In 1948, Dutch physicist Hendrik Casimir predicted that two parallel metal plates placed in a vacuum would attract each other with no external force acting on them. According to quantum mechanics, even the electromagnetic field's lowest-energy state is never perfectly still—it retains minute fluctuations known as vacuum fluctuations. These fluctuations create a pressure difference between the plates that manifests as a macroscopic force. Ever since Steve Lamoreaux experimentally confirmed the Casimir effect through precision measurements in 1997, vacuum fluctuations have been treated in physics textbooks as "real, but uncontrollable background noise."
Superconductivity, too, has a long history. The BCS theory proposed by Bardeen, Cooper, and Schrieffer in 1957 explained the mechanism by which electrons form pairs (Cooper pairs) mediated by phonons—quanta of lattice vibration—dropping into a state of zero electrical resistance. For more than half a century, conventional wisdom held that there were only two ways to raise the transition temperature : strengthen the electron-phonon coupling, or find a new pairing mechanism (such as magnetic fluctuations in cuprate high-temperature superconductors).
The research presented here suggests the possible emergence of a third option.
Why Free-Space Vacuum Fluctuations Are "Too Weak"
That vacuum fluctuations affect matter has long been known at the atomic scale. The subtle shift in hydrogen's energy levels measured by Willis Lamb in 1947 (the Lamb shift) is direct evidence of electrons interacting with vacuum fluctuations. The Casimir effect is another example.
However, these effects have all been confined to microscopic scales. The energy density of free-space vacuum fluctuations is extremely low—far too weak to shift the transition temperature of a macroscopic condensed system, such as a superconducting state in which billions of electrons behave cooperatively. As Professor ZENG Changgan of the University of Science and Technology of China stated in a press release from the Chinese Academy of Sciences, "Vacuum fluctuations in free space are usually too weak to produce observable effects in macroscopic condensed matter systems."
The means of overcoming this barrier is to confine the electromagnetic field within a resonator, or cavity. When light or electromagnetic waves are confined to an extremely small volume, the amplitude of the vacuum fluctuations in that mode is greatly amplified. Theoretical estimates suggest that in a cavity with a mode volume of around 1 nm³, the fluctuating electric field can reach magnitudes on the order of 10⁹ V/m. In recent years, this principle has given rise to a growing field known as "cavity materials engineering," which seeks to apply it to controlling the ground states of matter.
The Mechanics of the "Dark Cavity": Manipulating the Vacuum Without External Drive
To apply this principle to superconductivity, the experimental team at the University of Science and Technology of China, led by professors ZENG Changgan and CHENG Guanghui, used a split-ring resonator operating in the terahertz band. This is a cavity formed from a ring-shaped metallic structure with a gap, designed to confine electromagnetic modes at a specific frequency. Because no external light or current is injected into it, it is called a "dark cavity." The dark cavity amplifies vacuum fluctuations and reconfigures the electromagnetic environment.
The experimental team placed a six-layer thin film (approximately 4 nm thick) of NbSe₂, a layered transition metal dichalcogenide, inside the dark cavity, constructing a superconductor–dark cavity coupled device. As a point of comparison, they also measured the identical sample outside the cavity.
| Condition | Transition temperature | Critical current / critical field |
|---|---|---|
| Outside the cavity (free space) | Baseline | Baseline |
| Inside the cavity (dark cavity coupled) | Up to +5.4% increase | Markedly enhanced near the transition temperature |
| Detuned from resonant frequency | Enhancement disappears | Confirms frequency dependence of the resonance peak shape |
Given that prior research (Li et al., Nature Communications, 2017) reported a of about 4.2 K for five-layer NbSe₂, the of the six-layer sample used here is estimated to be roughly around 4 K. A 5.4% increase corresponds to about 0.2 K in absolute terms. Taken purely as a number, this may seem modest. But the significance of this result lies in the fact that superconducting stability was altered using nothing but quantum fluctuations of the vacuum, without injecting any external energy whatsoever.
What the Resonance Peak Reveals About "Coupling"
To confirm that the enhancement was not caused by environmental changes—such as mechanical strain, sample degradation, or electromagnetic shielding by the metal structure—the team conducted a wide range of control experiments. They systematically varied the cavity geometry, characteristic frequency, NbSe₂ film thickness, dielectric material, and the dimensions of the metal strips. The results showed that none of these conventional factors could account for the observed enhancement.
The decisive evidence was the frequency dependence of the enhancement. The degree of superconducting enhancement was not uniform with respect to the dark cavity's characteristic frequency but instead showed a sharp peak at a specific frequency. This strongly suggests that the cavity is not simply altering the material's surrounding environment, but is resonantly coupling to the superconducting state itself. As Professor Zeng put it, "This result is closely tied to the cavity's photonic characteristics, providing strong experimental evidence for coupling between the superconducting state and the dark cavity mode."
Virtual Photon Exchange Lowers Superconducting Energy: The Theoretical Model by Wilczek and Jiang Qingdong
The theoretical interpretation was jointly developed by Professor JIANG Qingdong of Shanghai Jiao Tong University and Frank Wilczek—who shared the 2004 Nobel Prize in Physics for the discovery of asymptotic freedom and holds joint appointments at MIT, Stockholm University, and Shanghai Jiao Tong University, among others.
Using the framework of Ginzburg-Landau theory, they described the coupling between the superconducting order parameter and a quantized cavity mode. In this model, the free energy of the superconducting state is lowered through the exchange of virtual photons between the superconducting state and the dark cavity. Put simply, when the superconductor "converses" with vacuum fluctuations, remaining in the superconducting state becomes energetically more favorable.
