The coldest experiment in the universe

  Where is the coldest place you can think of?
  In winter, the temperature in Antarctica is as low as minus 85°C; on the dark side of the moon, the temperature can reach minus 173°C; the coldest substance known in nature is liquid helium, and its temperature is minus 269°C.
  However, there is a place in the universe whose temperature is much colder than that of liquid nitrogen, only one billionth of a degree higher than absolute zero (the extremely low limit of the temperature that matter can reach by theory). This place is located in the world. The cold atom laboratory of the space station. As the name suggests, the Cold Atom Laboratory is where the “super-cold” atom cloud is made.
  However, why do scientists create atomic clouds in space that are billions of a degree, or even tens of billions of degrees, higher than absolute zero? This starts with Einstein, the most famous physicist.
  The new state under the coldest conditions
  In 1925, Einstein published a separate paper entitled “Quantum Statistics of Ideal Gas”. In this paper, Einstein for the first time predicted the Bose-Einstein condensate, which is a state of matter reached by atoms at extremely low temperatures. After solid, liquid, gaseous, and plasma states, it is Called the fifth state of matter. Why is it called Bose-Einstein condensate? Here is a little story.
  Usually, in our concept, the particles that make up matter are individual individuals, they are all doing their own irregular thermal motion, the size and direction of the motion are different, these particles are in different states, that is In other words, each particle can be distinguished.
  However, as early as 1924, when an Indian mathematical physicist, Bose, was studying photon statistics, he proposed an idea that microscopic particles are indistinguishable from each other. After arriving at this idea, Bose immediately wrote A related paper was written. However, because Bose at the time was an unknown scientist who had no doctorate, no journal was willing to publish his thesis. So, Bose got angry and directly sent the paper to Einstein. Einstein was very excited when he saw Bose’s paper, and personally translated Bose’s paper into German and published it in a German magazine. Later, Einstein proposed the phenomenon of Bose-Einstein condensate on the basis of Bose theory. To commemorate the two prophecies of this new state, people call it Bose-Einstein condensate.
  After talking about this Bose-Einstein condensate for a long time, what is going on with it?
  Particles in the microscopic world have wave-particle duality, that is to say, microscopic particles can not only be described in terms of particles, but also in terms of waves. Take the atom as an example. The atom can be seen as a particle or a wave. The characteristics of particle movement can be described by momentum (the product of particle mass and velocity), and the characteristics of particle fluctuation can be described by wavelength, and momentum and wavelength are inversely proportional, that is, the slower the particle’s movement speed, the longer the wavelength. At the same time, the temperature of matter comes from its own thermal movement. If the particle movement speed decreases, the temperature will naturally become lower.
  In general, the distance between atoms and atoms is very large, and their momentum is also relatively large. But as the movement speed of the atom gets slower and slower, that is, the temperature of the atom gets colder and colder, the atom slowly begins to show its own fluctuating nature. We can vividly compare the lengthening of the wavelength of atoms to the fact that atoms are getting fatter and fatter. When the wavelength of the atom itself approaches or exceeds the distance between the atoms, the atoms begin to “touch”, and if they continue to cool down, it becomes that there is me in you and you in me, and all atoms become A whole has the same state, namely the Bose-Einstein condensate. Atoms in this state will have some peculiar properties, and these properties can provide us with a new perspective for studying atoms or other physical phenomena.
  The “little box” for producing cold atoms
  On May 21, 2018, the National Aeronautics and Space Administration (NASA) sent a small device nearly the size of a drawer-Cold Atom Laboratory (CAL) to the International Space Station (jointly managed by 16 countries). After the delivery was successful For the past 7 months, scientists have produced ultra-cold atoms and observed the behavior of ultra-cold atoms through remote control every day. The cold atom laboratory consists of a laser, a vacuum chamber and an electromagnetic “knife”. It uses two technologies: laser cooling and evaporative cooling.
  Next, let’s take a look at these two technologies separately.
  If a particle is flying towards us, how can we stop it? The answer is naturally to apply a force to the direction of the particle’s movement to slow it down. Similarly, the photon in the laser has a certain momentum, and the direction is the direction of light propagation. If the photon collides with the atom (particle), it will also generate a force to slow the atom. Since the thermal motion of the atom is irregular, we do not know its specific direction of motion, therefore, it is necessary to use a laser to slow down the atom in all directions to reduce its temperature. This is the principle of laser cooling technology.
