Quantum Computing Concepts – Spin
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Quantum Computing Concepts – Spin


The simplest quantum bit we have in nature is called “spin”. All particles like electrons, protons and neutrons have the property of possessing an intrinsic magnetic dipole which acts like a microscopic compass needle. It’s called “spin” because it’s what you would obtain by taking an electric charge and making it spin onto itself, but in reality, the spin is an intrinsic quantum property of the particle
and does not actually involve any physical spinning. Just like a compass needle, the
spin has the tendency to align itself along a magnetic field. If we take an electron and place it in a magnetic field, it will have two basic quantum states – spin down and spin up. Since there’s only two of them, we can also call them 0 and 1 and use them to
encode quantum information. In a magnetic field, the 0 and 1 states of the spin have
different energy, just like a compass needle has lowest energy when it’s pointing along
the Earth’s magnetic field and highest energy when it’s pointing against it. If I prepared the spin in the 0 state, I can turn it towards the one state by irradiating it with an electromagnetic
field that oscillates at the frequency given by the energy difference between 1 and 0,
divided by the fundamental constant of quantum mechanics, the plank constant. This method
to control a spin is called “magnetic resonance” and it’s used in many practical devices such
as the MRI scanners found in hospitals. With the spin, it’s very easy to visualise what
a quantum super position looks like. A super position of 0 and 1 is obtained by rotating
the spin partially from down to up. For example, if I stop half way, the spin points horizontally. So what is special about the spin? Why is it different from the needle of a compass? To understand this, we need to look at what happens if we try to observe the direction of the spin and what happens if we place two spins close together. Then two striking phenomena
take place – quantum measurement and quantum entanglement.

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