Reinventing the Laser

  • By Jennifer Zheng
  • 28 September 2021

Congratulations to David Pekker and his team for publishing their paper, “Proposal for a continuous wave laser with linewidth well below the standard quantum limit,” in Nature Communications!

The standard quantum limit on coherence of laser light, or a laser's ability to maintain a pure color, was first obtained by Schawlow and Townes in 1958. Except for a small modification in 1999, which decreased this limit by a factor of two, the Schawlow-Townes limit has stood as the ultimate theoretical bound on laser linewidth for 62 years. In their Nature Communications paper they provide a theoretical blueprint for building a microwave laser with coherence that is better than the standard quantum limit by a factor equal to the number of photons in the laser cavity. This is a huge improvement and a significant new development in laser physics. Indeed, their design challenges the very notion of what is a laser, which is an acronym that stands for Light Amplification by Stimulated Emission of Radiation. Unlike conventional lasers, their design is able to generate ultra-coherent light precisely because it avoids the use of Stimulated Emission. They envision that the ultra-high coherence afforded by their design could be used to build more sensitive gravity wave detectors like LIGO and improve the control over qubits and amplifiers in quantum computer circuits.

Their microwave laser design uses the standard components of superconducting quantum computers: transmon qubits, Josephson junctions, inductors, capacitors, and transmission lines. Their work is related to a recent and concurrent work by the H. Wiseman group, results published in Nature Physics, that argues that the ultimate limit on laser coherence is n2 times better than the standard quantum limit, where n is the number of photons in the laser cavity.  Both papers argue that it is possible to circumvent the standard quantum limit on laser coherence by engineering the photon gain and loss processes. This results in highly squeezed light in the laser cavity and minimizes the phase noise generated by photons entering and leaving the laser cavity. These results were recently highlighted on Gizmodo.

This could be applied to several areas including the ultra-fine measurement of gravitational waves, as well as precise timekeeping and quantum computation. However, the team has yet to name this new technology, for no longer falls under the traditional definition of a laser. Among the proposed names are "maser", "mamer", and "lamer." Yeah, they're still workshopping it. But perhaps they will find the right name soon, since this new design will not stay theoretical for long. PhD student Maria Mucci is currently working on building one.

The team consists of three groups at the University of Pittsburgh. Michael Hatridge and his students Maria Mucci and Xi Cao from the Hatlab, which works on building superconducting quantum computers, provided the expertise in quantum computer circuits. Gurudev Dutt from the Duttlab, which works on experimental quantum optics, provided the expertise in classical laser physics and characterization of quantum light. David Pekker and his student Chenxu Liu, from the Pekker group, which works on the theory of quantum optics and quantum dynamics, used tremendous amounts of coffee and tea to put together superconducting quantum circuits and laser physics to make this development.