Quantum Computing Concepts – Entanglement
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Quantum Computing Concepts – Entanglement


To really understand what is special about
quantum bits and why they can provide such a large computational power, we have to look
at what happens when we take more than one qubit. Let’s take the example of a spin which
we know how to visualise. The super position of the 0 and 1 state of a single spin is simply the spin pointing to the right. So that doesn’t seem so strange. But now, let’s take two spins
and make them interact with each other. This can happen for example because each spin produces a tiny magnetic field in its vicinity and this field changes the energy of the other spin. Remember that we can rotate a spin by applying an oscillating magnetic field at the frequency proportional to the energy difference between the 1 and the 0 state but if there is another spin in the vicinity, the total energy depends also on the state of that spin. Let’s say I prepare both spins in the 0 state and then I put spin A in the super position of 0 and 1. Now I try to flip spin B by applying the frequency it would respond to if spin A is in the 0 state. But actually, since A is in both the 0 and the 1 state, then spin B both flips and doesn’t flip. The resulting state is a super position of 0-1 and 1-0. The 0-1 branch of the super position is created because spin B flips to 1 when spin A is 0 and the 1-0 branch is created because spin B remains 0 when A is in the state 1. This kind of super position is perfectly legitimate but it’s really special. In this state I have absolutely no idea of where the two spins are pointing but I do know that they point
in opposite directions. This is called an entangled state. If I separate the two spins after I have prepared the entangled state and I measure for example spin A and I find it in the 0 state, then instantly, spin B acquires the 1 state no matter how far apart
it is when the measurement takes place. This is what Einstein called dismissively, ‘The spooky action at the distance’ but now we know from many experimental tests, that this
is the way quantum mechanics really works. In the context of quantum computing, entangled states represent a strikingly non-classical computer code. There is no way you can write in your desktop computer a code where two bits have no value but have the opposite value,
but you can do so in a quantum computer and the more qubits you have, the more these entangled quantum computer codes proliferate and grow exponentially with the number of qubits. Gaining access to these entangled quantum states is the key to exploiting the exponentially large computational power of quantum systems. At the same time, entangled states are very fragile
and can easily be destroyed by unwanted interference from the outside world. This is why building a quantum computer requires so much effort and investment. We need to create a perfectly
controlled environment where large entangled states can be produced and manipulated without interference.

6 Comments

  • fanOmry

    is it possible to maintain an entanglment connection by a web of bonds?

    As in, one pair is measured, both particles are entangled with local particles. and then are reconnected to the connection web.

  • oosveluzo levso

    despite Quantum computer will be awesome, we actually need an entangled computer, not quantum at the moment, we need computers to communicate faster than the speed of light, AI and robots are the future of interplanetary explorations, we would be using robots as an avatars, humans will colonize mars first using robots and this is coming really soon. I'm on for mars explorations from Earth, it will be done using virtual reality. sending robots would be really easy, and far on the future it will be automatic, after mars colonization, other planets will be a piece of cake, since first robots will be sent to create more robots to colonize mars, some kind like bacteria reproducing and spreading through the universe.

  • Antonio Jesus

    For every particle created, there its its opposite, especially yours, on another universe in the multiverse; for we, are no different than clusters of particles

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