and-qutrit fridge could correct errors without any outside control.
Aamir Ali, also at Chalmers University of Technology, says this approach to resetting the qubit required less new hardware than more conventional methods – and yielded better results. Without any significant quantum computer redesign or introduction of new wires, the qubit’s starting state was correct 99.97 per cent of the time. In contrast, other reset methods typically only manage 99.8 per cent, he says.
This is a powerful example of how thermodynamic machines – which deal with heat, energy and temperature – can be useful in the quantum realm, says Nicole Yunger Halpern at the National Institute of Standards and Technology in Maryland, who worked on the project.
Conventional thermodynamic machines like the heat engine sparked a whole industrial revolution, but so far quantum thermodynamics hasn’t been very practical. “I’m interested in making quantum thermodynamics useful. This arguably useful autonomous quantum refrigerator is our first example,” says Yunger Halpern.
“It’s nice to see this machine implemented and useful. The fact that it is autonomous, so it does not require any external control, should make it efficient and versatile,” says Nicolas Brunner at the University of Geneva in Switzerland.
Michał Horodecki at the University of Gdańsk in Poland says that one of the most urgent problems for quantum computers built with superconducting circuits is making sure the machines don’t heat up and subsequently make errors. The new experiment opens a path for many similar projects that have been proposed but never tested, such as using qubits to build autonomous quantum engines, he says.
The researchers are already looking into whether they could build on their experiment. For example, they might create an autonomous quantum clock or design a quantum computer with other functions automatically driven by temperature differences.
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