Bose-Einstein condensate

History

In 1924, Indian physicist Satyendra Nath Bose sent Einstein an article on a new statistics for particles of light. Einstein immediately recognized the importance of the idea and extended it to atoms, showing that with sufficient cooling a gas should 'condense' into the lowest energy state. But experimental realization proved extremely difficult. Only in 1995 did Eric Cornell and Carl Wieman obtain the first condensate from rubidium atoms, cooling them to 170 nanokelvins. For this discovery, they (together with Wolfgang Ketterle) were awarded the Nobel Prize.

How it works

Imagine a group of musicians tuning their instruments. While it's warm (ordinary temperature), each plays its own note — complete chaos. But as it cools, their 'breath' slows down, and they begin to synchronize until they sound in unison as one pure note. So too atoms: at room temperature, they rush around at different speeds, but upon cooling to nanokelvins, their quantum waves smear out and overlap, forcing all particles to occupy a single energy level. The condensate behaves as a single object — it can even be 'shaken' and made to vibrate as a single drop.

History

In 1924, Indian physicist Satyendra Nath Bose sent Einstein an article on a new statistics for particles of light. Einstein immediately recognized the importance of the idea and extended it to atoms, showing that with sufficient cooling a gas should 'condense' into the lowest energy state. But experimental realization proved extremely difficult. Only in 1995 did Eric Cornell and Carl Wieman obtain the first condensate from rubidium atoms, cooling them to 170 nanokelvins. For this discovery, they (together with Wolfgang Ketterle) were awarded the Nobel Prize.

How it works

Imagine a group of musicians tuning their instruments. While it's warm (ordinary temperature), each plays its own note — complete chaos. But as it cools, their 'breath' slows down, and they begin to synchronize until they sound in unison as one pure note. So too atoms: at room temperature, they rush around at different speeds, but upon cooling to nanokelvins, their quantum waves smear out and overlap, forcing all particles to occupy a single energy level. The condensate behaves as a single object — it can even be 'shaken' and made to vibrate as a single drop.

History

The theoretical prediction of BEC dates back to the works of Satyendra Nath Bose (1924) and Albert Einstein (1925). Bose derived a distribution for photons without a classical limit, and Einstein applied this statistics to non-relativistic atoms. He showed that below a certain temperature, the gas should dramatically change its properties due to the accumulation of particles in the lowest energy state. For many decades, the search for the right conditions (necessary to avoid condensation into liquid and solid) yielded no results. The breakthrough came in 1995: the group of Eric Cornell and Carl Wieman from JILA (USA) first cooled a cloud of rubidium-87 atoms to a temperature of 170 nanokelvins and observed the phase transition to BEC, and soon Wolfgang Ketterle from MIT obtained a condensate from sodium atoms. These experiments opened the field of quantum gases and earned the authors the Nobel Prize in Physics in 2001.

How it works

A Bose gas at high temperatures is described by Maxwell-Boltzmann statistics. As the temperature decreases, the de Broglie wavelength of the particles increases: λdB ∝ 1/√T. When λdB exceeds the average interparticle distance d (i.e., when nλdB³ ~ 1, more precisely nλdB³ = ζ(3/2) ≈ 2.612), the wave packets of atoms begin to overlap, and one must apply quantum Bose-Einstein statistics. For bosons, unlike fermions, the Pauli principle does not prohibit multiple occupation of a single quantum state; moreover, at low temperatures this becomes energetically favorable. A phase transition occurs, and a macroscopic number of particles accumulates in the ground state. This condensate is described by a single wave function, which satisfies the second law of thermodynamics: the entropy of the system decreases during the transition, but is compensated by heat removal to the external environment. The Gross-Pitaevskii equation incorporates this wave function and accounts for weak interaction between atoms.

Open problems & fun facts

Creation of a stable condensate under gravity conditions and exclusion of all external fields.; Study of quantum turbulence in BEC and its connection to classical hydrodynamics.; Development of practically useful devices based on BEC — atomic lasers, precision gyroscopes, and quantum simulators.

💡 Bose-Einstein condensate can be used to create an 'atom laser' — a beam of atoms moving in perfect synchrony. Such a beam is already used in ultra-precise atomic clocks and interferometers.