What are Bose-Einstein Condensates?

3D Model of Bose Einstein Condensate

Core Concepts

This article will cover Bose-Einstein condensates, a unique state of matter, and how this condensate is prepared. This article will also describe the condensate’s unique properties and how scientists create the condensate.

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The Bose-Einstein Condensate and Boson Particles

Bose-Einstein condensate refers to a unique state of matter consisting of supercooled particles that display special quantum behaviours. After the particles are cooled to nearly absolute zero (or -273.1 degrees Celsius/-459.67 degrees Fahrenheit), the particles slow down significantly and display almost no movement. Clumping together and forming a singular “super-particle,” particles at this temperature enter the same quantum state and behave singularly as a unit. This same quantum state also enables the condensate to display wave-like properties, similar to photons.

All particles used in Bose-Einstein condensates are known as bosons – one of two major classes of submicroscopic particles that are categorised based on their quantum spin number. Bosons have a quantum spin that is a non-negative integer, such as 1 or 2. On the contrary, the other subatomic particle category, fermions, have an odd, fractional quantum spin number (1/2 or 3/2).

One major boson property that enables a Bose-Einstein condensate to form is their ability to share a single quantum or energy state. Unlike fermions, which obey the Pauli Exclusion Principle (how two electrons in the same orbital must have opposite spins and how no two electrons in an atom can have the same electornic quantum numbers), bosons behave like a wave and an unlimited number of bosons can occupy a singular quantum state. Thus, once boson particles are supercooled down to absolute zero, the particles can coalesce into a shared quantum state, creating the unique “superatom” or a “matterwave.” In fact, Bose-Einstein condensates is the only state of matter known for not obeying the Pauli Exclusion Principle.

Bosons vs. Fermions
Boson vs. Fermions

This means that all Bose-Einstein condensates contain particles with non-negative integer quantum spins.

Bose-Einstein Theory and Condensate Discovery

Unlike plasma, solid, gas, or liquid, scientists were unsure whether Bose-Einstein Condensates existed as a state of matter for decades. Until 1995, when scientists created the first Bose-Einstein condensate, physicists viewed this 5th state of matter as purely theoretical, with many claiming that achieving absolute zero conditions necessary to create this condensate was impossible.

Theoretically, the prospect of a Bose-Einstein condensate’s existence appeared in 1924, when renowned physicist Satyendra Nath Bose sent Albert Einstein his notes on photon behaviour. In his notes, Bose noted how boson particles differed from fermion particles and disobeyed Pauli’s Exclusion Principle once cooled down to a certain temperature. Impressed by Bose’s findings, Einstein expanded his notes to include atoms as well as light photons.

Satyendra Bose and Albert Einstein
Satyendra Bose and Albert Einstein

Both scientists found that, theoretically, cooling particles down to a hair of absolute zero would cause electrons to fall into the same energy level. Normally, electrons occupy discrete orbitals or quantum states in an atom, but in absolute zero conditions, these electrons fall into the same quantum level, making them indistinguishable from one another.

By the late 1990s, physicists Eric Cornell and Carl Wieman were able to cool rubidium atoms to 1.7 x 10^-7 K over absolute zero and observe the atomic behaviours. Taking advantage of unique quantum behaviours near absolute zero, Cornell and Wieman coalesced about 2,000 individual atoms into a “superatom” that they were able to see with a microscope.

How to Create A Bose-Einstein Condensate

Formation of a Bose-Einstein Condensate
Formation of a Bose-Einstein Condensate

With improved technology capable of dropping temperatures near absolute zero, scientists primarily use two methods to supercool a group of diffuse gas particles.

  1. Laser Cooling

By streaming six different lasers into the gas, an atom that moves towards the laser absorbs a photon and is thus slowed down. After absorption, the atom then releases the photon in a random direction. By repeating trials of absorption and emission multiple times, this cooling process reduces the overall speed of the atoms – and consequently, the temperature.

2. Evaporative Cooling

Known as skimming off the warmest atoms, evaporative cooling involves a magnetic device holding the diffuse gas particles in place and allowing particles displaying higher energy to escape. By removing particles with the greatest kinetic energy (and subsequently, temperature), the sample’s temperature rapidly decreases and approaches absolute zero.

Notable Properties of Bose-Einstein Condensates

At absolute zero, particles exhibit unique physical properties that physicists rarely observe. Therefore, these behaviours and properties make Bose-Einstein Condensate particles important for scientific research and development in the quantum physics field.

These characteristics can include:

  • Superfluidity– Bose-Einstein condensates are capable of flowing with no viscosity. The particles exhibit nearly no resistance to flow. This property is attributed to the coherence of matter waves within condensates, and the quantum states shared by all particles.
  • Macroscopic Quantum Phenomena – Due to the single quantum state shared by all condensate particles, all particles behave as if they are one entity, essentially losing all individual behaviour. This behaviour allows physicists to study quantum behaviour on a near-human scale.
  • Coherence/Interference – Bose-Einstein condensate matter waves exhibit coherence, or when the phase difference between their waves is consistent. Non-coherent waves are when the phase difference is random or inconsistent. Because of this coherence, the condensate can create interference patterns when interacting with two or more other waves.

Bose-Einstein Condensate Practice Problems

Problem 1

What are the five states of matter?

Problem 2

Which physicist reached out to Albert Einstein about photon behaviour?

Problem 3

Which Bose-Einstein condensate formation method involves atoms absorbing photons?

Problem 4

Which class of submicroscopic particles has a fractional quantum spin number?

Problem 5

What is absolute zero in Celsius?

Bose-Einstein Condensate Practice Problem Solutions

1: Gas, Liquid, Solid, Plasma, and Bose-Einstein Condensate

2: Satyendra Bose

3: Laser Cooling

4: Fermions

5: -273.1 degrees Celsius