7 Quantum Particles Act Like Billions in Weird Physics Experiment

Physicists have revealed that just seven quantum particles can behave as if they were in a crowd of billions.

At larger scales, matter goes through changes, called phase transitions, in which (for example) water turns into a solid (ice) or a vapor (steam). Scientists were used to seeing this behavior in large masses of molecules, but never in such a tiny cluster.

In a new study, detailed today (Sept. 10) in the journal Nature Physics, researchers witnessed these phase transitions in systems made up of just seven light particles, or photons, which took on an exotic physical state known as a Bose-Einstein condensate (BEC). That's the physical state that matter can reach at ultracold temperatures, in which particles begin to blend together and act in unison.

Because photons are packets of light, they're made of energy, not matter, which makes the idea of them going through a phase transition strange. But back in 2010, a team of German researchers showed that light particles could be induced to behave as a BEC would, just like their matter-particle cousins.

To trap the photons, those researchers built a small mirrored chamber and filled it with a colored dye. When the light particles banged into the dye particles, the dye particles would absorb them and re-emit them, so the photons took longer to move through the chamber — effectively slowing them down. When the photons struck the chamber's mirrored walls, the photons would bounce off without being absorbed or escaping. So the chamber was effectively a space where researchers could make photons sluggish and put them in close quarters. And in that situation, the physicists found, the photons would interact with one another like matter, and exhibit behaviors recognizable as those of a BEC.

In the more recent experiment, the researchers wanted to figure out the minimum number of photons necessary for that to happen. Using a fine-tuned laser, they pumped photons into a similar dye-filled mirror trap one at a time and observed the concoction to figure out when a BEC would emerge. They found that after an average of just seven photons, the photons formed a BEC — they began acting like one particle. That's a new low bar for particle counts necessary for a phase transition.

"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," lead author Robert Nyman, a physicist at Imperial College London, said in a statement.

There were some differences between the micro-BEC and phase transitions involving larger groups of particles, the researchers noted. When ice heats up past its melting point, it seems to go from solid to liquid form instantly, without any in-between stage. The same is true for most phase transitions of most chemicals. But the seven-photon BEC seemed to form a bit more gradually, the researchers said in the statement, rather than all at once.

Still, they wrote in the paper, the photon phase transition showed that even at very small scales, phase transitions are remarkably like what's common at larger scales. Physics is physics, all the way down.

Bose–Einstein condensate

A Bose–Einstein condensate (BEC) is a state of matter of a dilute gas of bosons cooled to temperatures very close to absolute zero. Under such conditions, a large fraction of bosons occupy the lowest quantum state, at which point microscopic quantum phenomena, particularly wavefunction interference, become apparent.

A BEC is formed by cooling a gas of extremely low density, about one-hundred-thousandth the density of normal air, to ultra-low temperatures.

This state was first predicted, generally, in 1924–1925 by Satyendra Nath Bose and Albert Einstein.

History

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Satyendra Nath Bose first sent a paper to Einstein on the quantum statistics of light quanta (now called photons), in which he derived Planck’s quantum radiation law without any reference to classical physics. Einstein was impressed, translated the paper himself from English to German and submitted it for Bose to the Zeitschrift für Physik, which published it in 1924.[1] (The Einstein manuscript, once believed to be lost, was found in a library at Leiden University in 2005.[2]).

Einstein then extended Bose’s ideas to matter in two other papers.[3][4] The result of their efforts is the concept of a Bose gas, governed by Bose–Einstein statistics, which describes the statistical distribution of identical particles with integer spin, now called bosons. Bosons, which include the photon as well as atoms such as helium-4 (4He), are allowed to share a quantum state.

Einstein proposed that cooling bosonic atoms to a very low temperature would cause them to fall (or “condense”) into the lowest accessible quantum state, resulting in a new form of matter.

In 1938 Fritz London proposed BEC as a mechanism for superfluidity in 4He and superconductivity.[5][6]

On June 5, 1995 the first gaseous condensate was produced by Eric Cornell and Carl Wieman at the University of Colorado at Boulder NIST–JILA lab, in a gas of rubidium atoms cooled to 170 nanokelvins (nK).[7]

Shortly thereafter, Wolfgang Ketterle at MIT demonstrated important BEC properties. For their achievements Cornell, Wieman, and Ketterle received the 2001 Nobel Prize in Physics.[8]

Many isotopes were soon condensed, then molecules, quasi-particles, and photons in 2010.[9] ...”

https://en.wikipedia.org/wiki/Bose%E2%80%93Einstein_condensate

States of Matter: Bose-Einstein Condensate

Jesse Emspak, Live Science Contributor
August 3, 2018

Of the five states matter can be in, the Bose-Einstein condensate is perhaps the most mysterious. Gases, liquids, solids and plasmas were all well studied for decades, if not centuries; Bose-Einstein condensates weren’t created in the laboratory until the 1990s.

A Bose-Einstein condensate is a group of atoms cooled to within a hair of absolute zero. When they reach that temperature the atoms are hardly moving relative to each other; they have almost no free energy to do so. At that point, the atoms begin to clump together, and enter the same energy states. They become identical, from a physical point of view, and the whole group starts behaving as though it were a single atom.