  Although laser cooling technology can reduce the temperature of atoms to an extremely low level, this temperature is not enough to allow atoms to reach the Bose-Einstein condensate. The further cooling of atoms requires the use of evaporative cooling technology. The basic principle of this technology is similar to the cooling of a cup of hot water. When people drink water, if the water is too hot, they have to blow it to cool the water. Due to the constant movement of molecules, some high-speed molecules (that is, molecules with high energy and relatively hot) on the surface of the water will leave the liquid water molecule group and run into the air. This part of the water molecules becomes water vapor, The hot water vapor molecules on the surface of the water blow away, and the water in the glass cools down. Similarly, evaporative cooling is to remove the hotter atoms from the atomic cloud (this part of the atoms has higher energy and easily escapes from the atomic cloud), and the temperature of the entire atomic cloud will drop.
  In fact, using these two technologies, laboratories on Earth can also produce ultra-cold atoms. But on the ground, gravity will act on the cooled atomic cloud, causing them to fall quickly, and the atomic cloud will immediately heat up again. During this period, scientists were able to observe the Bose-Einstein condensed atom cloud for only a fraction of a second. Although the magnetic field can be used to “capture” the atom cloud and keep it still, the natural motion behavior of condensed atoms cannot be observed in this way. Therefore, scientists want to produce cold atoms in space, because in the microgravity of space, cold atom clouds float for much longer, and scientists can have a deeper understanding of the behavior of condensed atoms.
  However, a cold atom laboratory that can be sent to space is not easy to make. Usually, the cold atom equipment on the earth is very large, can occupy an entire laboratory, and some switches are exposed outside, so that scientists can adjust the equipment at any time. First, the coldest laboratory sent to space is small; second, scientists can only remotely operate it on Earth. In fact, scientists and engineers started to make this space cold atom laboratory in 2012, but it was not successfully sent to space until 2018.
  Future transformation of new materials
  from Bose – Einstein condensate since been observed, cold atom experiments on concern because the results of cold atom experiments could lead to the development of many technologies, such as sensors, atomic clocks and interferometers Quantum computer, etc.
  Taking quantum computers as an example, the realization of quantum computers requires the use of quantum effects. In a classic computer, a bit can only be in one of the two binary states of 1 or 0. Then, two bits can represent one of the four numbers 0, 1, 2, and 3 (binary 1, 0 is decimal 2, binary 1 , 1 is decimal 3). We use the opening and closing of the circuit to represent 0 and 1, and then through the complex circuit, let the computer complete complex calculations. The basic storage unit of a quantum computer based on the laws of quantum mechanics is a qubit. Compared to the information stored in a bit, it can only be in two states of 0 or 1. Since the quantum is in a superposition state, the information stored in a qubit may be 1 or possible. It is 0, that is, the information stored by the qubit can be both 0 and 1. Therefore, one qubit can represent two numbers 1 and 0 at the same time, and two qubits can represent four numbers 0, 1, 2, and 3 at the same time. In fact, a quantum computer can express 0 and 1 through the transition of a cold atom between its own ground state and an excited state (a state with a certain energy higher than the ground state). As microscopic particles, cold atoms have quantum superposition characteristics, that is, cold atoms can be in either the ground state or the excited state. At the same time, cold atoms move slowly, have low energy, and have a more specific energy state than hot atoms (there are more than two energy states that hot atoms may be in). Therefore, cold atoms are a good candidate material for qubits. .
  In addition, cold atom experiments can also be used to improve the accuracy of atomic clocks. At present, our definition of time is determined by the frequency of photons emitted by cesium atoms. How to determine this frequency determines the accuracy of time. Scientists let cesium atoms pass through a microwave cavity (the microwave cavity can emit electromagnetic waves of a specific frequency, and researchers can adjust the frequency of the electromagnetic waves). When the frequency emitted by the microwave cavity is the same as the photon frequency of the cesium atom, resonance will occur. Scientists We can determine the photon frequency of cesium atoms. However, the average speed of cesium atoms at room temperature is about several hundred meters per second, and the time to pass through the microwave cavity is very short, which greatly limits the stability of determining the frequency. The motion speed of cold atoms in space is greatly reduced, and the time it takes to pass through the microwave cavity is longer, and the accuracy is naturally improved. The accuracy of the atomic clock originally had an error of 1 second/3 million years. The accuracy of the cold atomic clock can be increased to 1 second/300 million years, an increase of 2 orders of magnitude.
  Any physical phenomenon has far-reaching practical significance, and the key lies in whether we can discover and apply it in time. The cold atom experiment in space and the study of Bose-Einstein condensate must have a profound impact on physics and promote the progress of science and technology.