A piece of metal heated at one end produces a weak electric current: this has been known for nearly two hundred years. But what if, instead of a simple metal, you take a layered "sandwich": a superconductor, a thin insulator, and a flat layer of electrons? Through such a junction, electrons seep via quantum tunneling, like water through a porous wall, and in doing so they carry heat.
The authors calculated that if you tune the electric potential of the electron layer, the junction starts acting as a very strong generator: the voltage reaches 6.75 Δ₀/e, and the Seebeck coefficient hits 5 Δ₀/eK. This means that even a tiny temperature difference yields a noticeable voltage. In heat engine mode, the efficiency reaches 92% of the Carnot limit: that is, almost all the heat is converted into work rather than dissipated.
Such flexibility is needed for quantum technologies. Qubits, the quantum cells of computers, require temperatures lower than those in deep space. And detectors for dark matter or cosmic microwave background catch signals that drown in thermal noise. The new junction could become a compact and precise refrigerator for these instruments.
The quantum "sandwich" resembles an old thermoelectric element, but it works not on chaotic electron motion but on their ordered quantum leap. Heat is not just dissipated: it is almost entirely converted into electricity or directed cold.
The best modern thermoelectric materials have an efficiency of about 15–20% of the Carnot limit. The proposed system achieves 92%—four to five times closer to the ideal.
In Arthur C. Clarke's novel "Childhood's End," alien ships use energy sources that are incomprehensible to humans. Quantum heat engines operating with almost no losses could be a first step toward such technology.