Physicists have built the first quantum heat engine — and it could save quantum computers from cable chaos

Finnish physicists for the first time made a heat engine work according to the laws of the quantum world — at a temperature a hair's breadth from absolute zero. The device, about the size of a speck of dust, could solve one of the major engineering problems of future quantum computers: the millions of expensive wires that run from the chip to room temperature.

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Watt's steam engine, internal combustion engine, turbine at a power plant — they all convert heat into work according to the same laws of thermodynamics, derived back in the 19th century, reminiscent of xrust. A question that has long tormented physicists: what will happen to these laws if the engine is compressed to a size where it is not mechanics that rules, but quantum uncertainty?

A team from Aalto University (Finland) gave an experimental answer to this question for the first time. Scientists under the leadership of Professor Mikko Möttönen have assembled a superconducting circuit that behaves like a real cyclic heat engine — only instead of a piston and cylinder, it uses a qubit and a resonator, and instead of coal — crumbs of heat, which are only sufficient at temperatures a thousandths of a degree above absolute zero. The results were published in Nature Communications .

How the quantum engine works

The device is based on a transmon qubit (one of the basic elements of modern quantum processors) connected to a quantum refrigerator. The refrigerator is the team's key discovery: a conventional heat engine requires two different reservoirs, a hot and a cold one. Here, the same controlled chip alternately heats and cools the qubit on command.

“Our quantum refrigerator can be configured to both heat and cool a qubit on demand. Using precisely timed control pulses, we ran the motor through the Otto cycle and monitored the state of the qubit,” says first author of the study Tuomas Uusnäkki.

The Otto cycle is the same thermodynamic process that turns the pistons in a gasoline car engine. The difference is in scale: there the fuel burns, here controlled pulses drive heat between the states of a single quantum particle. Measurements have shown that for each cycle the system consistently produces positive work. This is the first experimental demonstration of a cyclic quantum heat engine on superconducting circuits — previously this was possible only in theory or in one-time, non-cyclic experiments.

Why is this needed: salvation from millions of wires

While this is a laboratory proof-of-concept, it has a very practical purpose. The Finns expect to bring the technology to a stand-alone device that can read the state of qubits directly on the chip — without a microwave pulse, which today has to be pulled by cable from the chip at a temperature of millikelvins to the equipment at room temperature.

This sounds like a technical detail, but it is one of the main bottlenecks on the path to truly large quantum computers. The Finnish national quantum strategy envisages the creation of a machine with a thousand logical qubits by 2035. In terms of physical qubits (including error correction), these are hundreds of thousands of elements — and, according to Möttönen, with current technologies, so many qubits will require millions of microwave cables, each of which costs about a thousand euros (about 100 thousand rubles at the current exchange rate). In addition to being expensive, cables also introduce excess noise into the system, which destroys already fragile quantum states.

An autonomous quantum engine built right into the circuit potentially solves both problems at once: no need to run a wire, no need to pay for it and no need to deal with the noise it brings.

What does this really mean

Here it is worth separating the spectacular headline from the real state of affairs. Scientists did not create an autonomous device — they proved that the principle works, and on this foundation they can build further. From the laboratory Otto cycle on one qubit to an independent built-in readout mechanism in an industrial processor is a distance of several years of research, and in the article itself the authors say this directly: the next step is to bring the design to complete autonomy.

Nevertheless, the formulation of the question itself is indicative. The race of quantum technologies has long ceased to be a dispute about who will collect more qubits on paper — now much more mundane things are being decided: how to cool the chip, how to read data from it and, as it turns out, even how to build into it a tiny “motor” that will be powered by literally crumbs of heat. For Russia, where quantum development is carried out by Moscow State University, the Russian Quantum Center and Rosatom structures, this topic is also not abstract: the same cable and thermal limitations face any laboratory that is trying to scale superconducting qubits — and the solution found in Finland is, in theory, applicable to the architectures that are being developed here.

The work was supported by the Research Council of Finland and the Finnish Cultural Foundation, and the experiment itself was carried out on the basis of the national infrastructure OtaNano.

Sources:

• aalto.fi
• nature.com

sciencedaily.com

Xrust Physicists have built the first quantum heat engine — and it can save quantum computers from cable chaos