Bose–Einstein condensate

A Bose–Einstein condensate forms when bosons are cooled until their quantum wave packets overlap and many particles enter one shared ground state. Bosons have integer-valued spin and are not restricted by the exclusion principle that applies to fermions. The prediction arose from work by Satyendra Nath Bose and Albert Einstein in 1924–1925, who showed that a sufficiently cold bosonic gas should undergo a phase transition into a highly occupied quantum state.¹

The condensate is not simply a collection of motionless atoms. Its particles are described by a coherent macroscopic wavefunction with a common phase, so measurements can reveal interference and collective dynamics. At finite temperature, a trapped gas generally contains both a condensate and a thermal cloud; the transition is therefore a change in the population and correlations of the gas, not the disappearance of all thermal motion.

Experimental condensates are produced by combining laser cooling, magnetic or optical trapping, and evaporative cooling. In 1995, Eric Cornell, Carl Wieman, and collaborators created a condensate of rubidium-87, while Wolfgang Ketterle and colleagues independently produced condensates of sodium-23.¹² Evaporative cooling removes the most energetic atoms from a trap, allowing the remaining gas to re-equilibrate at a lower temperature.

Early experiments identified condensation through time-of-flight imaging: after the trap is released, the condensate expands into a narrow, dense central feature, unlike the broader thermal distribution. Interference between separately prepared condensates provided a direct signature of phase coherence. The relevant temperatures are commonly in the nanokelvin range, although the exact transition temperature depends on density, atomic mass, interactions, and trap geometry.

Macroscopic phase coherence gives a Bose–Einstein condensate distinctive hydrodynamic and optical properties. A condensate can support superfluidity, in which flow persists with exceptionally low dissipation under suitable conditions, and it can form quantum vortices whose circulation is quantized rather than continuously variable.³

Researchers use condensates as controllable platforms for many-body physics, precision measurement, and quantum simulation. Optical lattices made from standing-wave laser fields can arrange atoms in periodic potentials resembling crystalline materials, allowing controlled studies of insulating and conducting phases. Tunable interactions, often adjusted with a Feshbach resonance, permit experiments ranging from weakly interacting gases to strongly correlated systems. Coherent outcoupling can also produce an atom laser, a directional matter-wave source analogous in some respects to an optical laser.³

Not every condensate is made from elementary atoms, and condensation can occur in several experimentally distinct settings. Ultracold molecules formed from paired atoms can behave as composite bosons, while exciton–polaritons in semiconductors have displayed condensate-like coherence at much higher temperatures than dilute atomic gases, though their driven and lossy environments differ from those of nearly isolated atomic condensates.³

Geometry also matters: reduced-dimensional gases can show strong phase fluctuations, and in two dimensions the relevant transition is associated with the Berezinskii–Kosterlitz–Thouless mechanism rather than a simple three-dimensional picture. Condensates may be engineered in microgravity, optical cavities, or lattices, and their collective modes can probe interaction strength and trap structure. These edge cases show why “condensate” describes an organized quantum phase, not a single material with one universal temperature or composition.³

Boson

A particle with integer spin that can share a quantum state with other identical bosons.

Ground state

The lowest-energy quantum state available to a physical system.

Phase coherence

A stable relationship between the phases of quantum wavefunctions across a system.

Evaporative cooling

Cooling by selectively removing the most energetic particles from a trapped gas.

Feshbach resonance

A resonance that permits controlled adjustment of interactions between ultracold atoms.