Frank Wilczek Helped Explain Why Empty Space Made a Superconductor Better. The Boost Was 5.4%.

A research team has strengthened superconductivity using nothing but the quantum fluctuations of empty space, in a result published in Nature on Aug. 19 that the authors describe as the first experimental demonstration of vacuum-fluctuation-enhanced superconductivity.

The physics starts with a fact that has been well established for 80 years and almost never exploited: a vacuum is not empty. Quantum electrodynamics, by way of the Heisenberg uncertainty principle, requires that even the ground state of a field is restless, with virtual particles continually created and annihilated. The Lamb shift, spontaneous emission and the Casimir effect are all consequences. What nobody had shown is that this background could be shaped to change a macroscopic quantum state of matter.

The work was led by Zeng Changgan and Cheng Guanghui of the University of Science and Technology of China, with Jiang Qingdong of Shanghai Jiao Tong University and Frank Wilczek of MIT. Zeng and Cheng's group had previously demonstrated direct control over vacuum fluctuations, reversing the Casimir force from attraction to repulsion under a magnetic field. That result raised the obvious follow-up question: if you can push the vacuum around, can you make it do something useful?

The experiment embedded niobium diselenide, a well-characterized layered superconductor, inside a terahertz split-ring cavity — a "dark" cavity, meaning nothing is shone into it. The cavity simply reshapes which vacuum modes exist in the space the material occupies. The superconducting critical temperature rose by up to 5.4%, with corresponding increases in critical current and critical magnetic field near the transition.

Proving that the vacuum did it required ruling out everything more mundane. The team ran systematic control experiments across cavity geometry and characteristic frequency, material thickness, dielectric materials and metallic strips, excluding strain, material degradation, inhomogeneity and metallic screening. The decisive evidence was the shape of the effect: the enhancement peaked resonantly as a function of the cavity's characteristic frequency. "This result, closely tied to the cavity's photonic properties, provides strong experimental evidence of the coupling between the superconducting state and dark-cavity modes," Zeng said.

Jiang's group and Wilczek supplied the theory. Within a Ginzburg-Landau framework, they proposed that the superconducting state exchanges virtual photons with the cavity, which lowers the energy of the superconducting state and thereby stabilizes it. "When the characteristic energy of the cavity mode matches the low-energy superconducting fluctuations, the NbSe2 device exhibits resonant enhancement, producing the peak in superconductivity enhancement," Jiang said.

Wilczek, who shared the 2004 Nobel Prize in Physics for asymptotic freedom, framed the result as a change in what the vacuum is for. "In most practical physics, the vacuum serves merely as the passive stage on which phenomena play out," he said. "This work shows that the background itself can become an actor — engineered to strengthen superconductivity and reshape the behavior of quantum matter."

Jiang's group has given the idea a name — "vacuumronics," engineered vacuum environments used to regulate electronic and photonic behavior. The practical appeal is that the control is entirely noncontact and requires no external driving, which avoids the heating that plagues laser-driven attempts to enhance superconductivity. A 5.4% bump is small. But it is a knob nobody had turned before. "With further optimization of cavity structures and material systems, vacuum-fluctuation coupling may enable more pronounced and widely applicable control of quantum states," Zeng said.