Tsinghua University's latest breakthrough in vacuum ultraviolet (VUV) laser technology has sent ripples through the global scientific community, marking a pivotal step toward realizing the long-anticipated nuclear clock. Researchers led by Associate Professor Shiqian Ding have developed a continuous-wave (CW) narrow-linewidth VUV laser source at 148.4 nanometers (nm), published in the prestigious journal Nature on February 11, 2026. This innovation addresses the final major technical hurdle in building a nuclear clock based on the thorium-229 (229Th) isomeric transition, promising timekeeping precision that dwarfs current atomic standards.
The achievement underscores China's accelerating dominance in quantum precision measurement, with Tsinghua at the forefront. By extending ultra-stable laser capabilities into the challenging VUV spectrum, the team not only paves the way for revolutionary clocks but also opens doors to advanced quantum applications. This development highlights the vital role of university-led research in higher education, training the next generation of innovators—like the paper's first author, an undergraduate student.
What Makes Nuclear Clocks a Game-Changer?
Nuclear clocks represent the next frontier in timekeeping, shifting from electronic transitions in atoms to nuclear transitions within atomic nuclei. Unlike optical atomic clocks, which rely on electron orbits and are susceptible to electric fields and environmental perturbations, nuclear clocks leverage the tiny charge radius of the nucleus for exceptional insensitivity to external influences. The 229Th nucleus features a uniquely low-energy isomeric state at approximately 8.28 electronvolts (eV), corresponding to a VUV wavelength of about 149 nm—ideal for laser excitation.
Discovered in the 1970s, the exact transition frequency eluded precise measurement until recent years. Initial spectroscopy efforts using pulsed VUV lasers achieved linewidths in the gigahertz (GHz) range, far too broad for coherent control. Tsinghua's CW laser, with its sub-100 Hz linewidth, enables Rabi oscillations and quantum manipulation of the nuclear state, essential for clock operation. Theoretical stability could exceed 10-19, meaning a loss or gain of just one second over billions of years.
In practical terms, this translates to transformative applications: ultra-precise GPS unaffected by relativistic effects, deep-space navigation, fundamental physics tests like probing dark matter or time-varying constants, and redefining the international second. For higher education, it exemplifies how university labs drive national innovation strategies in quantum technologies.
The Critical Challenge: VUV Lasers in the Deep Ultraviolet
Vacuum ultraviolet light (100-200 nm) is notoriously difficult to generate coherently due to strong absorption by air, materials, and nonlinear media limitations. Traditional harmonic generation in crystals fails below 190 nm, while gas-based pulsed sources suffer megahertz-to-GHz linewidths and spectral backgrounds unsuitable for precision spectroscopy. For the 229Th clock, a CW source with narrow linewidth (<1 kHz), milliwatt power, and tunability around 148 nm was the "last core bottleneck."
- Previous efforts: Pulsed excimer lasers (10 GHz linewidth), frequency quadrupled fiber lasers (30 MHz), limited power and coherence.
- Tsinghua solution: Resonantly enhanced four-wave mixing (FWM) in cadmium vapor, preserving input laser coherence.
This approach bypasses crystal damage thresholds, achieving unprecedented performance in a wavelength regime vital for nuclear physics and quantum sensing.
Tsinghua's Innovative Four-Wave Mixing Technique Explained Step-by-Step
The team's ingenuity lies in FWM, a nonlinear optical process where three photons interact to produce a fourth. Here's how they implemented it:
- Pump Lasers: Two Ti:sapphire lasers at 710 nm and 750 nm, each Pound-Drever-Hall (PDH) locked to a 10-cm ultra-low expansion (ULE) cavity for sub-Hz stability (1.05 Hz and 1.01 Hz full-width half-maximum, FWHM).
- Nonlinear Medium: Heated cadmium vapor cell (optimized temperature for resonant enhancement), where ω_VUV = 2ω_710 - ω_750.
