Out of two possible states, photons prefer the one already shared by many other photons
As far as particles of light are concerned, the collective is more important than the individual. When they get to decide between two states, they will favor the one that many of their fellow particles have already adopted. However, this collectivist tendency does not kick in until enough photons have assembled in the same place. These findings, revealed by University of Bonn physicists in a recent study, could aid the development of ultra-powerful laser sources, among other things. They have now been published in the journal “Physical Review Letters.”
Physics knows two fundamentally different kinds of particles, the fermion and the boson. Fermions are committed individualists: If you confine them in a tight space, they cannot assume the same state. Electrons surrounding a nucleus are one example of this. If two of them want to be in the same “cloud” (or “orbital,” to use the technical term), they have to have a different “spin” (essentially, be rotating in a different direction).
Bosons, by contrast, like to do things together and prefer to all share the same state. Photons are part of this group. If you cool enough of them down and shut them up together in a tiny space, they merge into a kind of gigantic “super-photon.” But what if you forced the light particles to take on one of two slightly different colors first? Would that make two differently colored super-photons? Or would they all pick the same color to satisfy their urge to conform?
Favoring company over solitude
This was the question investigated by the working group led by Professor Martin Weitz from the Institute of Applied Physics at the University of Bonn. “We started by using a certain method to create cooled photons,” explains Weitz, who is also a member of the University’s Matter Transdisciplinary Research Area (TRA) and its ML4Q—Matter and Light for Quantum Computing Cluster of Excellence. “We then shut these particles of light in a space in which they had to adopt one of two marginally different energy levels—slightly different colors, in other words.” Think of it as a restaurant with two long tables where diners can sit to eat.
The researchers then looked at which “table” the photons chose and found that the first few of them were distributed fairly randomly between the two. “Although the lower energy level was marginally more popular, the difference was so fine as to be virtually immaterial,” Weitz says. “However, that only remained the case while the number of photons was low.” As soon as the gathering numbered into the dozens, the new arrivals began to sort themselves, always being more likely to pick the table with more occupants. This trend continued to the extent that the emptier table was hardly ever selected again once it had gathered a few hundred photons.
Method could help design more powerful lasers
This collectivist behavior has already been demonstrated for gases containing various types of bosons. In gases, however, the particles always have a very wide variety of possibilities to choose from, rather than just two as in this case.
This principle could potentially be harnessed to design extremely powerful laser sources, because the energy in laser light can theoretically be increased by combining multiple sources of radiation. “However, this requires them all to be ‘in phase,’ meaning that their waves must always be exactly in synch,” Weitz points out. “If not, the peaks of the wave of the first laser beam may encounter the troughs of the second beam, and they would cancel each other out.”
Although aligning the light waves from two lasers so precisely is no easy task, it might be possible to exploit the photons’ penchant for collectivism to bring the beams together. “Our findings suggest this could work,” the researcher explains. “But there’s a long way to go until the technology is up and running.”
Funding:
The study was supported by the German Research Foundation (DFG), the European Research Council (ERC) and the German Aerospace Center (DLR) using funds from the Federal Ministry for Economic Affairs and Energy (BMWE).