Bose-Einstein Condensation

Bose-Einstein condensation is a phenomenon that occurs when a dilute gas of bosons is cooled to a temperature very close to absolute zero. At this temperature, the individual particles lose their distinct identities and merge into a single quantum state, forming a coherent matter wave.

To understand which of the given options can exhibit Bose-Einstein condensation, let's analyze each option:

a) Hydrogen
Hydrogen is composed of two particles, a proton and an electron. Both protons and electrons are fermions, not bosons. Fermions follow the Pauli exclusion principle, which states that no two identical fermions can occupy the same quantum state simultaneously. Therefore, hydrogen cannot exhibit Bose-Einstein condensation.

b) Lithium
Lithium is a chemical element with three protons, three electrons, and four neutrons. Similar to hydrogen, both the protons and electrons in lithium are fermions. Hence, lithium cannot undergo Bose-Einstein condensation.

c) Helium
Helium is composed of two protons, two neutrons, and two electrons. The two electrons in helium occupy separate quantum states due to the Pauli exclusion principle. However, the two helium-4 nuclei (also known as alpha particles) can be considered as bosons. At extremely low temperatures, these bosonic alpha particles can undergo Bose-Einstein condensation. This is known as superfluid helium-4.

d) Photon
Photons are elementary particles that are carriers of electromagnetic radiation. They are bosons and do not obey the Pauli exclusion principle. Photons can indeed exhibit Bose-Einstein condensation, which has been experimentally observed in certain systems such as optical lattices.

Therefore, among the given options, only photons (option d) can exhibit Bose-Einstein condensation, while hydrogen, lithium, and helium cannot.