In a study published in Nature on August 19, a team led by Profs. ZENG Changgan and CHENG Guanghui from the University of Science and Technology of China of the Chinese Academy of Sciences, along with Prof. JIANG Qingdong from Shanghai Jiao Tong University, Prof. Frank Wilczek from Massachusetts Institute of Technology, and other collaborators, for the first time, have achieved the enhancement of superconductivity by vacuum fluctuations, making a significant advance in the control of quantum states of matter.

Space free of matter is not truly empty. Vacuum is not void. In quantum electrodynamics, the Heisenberg uncertainty principle implies that even in the ground state there is irreducible activity, with the continual creation and annihilation of virtual particles. Thus, vacuum contains a dynamic "sea" of quantum fluctuations. Several celebrated phenomena, including the Lamb shift, spontaneous emission, and the Casimir effect, provide compelling experimental evidence for their existence.

In recent years, the team of Profs. ZENG and CHENG has focused on vacuum-fluctuation effects in condensed-matter systems. In an earlier study, the team achieved direct control over vacuum fluctuations: a reversible transition of the Casimir force from attraction to repulsion under a magnetic field. This study inspires a further question: Can vacuum fluctuations be harnessed to manipulate macroscopic quantum states?

Besides these experimental efforts, Prof. JIANG's team has conducted theoretical studies of quantum-vacuum control of states of matter. The team proposed the concept of "vacuumronics," in which engineered vacuum environments regulate electronic and photonic behaviors, which lays the theoretical foundation for interpreting the mechanism of vacuum-enhanced superconductivity reported in this study.

"Vacuum fluctuations in free space are generally too weak to produce observable effects in macroscopic condensed-matter systems," said Prof. ZENG. "To overcome this limitation, we introduced a terahertz split-ring resonator. Such a dark cavity can reshape the electromagnetic environment and substantially amplifies vacuum fluctuations."

In this study, the team of Profs. ZENG and CHENG embedded the superconductor NbSe₂ in the terahertz dark cavity, constructing a superconductor-dark-cavity coupled device. By systematically comparing superconductivity outside and inside the cavity, the team found a substantial increase in the superconducting critical temperature of NbSe₂.

"We observed that the critical temperature can increase by up to 5.4% in a six-layer NbSe₂ device, while the critical current and critical magnetic field are significantly enhanced near the superconducting transition," said Prof. CHENG. "This represents the first experimental observation of vacuum-fluctuation-enhanced superconductivity."

To clarify the origin of the superconductivity enhancement, the team carried out systematic control experiments on multiple parameters, including cavity geometry and characteristic frequency, material thicknesses, dielectric materials, and metallic strips. These experiments effectively ruled out trivial factors such as strain, material degradation, inhomogeneity, and metallic screening effects.

Notably, it was found that the superconductivity enhancement displays a resonant peak-like dependence on the characteristic frequency of the dark cavity. "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," said Prof. ZENG.

Moreover, Prof. JIANG's team and Prof. Wilczek jointly developed the theoretical model and interpreted the underlying mechanism. Within a Ginzburg–Landau framework, they proposed that the superconducting state exchanges virtual photons with the dark cavity, lowering the energy of the superconducting state and thereby strengthening superconductivity.

"When the characteristic energy of the cavity mode matches the low-energy superconducting fluctuations, the NbSe₂ device exhibited resonant enhancement, producing the peak in superconductivity enhancement," said Prof. JIANG. "In most practical physics, the vacuum serves merely as the passive stage on which phenomena play out. This work shows that the background itself can become an actor—engineered to strengthen superconductivity and reshape the behavior of quantum matter," said Prof. Wilczek.

This study employs cavity-engineered vacuum fluctuations to noninvasively enhance superconductivity without external driving. This approach offers a non-contact control knob for quantum states of matter. "With further optimization of cavity structures and material systems, vacuum-fluctuation coupling may enable more pronounced and widely applicable control of quantum states," said Prof. ZENG.