Article
Detangling Quantum Entanglement
29 November 2017 
Maryana Kartashevska  About a 4 minute read
Now that’s we’ve had time to wrap our heads around quantum super positioning and a hypothetical cat that’s both alive and dead simultaneously (see Cat, Bits and Quantum Computing), we need to consider another fundamental principle of quantum mechanics that is essential to quantum computing, quantum entanglement.
Quantum entanglement is a lack of independence between particles: the stronger the bond, the more influence the particles exert on each other. So much so that in a maximally entangled state particles cannot be described independently but must be considered as a combined system. Or as our quantum cat expert Schrodinger put it, when “the best possible knowledge of the whole does not necessarily include the best possible knowledge of all its parts”.
To Schrodinger, this bond was the defining characteristic of quantum mechanics. It does not occur in classical physics and remains true even when the particles are removed from each other – a phenomenon Albert Einstein termed a “spooky action in the distance”.
Spooky because regardless of the distance, the particles continue to interact and do so instantaneously; faster than any medium could possibly transmit information between them, even faster that the speed of light. This was particularly disturbing for Einstein because his own theory of special relativity maintained that nothing could travel faster than light.
For the sceptics amongst us, quantum entanglement is supported not only by theoretical mathematics but also by applied physics. Numerous experiments measuring the polarisation of pairs of entangled particles have demonstrated that observing one particle enabled us to consistently predict the behaviour of the other.
For example, measuring the spin of an electron as positive produced a negative spin measurement in its entangled counterpart and vice versa. When orbiting an atom two electrons with the same characteristics always have opposite spin directions and in a quantum entangled state this relationship can persist even if the electrons are removed from the atom’s orbit, or indeed from each other. Hence, if we observe one as spin positive, quantum theory stipulates that the other will be spin negative, regardless of the distance between them.
At first glance that’s no different to pulling a pair of socks out of a quantum entangled sock drawer, looking at the left and predicting that the other one will be the right. But it is different. Since electrons can assume a positive or a negative spin at random, it’s more like pulling any two socks out of a drawer and regardless of whether the first sock is the left or the right, knowing that the other one is its matching pair.
So how does this relate to quantum computing? To achieve the immense scalability and computational speedups, the building blocks of a quantum computer, its qubits, have to be in a quantum entangled super positioned state. This allows the bits to collaborate on running many more calculations simultaneously rather than sequentially as is the case in a classical computer. Essentially, the two entangled bits can be in four states at the same time, whereas classical bits must be in one of the four states at any point in time.
Imagine a 2 bit quantum computer that can run four states at the same time, i.e. 00, 01, 10 and 11. This can also be expressed mathematically as 2n, where n is the number of bits. Now imagine a 64 bit quantum computer. Solving the exponential 264 gives us a whopping 18,446,744,073,709,552,000 possible states simultaneously – something that a conventional computer would take hundreds of years to process sequentially.
Impressive and leads to questions such as where are we with the hardware and why aren’t we using quantum computers already? Well, a quantum state is very fragile; the ability to preserve it to process information is currently a major obstacle to working quantum devices. In the next blog, I explore existing solutions as well as the underlying concepts of decoherence and quantum error correction.
Until then, feel free to reach out and we can have a BohrEinstein like debate on whether quantum computing is the ‘answer to the ultimate question of life, the universe, and everything’.
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