The laser, a ubiquitous tool in modern technology, has always been bound by the Schawlow-Townes limit, a theoretical constraint on the purity of its light. This 1958 principle, derived by physicists Arthur Schawlow and Charles Townes, dictated the maximum coherence time of a laser, essentially defining how long its emitted photons could stay in sync. However, recent theoretical research from two independent teams challenges this long-held assumption, suggesting a path towards lasers with significantly enhanced coherence.
The implications are substantial, potentially revolutionizing applications from quantum computing to precision timekeeping. For decades, scientists and engineers have designed lasers within the confines of the Schawlow-Townes limit. This groundbreaking work suggests that this limit is not fundamental but rather a consequence of specific assumptions about laser operation.
The Schawlow-Townes limit arises from modeling a laser as a simple cavity where photons multiply and escape at a rate proportional to the internal light intensity, analogous to water draining from a barrel. However, the new research introduces a crucial element: control over the photon flow. By incorporating a “valve” to regulate photon escape, the coherence time can theoretically surpass the Schawlow-Townes limit.
Howard Wiseman of Griffith University in Australia, whose team published their findings in Nature Physics, asserts that their revised limit is the true quantum limit, dictated by the fundamental laws of quantum mechanics. David Pekker of the University of Pittsburgh, leading the other research group, concurs, stating that significantly more coherent lasers are now theoretically possible.
The key difference lies in the control of photon emission. While Schawlow and Townes’ model assumed a passive outflow, the new designs actively manage this process. This level of control was inconceivable in 1958 but is now feasible thanks to advancements in quantum computing technology.
Pekker and his collaborator, Michael Hatridge, are working to realize this novel laser design in the form of a maser, a microwave-emitting laser. Their goal is to integrate this maser into a superconducting quantum computer for qubit control. Although a complex undertaking, they believe they possess the necessary tools and expertise. Wiseman is also seeking collaborators to build his proposed maser design.
While the practical implications for commercial lasers remain to be seen, the theoretical implications are profound. Steven Touzard of the National University of Singapore notes that the Schawlow-Townes limit isn’t a primary design constraint for most laser applications. Nevertheless, this breakthrough represents a significant shift in our understanding of laser physics.
Intriguingly, these new laser designs deviate from another traditional concept: stimulated emission, the “s” and “e” in laser (light amplification by stimulated emission of radiation). They amplify light through different mechanisms, similar to the 2012 superradiant laser. This evolution highlights the changing definition of a laser, much like how “sailing” and “dialing” have retained their meaning despite technological shifts.
From its Cold War origins to its pervasive presence in modern life, the laser has continuously evolved. From vision correction to space communication, its applications are vast. Now, on the verge of another reinvention, the laser, a 60-year-old technology, continues to embody the promise of a futuristic world.
