Quantum physics is the branch of physics that describes the behavior of particles at the smallest possible scales. In the quantum realm, quantum mechanical effects become significant and particles can no longer be described using classical physics.
Quantum entanglement is a phenomenon observed at the quantum scale where entangled particles stay connected (in some sense) so that the actions performed on one of the particles affects the other, no matter the distance between two particles. Difficult to understand? It won’t be anymore. Keep reading the article.
Before understanding what quantum entanglement is, let us first have a look at the Double-slit experiment.
Double-slit experiment, first performed by Thomas Young in 1801 showed that when light passes through the double-slit assembly, it behaves exactly like a wave. The experiment which was initially a part of classical physics and aimed at demonstrating the wave nature of light later proved to be a revolutionary experiment in the field of quantum physics.
The adjacent image is a simulation of two waves created on the surface of water. It can be seen that when the two waves interfere, at regions where the crests (or elevations) of one wave falls on the crests of the other wave, the amplitudes add up and the resultant amplitude at those regions is maximum. On the other hand, when the crests of one wave falls on the troughs (or depressions) of the other wave, the amplitudes cancel out and the resultant amplitude at those regions is zero. Thus we can see alternate regions of maxima and minima formed in the image.
Exactly the same happens with light. When light passes through two slits, alternate regions of maximum and minimum amplitude are recorded on the screen. The double-slit assembly and the interference pattern developed using light is depicted below:
As one can see, the light from the light source is made to pass through the double-slit and the resultant interference pattern is recorded on the screen (which consists of alternate maxima and minima).
In the later versions of the double-slit experiment, when light was replaced by an electron beam, the interference pattern recorded on the screen was found to be wave-like. Not only electrons but even atoms and molecules were seen to behave like waves. Thus, it was found that at the quantum scale, objects can show characteristics of both particles and waves. The wave-particle duality hypothesis originally put forward by De Broglie was confirmed by physicists Davisson and Germer through their famous Davisson–Germer experiment.
Therefore, in the quantum world, particles like electrons can show wave-like characteristics. We can imagine this wave to be a wave of probabilities- the probability of finding the electron at a certain position. This is the same reason why electrons do not orbit the nucleus of an atom in the form of fixed orbits. Instead there is a probability of finding the electron in any specific region around the nucleus.
The probability of finding the electron can be expressed using a wave function. When the electron is not being observed, it behaves like a wave and produces a wave-like interference pattern on the screen but when a detector is located near one of the slits to observe the electron, the wave-function collapses and the electron behaves like a particle. Thus, when the electron is being observed, no (wave-like) interference pattern is produced on the screen.
Sounds weird? Well, what else can you expect? It is quantum physics!
Now, lets get back to the main reason why we are here: What is ‘Quantum entanglement?’
Inside an atom, when a photon (particle of light) interacts with the strong electric field around the nucleus, it is transformed into two particles (This is not the only way to create entangled particles). Since the photon has a zero charge and a zero spin, it is obvious that the two particles created in this process must have charges and spins in such a manner that the total charge and spin for the system is zero. (Since they must be conserved.)
The process mentioned above results into production of an electron (- charge) and a positron (+ charge) both having opposite charges (E = mc²). So the net charge of the system is zero. But what about their spins? Both electrons and positrons can have two types of spins: spin-up or spin-down. Since the resultant spin of the system must be zero, if one of the particles is spin-up, the other particle must be spin-down.
Thus the two possible states are:
- Electron (up) and positron (down)
- Electron (down) and position (up)
When the system of electron-positron is not being observed, the system can exist in both possible states at the same time (as seen previously for the electron). Thus the two particles act as if they’re one and exist in multiple superposition states at the same time. Such particles are said to be entangled. But as soon as one of the particles is observed, the wave function collapses and if the measured spin of electron is up, then the spin of the positron automatically takes a down value.
Even if you keep one of the particles (say positron) at the edge of the universe, as soon as you measure the spin of the other particle (electron), the spin of the particle at the edge of the universe becomes defined. This gives an impression that the information between the two entangled particles is transmitted instantaneously or faster than light. But this is not true. Quantum entanglement does not violate relativity because no actual information is passed when the entangled particles affect each other.
Note that this is the simplest explanation that can be given to a phenomenon as complex as quantum entanglement. If you still have queries, feel free to ask them in the comments.