To make a Bose-Einstein condensate, you start with a cloud of diffuse gas. Many experiments start with atoms of rubidium. Then you cool it with lasers, using the beams to take energy away from the atoms. After that, to cool them further, scientists use evaporative cooling. “With a [Bose-Einstein condensate], you start from a disordered state, where kinetic energy is greater than potential energy,” said Xuedong Hu, a professor of physics at the University at Buffalo. “You cool it down, but it doesn’t form a lattice like a solid.”

Instead, the atoms fall into the same quantum states, and can’t be distinguished from one another. At that point the atoms start obeying what are called Bose-Einstein statistics, which are usually applied to particles you can’t tell apart, such as photons.
Theory & discovery

Bose-Einstein condensates were first predicted theoretically by Satyendra Nath Bose (1894-1974), an Indian physicist who also discovered the subatomic particle named for him, the boson. Bose was working on statistical problems in quantum mechanics, and sent his ideas to Albert Einstein. Einstein thought them important enough to get them published. As importantly, Einstein saw that Bose’s mathematics — later known as Bose-Einstein statistics — could be applied to atoms as well as light.

What the two found was that ordinarily, atoms have to have certain energies — in fact one of the fundamentals of quantum mechanics is that the energy of an atom or other subatomic particle can’t be arbitrary. This is why electrons, for example, have discrete “orbitals” that they have to occupy, and why they give off photons of specific wavelengths when they drop from one orbital, or energy level, to another. But cool the atoms to within billionths of a degree of absolute zero and some atoms begin to fall into the same energy level, becoming indistinguishable.

That’s why the atoms in a Bose-Einstein condensate behave like “super atoms.” When one tries to measure where they are, instead of seeing discrete atoms one sees more of a fuzzy ball.

Other states of matter all follow the Pauli Exclusion Principle, named for physicist Wolfgang Pauli. Pauli (1900-1958) was an Austrian-born Swiss and American theoretical physicist and one of the pioneers of quantum physics.It says that fermions — the kinds of particles that make up matter — can’t be in identical quantum states. This is why when two electrons are in the same orbital, their spins have to be opposite so they add up to zero. That in turn is one reason why chemistry works the way it does and one reason atoms can’t occupy the same space at the same time. Bose-Einstein condensates break that rule.

Though the theory said such states of matter should exist, it wasn’t until 1995 that Eric A. Cornell and Carl E. Wieman, both of the Joint Institute for Lab Astrophysics (JILA) in Boulder, Colorado, and Wolfgang Ketterle, of the Massachusetts Institute of Technology, managed to make one, for which they got the 2001 Nobel Prize in Physics.

In July 2018, an experiment aboard the International Space Station cooled a cloud of rubidium atoms to ten-millionth of a degree above absolute zero, producing a Bose-Einstein condensate in space. The experiment also now holds the record for the coldest object we know of in space, though it isn’t yet the coldest thing humanity has ever created.

https://www.livescience.com/54667-bose-einstein-condensate.html

How do they produce and control single photons?

FWIW...

Although the concept of a single photon was proposed by Planck as early as 1900,[1] a true single-photon source was not created in isolation until 1974.

This was achieved by utilising a cascade transition within calcium atoms.[2] Individual atoms emit two photons at different frequencies in the cascade transition and by spectrally filtering the light the observation of one photon can be used to ‘herald’ the other.

The observation of these single photons was characterised by its anticorrelation on the two output ports of a beamsplitter in a similar manner to the famous Hanbury Brown and Twiss experiment of 1956.[3]

Another single-photon source came in 1977 which utilised the fluorescence from an attenuated beam of sodium atoms.[4] A beam of sodium atoms was attenuated so that no more than one or two atoms contributed to the observed fluorescence radiation at any one time.

In this way, only single emitters were producing light and the observed fluorescence showed the characteristic antibunching. The isolation of individual atoms continued with ion traps in the mid-1980s.

A single ion could be held in a radio frequency Paul trap for an extended period of time (10 min) thus acting as a single emitter of multiple single photons as in the experiments of Diedrich and Walther.[5]

At the same time the nonlinear process of parametric down conversion began to be utilised and from then until the present day it has become the workhorse of experiments requiring single photons.

Advances in microscopy led to the isolation of single molecules in the end of the 1980s.[6] Subsequently, single pentacene molecules were detected in p-terphenyl crystals.[7] The single molecules have begun to be utilised as single-photon sources.[8]

Within the 21st century defect centres in various solid state materials have emerged,[9] most notably diamond, silicon carbide [10] and boron nitride.[11] the most studied defect is the nitrogen vacancy (NV) centers in diamond that was utilised as a source of single photons.[12]

These sources along with molecules can use the strong confinement of light (mirrors, microresonators, optical fibres, waveguides, etc.) to enhance the emission of the NV centres.

As well as NV centres and molecules, quantum dots (QDs) can emit single photons and can be constructed from the same semiconductor materials as the light-confining structures.[13]

https://en.wikipedia.org/wiki/Single-photon_source

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