Creating a Bose-Einstein condensate (BEC) of helium involves a complex experimental setup and specialized techniques. Helium can exist in two isotopic forms, helium-4 (4He) and helium-3 (3He). The process of making a BEC typically refers to creating a BEC of helium-4. Here's a general outline of the steps involved:
Cooling the helium gas: The first step is to cool the helium gas to extremely low temperatures. This is typically done using a combination of techniques, such as dilution refrigeration and adiabatic demagnetization. These methods involve reducing the temperature of the helium gas by exploiting the properties of magnetic fields and heat exchange.
Cooling to the lambda point: As the helium gas is further cooled, it reaches a critical temperature known as the lambda point, which is around 2.17 Kelvin for helium-4. At this temperature, helium starts to exhibit quantum behavior and undergoes a phase transition. Helium-4 undergoes a transition from the normal state to a superfluid state, where it can flow without friction.
Evaporative cooling: To create a BEC, the helium gas is further cooled through evaporative cooling. This process involves selectively removing the hottest and most energetic atoms from the gas using radiofrequency or laser-induced evaporation techniques. By removing these atoms, the remaining atoms become colder and closer to condensing into a BEC.
Reaching the critical temperature for BEC: By continuing the evaporative cooling process, the temperature of the helium gas is lowered until it reaches the critical temperature for Bose-Einstein condensation. In the case of helium-4, this critical temperature is approximately 2.17 Kelvin.
Formation of the BEC: Once the critical temperature is reached, the remaining helium atoms undergo a transition and condense into a single quantum state, forming a Bose-Einstein condensate. At this point, the helium atoms exhibit quantum phenomena, such as wave-like behavior and coherence, where the individual atoms behave collectively as a single entity.
Creating a BEC of helium is a challenging process that requires precise temperature control, sophisticated cooling techniques, and careful experimental setup. It is an active area of research in the field of quantum physics and has provided valuable insights into the study of quantum matter.