- Phase Matching: Precise alignment ensures efficient energy transfer, yielding >100 nW CW VUV power (290 nW inferred), tunable over 140-175 nm.
- Linewidth Verification: Heterodyne with optical frequency comb (0.25 Hz reference); novel spatially resolved homodyne bounds FWM phase noise, projecting <100 Hz linewidth—five orders better than priors.
- Power Scaling: Potential to µW levels by amplifying pumps, sufficient for tight focusing (2 µm spot) to drive nuclear transitions at kHz rates.
This platform's coherence rivals visible lasers, revolutionizing VUV applications.Read the full Nature paper
Spotlight on the Tsinghua Team: Empowering Young Talent
Led by Shiqian Ding from Tsinghua's State Key Laboratory of Low-Dimensional Quantum Physics, the team includes collaborators from Beijing Academy of Quantum Information Sciences and National Institute of Metrology. Notably, first author Qi Xiao, a 2021 undergraduate entrant, demonstrates Tsinghua's mentorship model—fostering PhD-level research in bachelor's programs. Co-authors like Gleb Penyazkov (international PhD) highlight global appeal.
This success reflects China's higher education emphasis on quantum training, with Tsinghua producing leaders in research jobs and innovation. For aspiring physicists, it underscores the value of hands-on lab experience; explore academic CV tips to join such teams.
Global Context: How Tsinghua Leads the Nuclear Clock Race
While groups at PTB (Germany), JILA (USA), and RIKEN (Japan) advanced 229Th spectroscopy, no one cracked CW VUV until now. Pulsed systems enabled initial excitations but lacked coherence for clocks. Tsinghua's five-order linewidth leap positions China ahead, aligning with national quantum initiatives.
Recent milestones: 2024 Th-doped crystals for ensemble averaging; this laser completes the toolkit. Internationally, it spurs collaboration opportunities for Chinese universities.
- USA: Frequency metrology focus, but VUV lags.
- Europe: Ion trapping advances.
- China: Integrated hardware-software edge via state funding.
Precision Timekeeping Revolution: Atomic vs. Nuclear Clocks
Current optical clocks (e.g., Sr/Yb lattices) reach 10-18 stability but falter in fields. Nuclear clocks promise:
| Aspect | Atomic Clock | Nuclear Clock |
|---|---|---|
| Transition | Electronic | Nuclear (229Th) |
| Stability Potential | 10-18 | >10-19 |
| Environmental Sensitivity | High (E-fields) | Low |
| Size/Portability | Lab-scale | Crystal-based, compact |
| Laser Need | NIR/IR | 148 nm VUV |
Such comparisons illustrate why Tsinghua's laser is transformative.Tsinghua announcement
Beyond Clocks: Quantum and Industrial Impacts
The VUV platform extends to:
- Quantum info: Nuclear qubits with long coherence.
- Condensed matter: Angle-resolved photoemission spectroscopy (ARPES).
- Metrology: Semiconductor VUV standards, self-reliant supply chains.
- Navigation: Autonomous systems, space probes.
In China, it bolsters high-tech manufacturing; globally, accelerates fundamental probes.
China's Quantum Higher Ed Ecosystem
Tsinghua exemplifies China's ascent: Over 40 quantum labs, massive funding (e.g., National Quantum Lab). Enrollment in physics surges 15% yearly, with international partnerships. This Nature paper boosts Tsinghua's QS ranking, attracting talent. For jobs, check China university positions or faculty roles.
Future Outlook: From Lab to Legacy
Next: Integrate laser with Th-doped SrF2 crystals for prototype clock by 2028. Challenges remain—nuclear lifetime measurement, clock interrogation—but momentum is strong. This could redefine time, physics, and tech. Aspiring researchers: Build skills via postdoc advice.
In summary, Tsinghua's VUV laser not only unlocks nuclear clocks but elevates Chinese higher education globally. Explore Rate My Professor, Higher Ed Jobs, Career Advice, University Jobs.
Photo by Artur Adilkhanian on Unsplash
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