Lasers are a well-known technology that have found myriad applications in all aspects of our lives, from sensors used in homes and stores, to advanced physics probes like LIGO that detected the first gravitational waves, and of course information technologies involving memory storage, retrieval, and data transmissions, to name but a few examples. Laser is an acronym for light amplification by stimulated emission of radiation, a technique that utilizes the wave-like nature of light, in which photon wave-packets that are of the same wavelength and phase (matching wave crest-to-crest and trough-to-trough, called constructive interference) can be combined and amplify the magnitude or strength of the light. The electromagnetic radiation is in a coherent state, and this is possible as well because photons obey what are known as Bose statistics, a quantum mechanical property of matter that allow Bose particles—like the photon— to occupy the same quantum state, something that is not possible with Fermi particles because of Pauli Exclusion.
The Duality of Matter: Bose and Fermi Particles
What categorizes a particle as Bose or Fermi depends on its spin, so the property is not exclusive to light but extends as well to what we more commonly consider as elementary particles of matter. So, the wave-like nature of matter can be utilized in much the same way as the wave-like nature of light, and the matter waves of Bose particles can be combined to form a single wave-state, a state of matter known as a Bose-Einstein condensate (BEC). This means that we can perform mater-wave amplification by stimulated emission of a BEC, just like a laser but with elementary particles of matter instead of light and can use the resulting coherent matter waves as probes and sensors.
Even though the concept of a BEC was proposed in 1924–1925 [1,2], it was not realized until 1995 [3], after two types of cooling (laser and evaporative) had been invented. Since then, the pulsed matter waves associated with BECs have been widely used in atom interferometry. Atom interferometers use laser beams to split up matter waves and then recombine them to produce interference patterns. These patterns are sensitive to vibrations, changes in temperature and other perturbations that can give us revealing insights into the nature of matter interactions and fundamental forces. We have previously reported on atom interferometers used for quantum tests of the Einstein equivalence principle and gravity [4].
Production and application of a Bose–Einstein condensate. (A) In quantum physics, matter can behave like a wave that has a particular wavelength. For a cloud of hot atoms, these wavelengths are so short that each atom can be regarded as an individual object. If the atoms are cooled, the wavelengths become longer. And if the atoms are cooled to a critical temperature, the wavelengths are large enough to cover the extent of the atomic cloud. Most of the atoms condense into a state known as a Bose–Einstein condensate (BEC), in which they can be regarded as a single matter wave (red)… (B) BECs can be used in sensors known as atom interferometers, in which laser beams cause a matter wave to split into two and then recombine to generate an interference pattern that is sensitive to external perturbations. Image and image description from [5] L. Liu, “Exploring the Universe with matter waves,” Nature, vol. 562, no. 7727, pp. 351–352, Oct. 2018, doi: 10.1038/d41586-018-07009-5
An exciting and remarkable application of this technology would be the development of a continuous, versus a pulsed matter wave probe. This had been recognized, in principle, as a possibility for decades, however constructing a quantum probe using controlled emissions of BEC matter waves was technologically challenging, because of the difficulties associated with maintaining a constant BEC reservoir from which continuous emission could be generated. Previously, the best that could be achieved was a pulsed stimulated coherent emission—the laser followed the same developmental trajectory; however, it was only a few months after the generation of the first pulsed laser emission that a continuous version was invented.
From Pulsed to Continuous Matter-Wave Lasers
Now, for the first time a team has reported in Nature the generation of a continuous coherent-matter-wave device, a matter wave analog of a continuous wave laser [6]. The team overcame the technological hurdles of transitioning from a pulsed BEC coherent-matter wave to a continuous wave (CW) by addressing the long-standing constraint for quantum gas devices that required cooling stages (to regenerate the BEC) to be performed in a sequence of steps, rather than providing continuous cooling for constitutive BEC regeneration. The team achieved continuous Bose-Einstein condensation by creating a CW condensate of strontium atoms sustained by amplification through Bose-stimulated gain of atoms from a thermal bath (see figure below).
The team discovered that the key to realizing a CW BEC of atoms was to continuously amplify the atomic matter wave while preserving its phase coherence, so that the BEC can be continuously sustained even while undergoing inexorable losses, like from strontium atoms reverting to the “individualistic” molecule state (versus the collective phase of the BEC), and to replace the atoms that are coupled out of the BEC for sustaining the atom laser. The team solved the gain amplification by providing the BEC with a continuous supply of ultracold, dense gas strontium atoms that provided gain via elastic collisions between thermal atoms (in the dense ultracold gas) without sacrificing phase coherence, successfully generating the proof-of-principle methodology for a CW BEC atom laser. As the team describes it:
Continuous operation is advantageous for sensors as it eliminates dead time and can offer higher bandwidths than pulsed operation. Meanwhile, sensors using BECs benefit from their high phase-space density and unique coherence properties. Combining these advantages, a CW atom laser beam outcoupled from a CW condensate could be ideal for many quantum sensing applications. In the long term, CW atom lasers could benefit applications ranging from dark-matter and dark-energy searches, gravitational-wave detection, tests of Einstein’s equivalence principle to explorations in geodesy. In the short term, the CW BEC offers a platform to study quantum atom optics and new quantum phenomena arising in driven-dissipative quantum gases. [6]
RSF in Perspective
It is important to keep in consideration that when we discuss matter at the elementary scale we are talking about waveforms, and while having quantized particle-like properties (wave-packets, or de Broglie waves), are nevertheless essentially resonances. Resonance, and the constructive / destructive combining of waveforms underlies all of physics and the behavior of matter and energy in the universe. Furthermore, as our new research will demonstrate mathematically, the phase of these quantum wave resonances, whether in the Bose, mixed Bose-Fermi, or purely Fermi state determines the fundamental properties and forces of elementary particles, like the mass of the proton, electron, and the associated nuclear binding force and electromagnetic interaction. Having a continuous coherent-matter-wave atomic laser will in principle enable certain tests to be made of quantum gravity and probe the fundamental nature of matter resonances and phases.
References
[1] Bose, S. N. Z. Phys. 26, 178–181 (1924).
[2] Einstein, A. Phys. Math. Klasse 1, 3–14 (1925).
[3] Anderson, M. H., Ensher, J. R., Matthews, M. R., Wieman, C. E. & Cornell, E. A. Science 269, 198–201 (1995).
[4] Urdaneta, I., Brown, W. Measuring the Curvature of Spacetime using Time Dilation at Atomic Scale. The Resonance Science Foundation. Available online at https://www.resonancescience.org/blog/measuring-the-curvature-of-space-time-using-time-dilation-at-atomic-scale
[5] L. Liu, “Exploring the Universe with matter waves,” Nature, vol. 562, no. 7727, pp. 351–352, Oct. 2018, doi: 10.1038/d41586-018-07009-5
[6] C.-C. Chen, R. González Escudero, J. Minář, B. Pasquiou, S. Bennetts, and F. Schreck, “Continuous Bose–Einstein condensation,” Nature, vol. 606, no. 7915, pp. 683–687, Jun. 2022, doi: 10.1038/s41586-022-04731-z
