Bose-Einstein Condensate

A Bose-Einstein condensate (BEC) is a state of matter that only exists at temperatures unbelievably close to absolute zero. At these temperatures (a fraction of a degree above the coldest possible), some atoms can lose their separate identities and merge into a single quantum entity that behaves like one giant atom. BECs were predicted by Albert Einstein and the Indian physicist Satyendra Bose in 1924, but they were not actually produced in a laboratory until 1995. They are one of the strangest states of matter ever discovered and have already led to several Nobel Prizes.

  • What it isFifth state of matterAtoms merge into one quantum blob
  • Required temperatureNear absolute zeroA few billionths of a degree
  • Predicted1924By Bose and Einstein
  • First made1995Won Nobel Prize in 2001
  • Atoms usedRubidium, sodium, lithiumSpecial bosonic isotopes
  • Where seenOnly in special labsNot in nature anywhere

Setting the scene

To understand a BEC, you need to know two things from quantum physics:

  • Every particle has a "wave" aspect, with a wavelength that depends on its speed. Fast particles have short wavelengths; slow particles have long wavelengths.
  • Particles called bosons (like photons, helium-4 atoms, certain isotopes of rubidium and sodium) can occupy the same quantum state. Other particles called fermions (electrons, protons, neutrons) cannot.

Cool a gas of bosonic atoms far enough and their wavelengths get long enough to overlap. At a certain critical temperature, a huge number of atoms suddenly all drop into the lowest possible quantum state, all behaving as one synchronised wave. This is the BEC.

The recipe for a BEC

Making a BEC takes some of the most precise apparatus in physics:

  • Start with a thin gas of bosonic atoms (often rubidium-87 or sodium-23).
  • Cool it using laser cooling. Carefully tuned lasers hit the atoms with photons that slow them down. This brings them down to about a millionth of a degree above absolute zero.
  • Trap them with magnetic fields. The cold atoms are held in a kind of magnetic bowl so they do not fall into the walls of the container.
  • Cool further by evaporative cooling: let the hottest, fastest atoms escape, leaving only the coldest behind. Like blowing on a cup of tea, but in reverse.
  • The remaining atoms reach a few nanokelvins (billionths of a kelvin) above absolute zero.
  • Suddenly, in a sharp transition, most of the atoms drop into the same quantum state. A BEC has formed.
Fact The coldest temperature ever produced on Earth was achieved with a BEC: about 38 picokelvin (3.8 x 10^-11 K) in a German laboratory in 2021. That is colder than anything ever measured anywhere else, including deep interstellar space (which is around 2.7 K, warmed by the leftover Big Bang radiation).

Why BECs are so strange

In a BEC, the atoms have lost their individual identities. They all share one quantum "wave function". This gives BECs properties unlike anything in normal matter:

  • Superfluidity: a BEC can flow with zero viscosity, climbing the walls of its container in defiance of gravity.
  • Quantum interference: if you split a BEC into two halves and bring them back together, they interfere like overlapping water waves, producing dark and bright stripes.
  • Slow light: BECs can be used to slow light down. In one famous experiment, light was slowed to just 17 metres per second (about the speed of a cyclist) inside a BEC. Light usually travels at 300 million metres per second.
  • Quantum vortices: stir a BEC and instead of swirling smoothly, it develops only certain allowed patterns of tiny whirlpools, like an army of quantum hurricanes.

The Nobel Prize story

BECs were predicted by Einstein and Bose in 1924, but the technology to make them did not exist for 70 more years. In 1995, two American teams (one led by Eric Cornell and Carl Wieman, the other by Wolfgang Ketterle) succeeded within months of each other in producing the first BECs. All three shared the 2001 Nobel Prize in Physics. Since then, BECs have been made in dozens of labs around the world.

Did you know? In 2018, NASA put a BEC experiment on board the International Space Station. In microgravity, the trapped atoms do not fall, so they can be held still for much longer than on Earth. This lets scientists study BECs in even more detail. The experiment is called the Cold Atom Lab, and it is the coldest known place in the entire solar system.

What BECs are used for

BECs are still mostly a research tool, but they are being explored for many practical uses:

  • Atomic clocks: ultra-cold atom clouds are the basis of the worlds most accurate clocks, which lose less than one second in tens of billions of years.
  • Quantum computers: BECs may help build the next generation of quantum computers, which could solve some problems much faster than ordinary computers.
  • Precision sensors: BECs are extraordinarily sensitive to gravity, magnetic fields and rotation. They are being developed as new tools for navigation, geology and even searching for gravitational waves.
  • Fundamental physics: studying BECs has helped scientists test ideas about quantum mechanics, superfluidity, superconductivity and even black holes.
Try this You cannot make a BEC at home (the equipment costs millions of pounds and fills a room), but you can look up videos online of the first BECs being made. Search for "Bose-Einstein condensate explained". You will see the temperature drop, the atomic cloud condense into a tiny dense dot, and the slow-motion behaviour of the resulting quantum blob. Some images of BECs are among the most famous photos in physics.
Deeper dive: why some particles can crowd together and others cannot

The BEC effect happens only with certain types of particle called bosons. Other particles, called fermions, behave completely differently.

The distinction comes from a property called spin. Every fundamental particle has a quantum spin: a built-in amount of angular momentum, measured in units of Plancks constant.

  • Fermions: spin is a half-integer (1/2, 3/2, etc.). Examples: electrons, protons, neutrons. They obey the Pauli exclusion principle, which means no two fermions can occupy the same quantum state. This is why electrons in an atom have to spread out into different shells, building the structure of every atom.
  • Bosons: spin is an integer (0, 1, 2, etc.). Examples: photons, helium-4 atoms (4 protons + 2 neutrons all combine to integer spin), gluons. Many bosons can pile into the same state with no problem. This is why laser light works (millions of photons in the same state) and why a BEC is possible.

Curiously, an atom can act as a boson or fermion depending on its makeup. Helium-4 (2 protons + 2 neutrons + 2 electrons = 6 fermions = even = bosonic) can form a BEC. Helium-3 (2 protons + 1 neutron + 2 electrons = 5 fermions = odd = fermionic) cannot. The basic chemistry of helium-3 and helium-4 is the same; their quantum behaviour at ultra-low temperatures is completely different.

This boson/fermion distinction reaches surprisingly deep into our world. The fact that every atom has a unique structure (rather than collapsing into a tiny ball) is because electrons are fermions. The fact that lasers can shine bright is because photons are bosons. The bizarre quantum world is the foundation that everyday chemistry, biology and engineering all sit on top of.

For more, see plasma and what are states of matter.