Quantum
Entanglement
Every object around us is made up of massive particles.
Collectively, we refer to these particles as matter. However, there is a deeply
similar entity in the universe that we do not encounter on a daily basis –
antimatter. Antimatter is composed of antiparticles, which have the same mass
as their particle counterparts but are oppositely charged. For example, the
antiparticle of electron, called positron, is positively charged, while the
electron is negatively charged. When a particle comes in contact with an
antiparticle, both of them are destroyed while releasing an enormous amount of
energy. This process is called annihilation.
Let us imagine a situation where a particle collides with its
antiparticle, electron with a positron, for instance, while the electron has a
spin opposite to the spin of the positron at the time of the collision, so that
their overall spin is zero. Once they collide, annihilation occurs instantly.
In this case, the annihilation energy is released in the form of two photons of
gamma radiation. Let us label the photons as photon A and photon B.
As mentioned earlier, spin represents the intrinsic angular momentum.
That is to say that spin obeys the law of conservation of angular momentum,
which states that the total angular momentum of a system does not change over
time. In other words, if the total spin of the system of the electron and the
positron was zero, the total spin of the photons A, B has to be zero as well.
Photon A therefore must have a spin that is opposite to the spin of photon B.
For illustration, let us label the spins of the photons as spin 1, spin 2.
However, remember that unless a quantum object is observed, it is in a
superposition of all possible states. Photon A is therefore in a superposition
of spin 1 and spin 2.
The same thing applies to photon B. The spin of neither of the photons is
defined, but it is given that the spin of one photon must be opposite to the
spin of the other photon.
If somebody observes one of the photons (say, photon A) and tries
to measure its spin, its wave function collapses, and the photon obtains only
one spin (say, spin 1). To fulfil the law of conservation of angular momentum,
immediately after the wave function of photon A has collapsed, the wave
function of photon B must collapse as well, so that the total spin of photons A
and B stays zero.
In other words, the photons are in a state wherein an observation of
photon A immediately influences the state of photon B, regardless of the
distance between the photons. This state of a kind of superposition, where
observation of one object determines the state of another object, is called
quantum entanglement. Mathematically we can write the entangled state of
photons A, B with spins 1, 2 as follows:
|𝛙⟩ = |𝟏𝑨 ⟩|𝟐𝑩⟩ + |𝟐𝑨 ⟩|𝟏𝑩⟩