This effect is maximized when the characteristic energy of the cavity mode (determined by its characteristic frequency) matches the energy of NbSe₂'s low-energy superconducting fluctuations. The resonance peak observed in the experiment is explained as a reflection of this matching condition.
Professor Jiang's group has also proposed a broader concept called "vacuumronics"—controlling the behavior of electrons and photons through engineered vacuum environments (Jiang et al., published in a review journal in the Nature Reviews Physics family in 2025)—and this experiment is positioned as the first concrete demonstration of that concept.
A Contrasting Case: "Suppression" Reported in NbN
Regarding the relationship between vacuum fluctuations and superconductivity, a result opposite to this enhancement has also been reported. A paper posted to arXiv in January 2026 (Kuznetsov et al., arXiv:2601.08191) enclosed a thin film of NbN within a high-Q terahertz photonic crystal cavity and examined its response using terahertz time-resolved spectroscopy. That experiment showed that the cavity's vacuum fluctuations reduced the superfluid density by about 13% and shrank the superconducting gap by about 2%—that is, the vacuum fluctuations worked to suppress superconductivity. (The original paper does not report absolute values for the superfluid density or gap, only the percentage changes.)
| This study (NbSe₂ + dark cavity) | Kuznetsov et al. (NbN + high-Q photonic crystal cavity) | |
|---|---|---|
| Material | Six-layer NbSe₂ (layered, coexisting with charge density wave) | NbN thin film (conventional BCS superconductor) |
| Cavity | Terahertz split-ring resonator (dark cavity) | One-dimensional photonic crystal cavity (high-Q) |
| Observed effect | up to +5.4%, enhanced critical current/field | Superfluid density decreased ~13%, gap decreased ~2% (absolute values not reported) |
| Interpretation | Stabilization of superconducting state via virtual photon exchange | Virtual photons break Cooper pairs, increasing quasiparticles |
The fact that the same phenomenon—vacuum fluctuations—produced enhancement in one case and suppression in the other indicates that the effect is highly sensitive to a material's electronic structure and the presence of competing orders.
Tension with a "No-Go Theorem": "Enhancement Is Impossible Under Minimal Coupling"
The theoretical debate runs deeper still. A paper posted to arXiv in August 2026 (arXiv:2608.14784) started from minimal coupling between Ginzburg-Landau theory and a quantized cavity mode and proved a theorem. In a passive cavity, vacuum fluctuations generate a positive diamagnetic contribution (arising from the coupling) and a negative paramagnetic exchange contribution (arising from virtual photon processes), but the magnitude of the latter can never exceed that of the former. The content of this "no-go theorem," therefore, is that within the minimal coupling framework, vacuum fluctuations can only act to suppress .
If this theorem holds, the enhancement observed in this experiment would need to be explained as an indirect effect arising from physics beyond minimal coupling—for example, contributions from collective excitation modes coupled to the cavity, or suppression by the cavity of an order competing with superconductivity (a candidate being the charge density wave present in NbSe₂). The authors of the no-go theorem cite these two pathways as possible routes to enhancement.
This theoretical tension does not diminish the significance of the present discovery. Rather, it means that identifying exactly which physical pathway underlies the observed enhancement has emerged as the next research challenge. NbSe₂ is a representative system in which superconductivity and charge density waves coexist and compete, so it is plausible that the "suppression of competing order" pathway is at work—though this remains unconfirmed at present.
"The Vacuum Can Be an Actor, Not Just a Stage"
Wilczek stated in the Chinese Academy of Sciences press release: "In most of practical physics, the vacuum is merely a passive stage on which phenomena unfold. This work shows that the background itself can become an actor. The vacuum can be engineered to strengthen superconductivity and reshape the behavior of quantum matter."
This metaphor succinctly captures the shifting status of the vacuum throughout the history of physics. When Casimir predicted his force in 1948, the vacuum was the stage for a paradox—a force arising in space that was supposed to contain nothing. When the Zeng-Cheng team reversibly switched the Casimir effect from attractive to repulsive using a magnetic field in 2024 (Nature Physics, 2024, DOI: 10.1038/s41567-024-02521-0), the vacuum became, for the first time, an object of control. And with this latest experiment, the vacuum has been elevated further still—into an active means of altering the state of matter.
Questions That Remain Before Practical Application
This discovery does not open a path to room-temperature superconductivity. The reported increase in was at most 5.4% (about 0.2 K), and the experiment was conducted using a specific material—NbSe₂—under specific dark cavity conditions. Whether the effect can be applied to other superconductors, particularly high-temperature superconductors, remains untested. Systematic research will be needed to determine how far the enhancement ratio can be pushed by optimizing the dark cavity design, and how the effect changes with different layer numbers or materials.
Beyond that, unresolved theoretical questions remain. How should the results be reconciled with the no-go theorem? Is the microscopic origin of the enhancement the suppression of a competing order, the amplification of a collective mode, or some other mechanism entirely? The answer to this question will determine just how far vacuum fluctuations can be exploited as a "designable resource."
Applications to superconducting qubits and quantum sensors also remain distant at this stage. The principle advantage—being able to manipulate quantum states non-invasively without external driving—is attractive for quantum technology, but translating this laboratory effect into a device will require overcoming many engineering challenges related to integration and reproducibility. Professor Zeng noted that "further optimization of cavity structures and material systems may enable vacuum-fluctuation coupling to achieve more pronounced and broadly applicable control of quantum states"—but whether this "possibility" becomes "reality" will depend on years of follow-up experiments and theoretical clarification to come.