Who is it? Making countless atoms sing in unison (Part 1)

Before the establishment of quantum theory, people generally believed that at room temperature, atomic gas was always in a state of irregular high-speed thermal motion, just like a group of billiard balls running around, constantly colliding with each other, and the interactions between atoms followed the various laws of motion of classical mechanics.

However, when these atoms are cooled by laser, their temperature approaches absolute zero (uK (microkelvin) level). The speed of atomic movement decreases sharply, quantum effects become very significant, and the interaction between atoms can be simply described by " De Broglie waves".

0 1 "Bose-Einstein Condensate" - the wonderful fifth form of matter

As early as 1924, French physicist de Broglie proposed a bold conjecture: since light has the dual nature of wave and particle, all physical particles (such as atoms and electrons) should also have their own wave properties. In other words, physical particles are actually a kind of "matter wave" and have a wavelength corresponding to their own energy:

Specifically, the wavelength of the matter wave corresponding to the atom satisfies:

The physical relationship between

The parameters m and v are the mass and speed of the atom respectively.

It is not difficult to find that as the speed of atoms decreases, the de Broglie waves corresponding to the atoms will increase accordingly. For example, at room temperature (about 300 K), the de Broglie waves corresponding to helium atoms are about 0.09 nm (nanometers). This wavelength is much smaller than the average spacing between helium atoms, and therefore cannot reflect macroscopic quantum properties; and when helium atoms approach absolute zero (μK level), the de Broglie waves will increase to about 1600 nm, which is close to the average spacing of atoms. Therefore, atomic gases under low temperature conditions will experience a "coherent superposition" of matter waves, thus forming a macroscopic quantum superposition state different from that in the classical world, namely the "Bose-Einstein condensate ( BEC )".

At this time, individual atoms no longer move independently, but act together with a large number of atoms to show collective macroscopic quantum characteristics. All atoms are uniform like a "super atom". This wonderful form of matter is also called the fifth form of matter besides solid, liquid, gas, and plasma.

So, how was this wonderful state of matter that makes countless atoms "sing in unison" predicted by Einstein a hundred years ago? What interesting story is there between its discovery and physicist S.N. Bose? Next, let's open the scientific letter from a hundred years ago.

02 A scientific conjecture from a hundred years ago - Bose's letter

In 1924, Indian physicist Bose used a new statistical method to re-derive the Planck distribution law describing photons (i.e., the blackbody radiation formula). However, Bose's research results were rejected by many physics journals. Therefore, Bose, unwilling to fail, sent the research results directly to Einstein in Germany, hoping that he could scientifically evaluate the research results.

In 1924, Einstein received an academic manuscript from Bose (translated into German)

(Image source: Springer Nature) References [1]

After reading Bose's letter, Einstein realized the importance of this research result, translated it into German and submitted it to the German academic journal "Journal of Physics" for publication. Then in 1925, Einstein extended Bose's theory to the ideal case of massive monatomic gas, further supporting Bose's theory.

In 1925, Einstein published an academic paper entitled "Quantum Theory of Monatomic Ideal Gases"

(Image source: Wiley Online Library) References [2]

During Einstein's deduction process, he became keenly aware of an interesting phenomenon - if the total number of atomic gas remains constant, even if there is no interaction between the atoms, at a low enough temperature, these atoms will still undergo a transformation of their material form, so that all the atoms will condense into the same quantum state with the lowest energy.

In this wonderful state of matter, the de Broglie waves between the atomic gases will superimpose on each other, and their respective motion states will be completely synchronized. At this time, all atoms have completely consistent physical properties, and a single wave function can be used to describe the macroscopic quantum properties of the entire atomic gas. Therefore, all atoms are like condensed into a "super atom" - Bose-Einstein condensate.

Schematic diagram of the atomic state in "Bose-Einstein condensate".

The corresponding characteristic temperatures are 400nK, 200nK and 50nK respectively.

(Image source: JILA, University of Colorado, Boulder) References [3]

Conclusion

So, can all kinds of atoms achieve this wonderful "Bose-Einstein condensate" as long as the temperature reaches a low enough level? Have physicists already achieved "Bose-Einstein condensate" experimentally?

Let us go with curiosity and questions and uncover more interesting stories about "Bose-Einstein Condensate" in the next journey!

author:

Luan Chunyang, Quantum Hardware Researcher at China Mobile Communications Corporation, PhD in Physics at Tsinghua University

Wang Yutong, PhD, Department of Physics, Tsinghua University

Reviewer: Luo Huiqian, Researcher, Institute of Physics, Chinese Academy of Sciences

Planning: Zhai Guoqing

Produced by: Science Popularization China

References

[1] Bose. Plancks gesetz und lichtquantenhypothese[J]. Zeitschrift für Physik, 1924, 26(1): 178-181.

[2] Einstein A. Quantentheorie des einatomigen idealen Gases. Zweite Abhandlung (Quantum theory of monatomic ideal gases, part two)[J]. SBd Preuss. Akad. Wiss. Ber, 1925, 1(3).

[3] JILA - University of Colorado Boulder. URL https://jila.colorado.edu/.

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