Researchers have demonstrated for the first time that quantum fluctuations in a vacuum can strengthen superconductivity, opening a new path for controlling unusual states of matter.
The study, published in Nature, was led by Profs. Changgan Zeng and Guanghui Cheng of the University of Science and Technology of China of the Chinese Academy of Sciences. The collaboration also included Prof. Qingdong Jiang of Shanghai Jiao Tong University, Prof. Frank Wilczek of the Massachusetts Institute of Technology, and other researchers.
Empty Space Is Not Truly Empty
In everyday language, a vacuum may sound like completely empty space. Quantum physics paints a very different picture.
According to quantum electrodynamics and the Heisenberg uncertainty principle, even the lowest energy state is never perfectly still. Virtual particles continually appear and disappear, producing an ever present background of quantum fluctuations.
These fluctuations are not merely theoretical. Their effects have been observed through well established phenomena such as the Lamb shift, spontaneous emission, and the Casimir effect.
For several years, the teams led by Zeng and Cheng have investigated how these vacuum fluctuations affect condensed matter systems. In an earlier study, the researchers showed that they could directly manipulate vacuum fluctuations by using a magnetic field to reversibly switch the Casimir force between attraction and repulsion.
That result raised a larger question: Could quantum vacuum fluctuations also be used to control macroscopic quantum states?
Turning the Vacuum Into a Control Tool
Jiang’s group has approached the same problem from a theoretical perspective, studying how engineered quantum vacuum environments could influence different states of matter.
The researchers introduced the concept of “vacuumronics,” in which specially designed vacuum environments are used to control electronic and photonic behavior. That theoretical work helped provide a framework for understanding the superconductivity enhancement observed in the new experiments.
“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 amplify vacuum fluctuations.”
To test the idea experimentally, the researchers placed the superconductor NbSe2 inside a terahertz dark cavity. This created a coupled system in which the superconducting material interacted with the specially engineered electromagnetic environment inside the cavity.
The team then compared the material’s superconducting behavior inside and outside the cavity. The results showed that placing NbSe2 in the cavity significantly increased its superconducting critical temperature, the temperature below which the material enters its superconducting state.
Superconductivity Gets Stronger
“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.”
The researchers conducted a series of control experiments to determine whether the effect could be explained by more conventional changes to the material.
They varied several properties of the setup, including the cavity geometry, characteristic frequency, material thicknesses, dielectric materials, and metallic strips. The results allowed them to rule out explanations involving strain, material degradation, inhomogeneity, and metallic screening effects.
One of the strongest clues came from the relationship between the superconductivity enhancement and the characteristic frequency of the dark cavity. Instead of changing smoothly, the enhancement showed a resonance-like peak at a particular 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,” said Prof. Zeng.
Virtual Photons May Explain the Effect
To explain what was happening, Jiang’s team and Wilczek developed a theoretical model based on the Ginzburg-Landau framework.
Their interpretation suggests that the superconducting state exchanges virtual photons with the dark cavity. This interaction lowers the energy of the superconducting state, making superconductivity more stable and effectively strengthening it.
“When the characteristic energy of the cavity mode matched the low-energy superconducting fluctuations, the NbSe2 device exhibited resonant enhancement, producing the peak in superconductivity enhancement,” said Prof. Jiang.
The result changes the usual way physicists think about the vacuum. Rather than serving only as an empty backdrop, the quantum vacuum can potentially be engineered to influence the behavior of matter itself.
“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.
A New Way To Control Quantum Matter
The approach strengthens superconductivity without directly driving the material with an external energy source. Instead, the researchers reshape the surrounding vacuum environment, providing a noncontact method for influencing a quantum state of matter.
That could eventually make engineered vacuum fluctuations a useful new control mechanism for superconductors and other quantum materials.
“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.