Bose–Einstein condensate — the fifth state of matter

What just happened?

We cooled the cloud of bosonic atoms far below the critical temperature $T_c$. Instead of each moving chaotically on its own, the atoms “merged” into one shared, lowest-energy quantum state — this is the Bose–Einstein condensate.

Condensate fraction:

— N₀ is the number of atoms in the condensate, N is the total number of atoms. The further below $T_c$, the larger the fraction of atoms that “fall” into the common ground state.

Why does this happen only near zero?

Each atom corresponds to a matter wave whose de Broglie wavelength grows as the atom slows down:

When $T$ drops to nanokelvins, $\lambda_{dB}$ becomes comparable to the spacing between atoms — their matter waves begin to overlap. The atoms lose their individual identity and are described by one shared wave function.

Why is it the fifth state of matter?

It is not a gas, liquid, solid, or plasma — the BE condensate is a macroscopic quantum object: millions of atoms behave like a single “superatomic” wave, with phase coherence and superfluid properties.

Nobel Prize 2001:

The first Bose–Einstein condensate was created in 1995 by Eric Cornell and Carl Wieman (rubidium atoms), and independently by Wolfgang Ketterle (sodium atoms), combining laser cooling with evaporative cooling. For this achievement they received the Nobel Prize in Physics in 2001. Today BE condensates are used in atomic clocks and precise quantum sensors.

Why you need this for the exam:

This extends the kinetic-molecular theory of gases and wave–particle duality (de Broglie). Trick: check whether $T$ is above or below $T_c$ — that decides whether the gas is described by the Maxwell–Boltzmann distribution or by a single condensate wave function.

Remember: Bose–Einstein condensation occurs only below the critical temperature T_c, close to absolute zero — it is the fifth state of matter, in which atoms lose their individual identity and behave like one quantum wave.