# Aalto University physicists run a single-qubit heat engine, reaching over 25% of Otto efficiency

- Published: 2026-11-14 07:33 UTC
- Section: Digital & Computing › Quantum technology
- Tags: quantum heat engine, single-qubit engine, transmon qubit, quantum Otto cycle, Aalto University, Tuomas Uusnäkki, superconducting circuit, quantum thermodynamics, Nature experiment
- URL: https://realtech.media/news/rt-2448e2808d2d05a229d132b4_aalto_university_physicists/

**In a lab experiment published in Nature, a superconducting transmon qubit performed work in a quantum Otto cycle, showing that a single quantum system can act as a heat engine.**

![Aalto University physicists run a single-qubit heat engine, reaching over 25% of Otto efficiency](https://realtech.media/media/2026/11/6656aae0018bd73d.jpg)
*SachaAB, Wikimedia Commons, CC BY 4.0*

Physicists led by Tuomas Uusnäkki at Aalto University in Finland have built a laboratory engine that does work without anything rotating, moving or burning inside it. According to spektrum.de, the experiment is described in the journal Nature [1]. Its core is a tiny superconducting circuit [1]. The result shows that even a single quantum system can be operated as a thermodynamic machine [1].

## Setup and the quantum Otto cycle

The experiment had two components: a working medium that served as the "piston" of the quantum engine, and a quantum circuit that acted as the heat bath [1]. The working medium was a transmon qubit, a superconducting circuit that behaves like an atom with discrete energy states [1]. The qubit was coupled to the circuit, which, depending on the applied voltage, could deliver heat to the qubit or remove it [1].

This realized a quantum Otto cycle, which, like the classical version, consists of four steps [1]:

- Heating: the circuit supplies heat to the qubit, so a higher energy level becomes occupied.
- Expansion: instead of enlarging the volume of a gas, the researchers reduce the qubit's energy spacings with a magnetic flux, and the system delivers work.
- Cooling: the circuit extracts heat from the qubit, and the occupation of higher energy states decreases.
- Compression: the energy spacings are increased again, which prepares the initial state of the engine.

## Measured result and limits

The team ran the engine through the cycle many times and used energy measurements to show that it absorbed heat and delivered work [1]. This work does not come from visible motion like that of a piston or rotor [1]. Instead, a magnetic field was used to change the energy spacings between the quantum states of the artificial atom [1]. Because the occupation of the states stays the same during this step, the qubit's mean energy changes, and that change corresponds to the work the system gives off [1]. Overall, the engine reached more than 25 percent of the theoretically possible Otto efficiency [1].

Such a quantum engine will not drive a machine, replace a turbine or power a quantum computer for now, because the setup is too complex and the work obtained is far too small [1]. The researchers did show, however, that a single quantum system can be run like a heat engine [1]. This opens a research field focused not only on the classical motion of matter but on deliberate control of its quantum states [1].

## Sources

1. [spektrum.de](https://www.spektrum.de/news/physiker-bauen-motor-mit-quanten-antrieb-im-labor/2354191)

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Source: RealTech, realtech.media — https://realtech.media/news/rt-2448e2808d2d05a229d132b4_aalto_university_physicists/