In a finding that reveals the social lives of light particles, physicists have demonstrated that photons behave as a collective, aligning into a single quantum state, but only after their numbers surpass a distinct threshold. Researchers at the University of Bonn observed that when forced to choose between two available energy states, the first few dozen photons act randomly, but subsequent particles overwhelmingly join the more crowded option, confirming a long-predicted feature of quantum mechanics with unusual clarity.
This tendency for particles to act in unison is a defining characteristic of a class of particles known as bosons, to which photons belong. The study, published in the journal Physical Review Letters, provides a clear illustration of this quantum principle by limiting the particles’ options to a simple binary choice. Beyond its fundamental importance, the discovery that this collective behavior turns on at a specific population density could inform the development of a new class of ultra-powerful lasers, which would require multiple beams of light to synchronize their waves perfectly.
The Two Families of Particles
The universe organizes fundamental particles into two distinct families with opposite behaviors: fermions and bosons. Fermions are staunch individualists, governed by a rule that prevents any two of them from occupying the same quantum state in the same place. This principle forces electrons, which are fermions, to arrange themselves in sequential energy shells around an atomic nucleus, giving rise to all of chemistry.
Bosons, in contrast, are gregarious. They prefer to exist in the exact same state, and the more bosons that occupy a state, the more likely new ones are to join them. Photons, the fundamental particles of light, are bosons. This property allows them to be packed together in vast numbers with identical properties, which is the basic principle behind a laser beam, where countless photons march in lockstep with the same frequency and direction.
Creating a Condensate of Light
The research team, led by Professor Martin Weitz at the University of Bonn, has long specialized in coaxing photons into an exotic state of matter known as a Bose-Einstein condensate. First achieved with atoms, such a condensate occurs when bosons are cooled to temperatures near absolute zero and confined in a small space. Under these conditions, the particles lose their individual identities and merge into a single quantum object, which the Bonn team refers to as a “super-photon.”
Creating a photon condensate is uniquely challenging because, unlike atoms, photons are massless and are easily created or destroyed. If one tries to cool them down in a box, they will simply be absorbed by the walls. The Bonn experiment overcomes this by using a system of mirrors to trap and accumulate photons, allowing them to thermalize and reach the density required for condensation without being annihilated.
Observing the Critical Choice
The core of the new experiment was to investigate what happens when photons in this system are not given one state to occupy, but are forced to choose between two slightly different energy levels, or colors. Professor Weitz’s team set up their apparatus to present the condensing photons with this binary option, analogous to guests arriving at a party and having to choose between two tables.
A Shift in Preference
Initially, with only a few dozen photons in the system, the particles showed no preference, distributing themselves evenly between the two available states. This corresponds to a random choice. But as the researchers injected more photons, a clear trend emerged. Once the population surpassed this critical threshold, newly arriving photons began to strongly favor the “table” that already had more occupants.
This collectivist behavior grew stronger as the number of photons increased. Once a few hundred particles had accumulated in the system, the preference became nearly absolute. At that point, the less-populated state was almost completely ignored, with nearly all subsequent photons joining the dominant group. This demonstrated that the transition from individual to collective action is not gradual but occurs after a critical mass is achieved.
Implications for Future Technology
This fundamental insight into photon behavior could have significant long-term applications. One of the most promising is in the design of high-energy lasers. The power of a laser could theoretically be increased by combining the light from multiple laser sources into a single, more powerful beam.
However, this requires the light waves from every source to be perfectly aligned, or “in phase,” so that their wave crests and troughs match exactly. If they are out of phase, they can cancel each other out, weakening the beam instead of strengthening it. Professor Weitz suggests that the photons’ natural tendency to join a crowd could be harnessed to solve this problem, potentially allowing multiple laser beams to spontaneously synchronize into a single coherent state.
“Our findings suggest this could work,” Professor Weitz stated, while also noting that significant technical hurdles remain. “But there’s a long way to go until the technology is up and running.” The work first provides a clear and elegant confirmation of how individual particles make the leap to becoming a collective whole.