A team of Chinese researchers has demonstrated a new crystal that can generate some of the shortest-wavelength laser light ever produced by a solid-state material, clearing a major technical hurdle on the path to nuclear clocks stable enough to navigate without GPS. The crystal, ammonium borate fluoride (ABF), was described in a study published in Nature in early 2025, and its potential military applications have drawn attention from defense analysts tracking China’s push for satellite-independent positioning technology.
At stake is a class of ultra-precise timekeeping devices that could allow submarines, missiles, and aircraft to hold accurate position fixes for hours or days using only onboard sensors, eliminating their reliance on GPS signals that adversaries can jam or spoof.
Why this crystal matters
The core problem ABF solves is deceptively simple: there has been no good way to produce coherent laser light below 200 nanometers, in the so-called vacuum-ultraviolet range. That spectral window is exactly where thorium-229, a rare isotope, has a unique nuclear transition that can serve as the “tick” of an extraordinarily stable clock.
Existing nonlinear crystals, the optical components used to convert laser light to shorter wavelengths, tend to absorb, scatter, or degrade when pushed into the vacuum-ultraviolet. The Chinese team engineered ABF with a crystal structure and wide bandgap that maintain transparency and efficient frequency conversion deep into that regime. According to the Nature paper, the crystal withstood the required laser intensities without the rapid damage that has sidelined earlier materials.
A separate Nature paper published around the same time reported that solid-state thorium-229 nuclear clocks, built by embedding thorium ions in crystal hosts, have now achieved frequency stability comparable to leading optical atomic clocks under laboratory conditions. Together, the two papers show that the light source and the clock architecture are advancing on parallel tracks, each solving a piece of a puzzle that neither could complete alone.
The GPS-free navigation connection
GPS works by comparing time signals from multiple satellites. Any drift in a receiver’s onboard clock translates directly into positional error, roughly 300 meters for every microsecond of accumulated drift. Today’s best military inertial navigation systems, which use ring laser gyroscopes and accelerometers, can hold position for limited periods without GPS, but their accuracy degrades over time as small measurement errors compound.
A nuclear clock stable enough to keep time with near-zero drift for extended periods would dramatically slow that error accumulation. Paired with high-quality inertial sensors, it could let a platform track its own position autonomously, with no external signals to intercept or disrupt.
This is not speculative framing. The U.S. National Institute of Standards and Technology has explicitly described thorium-229 nuclear clocks as enabling “navigation with or without GPS” in a September 2024 announcement about its own nuclear clock research. NIST scientists have also built a tunable vacuum-ultraviolet frequency comb, a precision laser tool with evenly spaced spectral lines, designed specifically for thorium-229 spectroscopy. That work, published in Optics Letters, targets the same source requirements that a crystal like ABF would need to satisfy in a working clock.
A global race, not just a Chinese one
China is not working in isolation. The European Union has funded the ThoriumNuclearClock consortium, a multi-institution effort to build a prototype thorium-229 clock. In the United States, NIST, JILA at the University of Colorado Boulder, and defense-linked laboratories have been pursuing the same goal for years. DARPA and the Army Research Laboratory have both expressed interest in compact, ruggedized atomic and nuclear clocks for military platforms.
What makes the ABF crystal noteworthy is that it addresses a specific bottleneck, the vacuum-ultraviolet light source, that has constrained all of these programs. If the material performs as described and can be reproduced reliably, it could benefit nuclear clock development worldwide, not just in China. But the fact that a Chinese team published the breakthrough, and that Chinese state-affiliated outlets have highlighted its strategic implications, has sharpened the competitive framing.
What remains uncertain
No Chinese government or military statement has confirmed plans to integrate ABF crystals into submarine or missile navigation systems. The connection between the Nature-published research and specific weapons platforms is inferred from the technology’s military relevance, not from any procurement announcement or defense white paper.
Key scientific parameters also remain unsettled. The exact wavelength of the thorium-229 nuclear transition has been measured at approximately 148.71 nanometers in calcium fluoride and magnesium fluoride hosts, based on observations reported in preprint literature and in a doctoral thesis from the JILA Physics Frontiers Center at the University of Colorado Boulder. However, because neither the preprint nor the thesis is linked directly in the sources reviewed for this article, readers seeking to verify those specific figures should search the arXiv repository for recent thorium-229 isomer decay measurements and the University of Colorado Boulder’s electronic thesis archive for JILA dissertations on thorium-229 nuclear spectroscopy. Environmental factors like temperature, mechanical strain, and electric fields shift the transition in ways that have not been fully characterized. Until those systematic effects are pinned down and compensated, any nuclear clock will face performance limits that prevent deployment-grade accuracy.
Scaling ABF from laboratory samples to ruggedized military components is its own challenge. The Nature paper demonstrates optical performance under controlled conditions, using crystals grown and handled in a research setting. Surviving the vibration, temperature swings, and pressure changes inside a submarine hull or missile airframe is a different problem entirely. No published source provides timelines, manufacturing yields, or cost estimates for that transition.
Building a complete navigation-grade system would also require compact pump lasers, thermal management, radiation shielding, and autonomous control electronics, each introducing potential failure modes. Competing approaches, particularly improved optical atomic clocks that are further along in miniaturization, could prove easier to harden for field use.
Where the evidence stands as of May 2025
The strongest claims in this story rest on peer-reviewed Nature papers with named research teams and reproducible experimental data. ABF works in the lab as a vacuum-ultraviolet frequency converter. Thorium-229 solid-state clocks produce measurable frequency stability under controlled conditions. NIST has publicly tied nuclear clock development to GPS-free navigation.
The military application layer is plausible but unconfirmed. No open-source evidence identifies a specific Chinese defense program, budget line, or integration contract tied to ABF or thorium-229 clocks. The leap from a crystal paper in Nature to operational hardware in a submarine requires engineering breakthroughs, procurement decisions, and political commitments that have not been documented publicly.
What is clear is that the physics underpinning GPS-free nuclear clock navigation is advancing faster than many analysts expected, and that Chinese researchers are now contributing critical pieces of the puzzle. Whether that translates into a deployed military capability in the next decade depends on engineering realities that no single crystal, however promising, can resolve on its own.
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*This article was researched with the help of AI, with human editors creating the final content.
