A New Era of Precision: The Continuous-Wave Vacuum Ultraviolet Laser
For decades, the dream of a nuclear clock—a timekeeping device based on the incredibly stable oscillations of atomic nuclei—has tantalized physicists. Its potential accuracy dwarfs even the most sophisticated atomic clocks, promising breakthroughs in fundamental physics and advanced technologies. But a crucial piece of the puzzle has remained frustratingly elusive: a laser capable of precisely interacting with the nucleus of thorium-229, the element at the heart of this ambitious endeavor. Now, a team of researchers from Tsinghua University and the National Institute of Metrology in China has achieved a landmark breakthrough, creating the first continuous-wave vacuum ultraviolet (VUV) laser capable of precisely targeting the thorium-229 nucleus, a feat previously thought impossible.
Unlocking the Nucleus: Thorium-229 and the Quest for Precision
The nucleus of thorium-229 possesses a unique property: an isomeric transition, a low-energy shift between nuclear energy states. This transition, occurring at a wavelength around 148.4 nanometers, falls within the realm of optical accessibility, a fact that has ignited the quest for a nuclear clock. The exceptionally stable nature of this transition promises a clock far more accurate than any current timekeeping technology, offering potential precision reaching the 19th decimal place. This level of accuracy would revolutionize various scientific fields, allowing us to test fundamental theories of physics with unprecedented rigor, and unlock possibilities in navigation, communication, and more. But to achieve this, a laser with exceptional precision and power was needed—a laser that could precisely and persistently interact with the nucleus without causing disturbances.
A Technological Leap: Overcoming the VUV Barrier
The challenge lay in the vacuum ultraviolet (VUV) region of the electromagnetic spectrum. VUV light is notoriously difficult to generate with the necessary precision and power. Existing techniques often yielded lasers with linewidths (a measure of the laser’s spectral purity) in the gigahertz range—far too broad for the delicate task of coherent nuclear manipulation. This limitation, akin to trying to tune a radio to a specific station with a hopelessly fuzzy dial, prevented precise control of the thorium-229 nucleus. The breakthrough achieved by researchers at Tsinghua University, led by Shiqian Ding, is a five-orders-of-magnitude improvement in linewidth, achieving a linewidth well below 100 Hz. This is equivalent to transitioning from a cacophony of static to a crystal-clear signal.
The Power of Four-Wave Mixing: A Novel Approach
The researchers employed a technique known as four-wave mixing (FWM) in cadmium vapor. In this method, three laser beams with different wavelengths interact within the cadmium vapor, generating a fourth beam at the desired VUV wavelength. By carefully controlling the parameters of the interaction, the researchers were able to generate a continuous-wave laser beam at 148.4 nm—the precise wavelength needed to excite the thorium-229 nuclear transition. This method cleverly circumvents many of the limitations of conventional approaches, effectively achieving what previously seemed impossible.
Sub-Hertz Precision: A New Standard of Coherence
The researchers didn’t merely create a VUV laser; they created one with unprecedented coherence, meaning the laser’s light waves maintain a stable phase relationship over extended periods. Through a novel spatially resolved homodyne technique, they demonstrated that the FWM process itself introduced minimal phase noise, effectively maintaining sub-hertz linewidth capability. This level of coherence is akin to having a perfectly synchronized orchestra, where every instrument plays in perfect harmony with incredible accuracy. This precision is far beyond what has previously been possible at these wavelengths. The phase noise, the random fluctuations that limit a laser’s accuracy, was rigorously measured and shown to be exceptionally low. The laser produced by the team produces 100 nanowatts of power—a seemingly small amount, but sufficient for the task and a tremendous accomplishment in VUV laser technology. This amount of power, combined with the incredibly narrow linewidth, represents a massive improvement over previous VUV laser sources.
Implications Beyond Timekeeping: A Versatile VUV Platform
The implications of this breakthrough extend far beyond the realm of nuclear clocks. This new, highly versatile VUV laser platform promises advancements across a spectrum of fields. Its high spectral purity and broad tunability will revolutionize high-resolution VUV spectroscopy, enabling detailed studies of molecular structures and dynamics. In condensed matter physics, it promises enhanced energy resolution in angle-resolved photoemission spectroscopy (ARPES), providing insight into complex materials like high-temperature superconductors. This new laser may also have applications in advancing quantum computing. Its precision could also propel further progress in quantum information science by enabling more efficient and precise control of quantum states, particularly within the emerging Rydberg-ion platform for universal quantum computing.
The Future of Time and Beyond: A New Chapter
The creation of this continuous-wave VUV laser marks a watershed moment in precision metrology and beyond. This new technology paves the way for the realization of a 229Th-based nuclear clock, a device that promises to redefine our understanding of time itself. It also has the potential to reshape our approach to scientific exploration, enhancing the accuracy and precision of experiments across numerous fields. The possibilities opened by this innovation are as vast and intricate as the intricacies of the universe itself.
