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Physicists Observe Phase Transition In Cloud Of Quantum Particles

It is well known that the quantum world is weird. Most news pieces about quantum mechanics consist of some new study showing how some quantum phenomenon behave utterly unlike ordinary macroscopic phenomena. So, it is slightly ironic that a new study has been making the rounds where the punchline is some quantum systems exhibit similar behavior to macroscopic systems of particles.

In a new study published in Nature, a team of physicists from Imperial College London reports observing a phase transition in the state of a Bose-Einstein condensate composed out of less than 10 supercooled photons. As individual photons were added to the condensate, the coherence between the individual photons would increase, until reaching a critical point of about 7 photons at which coherence would begin to decrease rapidly with respect to increasing photon population. Such phase transitions—abrupt, non-linear changes in the behavior and dynamics of a system, are common in macroscopic objects with large ensembles of particles.

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The discovery that such changes also occur in quantum systems with very few particles will allow physicists to better investigate the quantum properties of matter, light, and the fundamental interactions between the two. “With the best of two distinct worlds—the physics of phase transitions and the accessibility of small systems—this unusual light source has potential applications in measurement or sensing.” lead co-author Dr. Florian Mintert told phys.org.

Phase Transitions and Bose-Einstein Condensates

Most substances studied by scientists are composed out of an unfathomably large amount of particles. These numbers are so large that they generally make little difference to the macroscopic behavioral properties of the system—both a single drop and a bucket full of water boil at 100 degrees Celsius and freeze at 0 degrees Celsius. These changes in state are called phase transitions, the most common of which are the transition of matter from solids to liquids to gases, but more exotic states of matter and phase transitions exist.

In simpler terms, a phase transition occurs when a system hits a specific “critical value” where new behavior emerges rapidly. Lessons from the phenomena of phase transitions in chemistry have found applications in evolutionary game theory, stock markets, and even traffic jams. In macroscopic systems where large numbers of particles seem to act all at once, producing the abrupt and non-linear change. These phase transitions occur in systems with high numbers of particles where it appears all the particles act at once to generate the large-scale change in behavior.

Most quantum systems studied by physicists, in contrast, consist only of a handful of particles. So the question naturally arises, do the same rules of phase transitions hold for the quantum sphere? To figure out, the team used a system of micro dyes and mirrors to create a supercooled Bose-Einstein condensate (BEC) out of fewer than 10 photons. A BEC forms when supercooled particles begin to occupy the lowest quantum state.

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At those levels, quantum effects like wave-particle duality become very apparent between groups of quantum and the ensemble of particles begins to behave like a singular “superatom.” BECs are notoriously difficult to produce and contain but their unique properties make them ideal subjects for research into quantum ensembles.

Previous work on the matter involving BECs utilized BECs composed from ensembles of thousands of quantum particles. This current study is unique in that the team achieved BEC condensation with only 7 photons, the smallest number recorded. The previous smallest number recorded had been 70 photons, achieved by a German team of scientists in 2014.

Using their setup of mirrors, the team slowly added photons together in an ensemble. They observed that initially, as more photons were added coherence of the particles increased. Once the system reached about 7 particles through, this pattern broke and the coherence began to decrease as more particles were added. According to the researchers, this point represented a “condensation” of the BEC, a phase transition. Although the transition was not as abrupt as those seen in macroscopic systems, it still occurred at a predictable and reliable point, mimicking the phase behavior of macroscopic systems.

The team theorizes that when the number of photons reaches the critical point of around 7±2, some mechanism triggers a sudden decrease in coherence across the photons in the microcavity. According to the study, “in a multimode condensate, photons in one condensed mode could act as reservoirs of excitations for other modes, enhancing number fluctuations and hence decreasing phase correlations.” The general idea seems to be that at a certain point, feedback amongst particles upsets the current stable configuration, leading to decreased coherence. The authors predict that when the number of photons is scaled up appropriately, the result would be non-classical states of light that cannot be described with classical electromagnetic theory.

So what does this mean for us? Well, knowing how particles behave at such small scales gives us more room to manipulate them to do what we want. The new field of quantum computing relies on extremely precise quantum interactions to compute things and having more accurate models of quantum interactions will further that goal. Additionally, the extension of the concept of phase transition to the quantum realm opens up avenues for research. According to Robert Nyman, lead author of the study, “Now that it’s confirmed that ‘phase transition’ is still a useful concept in such small systems, we can explore properties in ways that would not be possible in larger systems.”

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