This post is to succinctly describe the key ideas that are presented by Prof. Vazirani in his QMQC course on EdX (I saw this when it was originally offered on coursera) related to Bell's idea to show that quantum randomness is (or is not) inherent in nature.
So, as usual, we have two players Alice and Bob who can initially agree on a strategy and are then separated by a huge distance over which we will assume that they cannot communicate. Now, the game is that both Alice and Bob receive a binary input and produce a binary input. They win when they generate different inputs only when both their inputs are 1, otherwise they should produce the same output. Classically, we can see that the best that Alice and Bob can do is win with a probability of 0.75.
However, if they can share an entangled Bell state then they can achieve a better probability of success. An entangled state of two particles is one which cannot be expressed as a product of the states of the two particles. So, as an example,
\[ \lvert \Psi \rangle = \frac{1}{\sqrt{2}} \left( \lvert 00 \rangle + \lvert 11 \rangle \right) \]
is an entangled state. It is instructive to see that this state cannot be written as a tensor product of the individual states of the two particles. Now, the next fun thing to note is that the representation of this state when written in any other orthonormal basis \(u, u^{\perp} \) does not change (again, expand out the tensor products to see this). So, our state above can be expressed as
\[ \lvert \Psi \rangle = \frac{1}{\sqrt{2}} \left( \lvert uu \rangle + \lvert u^{\perp}u^{\perp} \rangle \right) \]
Now, let Alice and Bob share the entangled state with one particle each before separation and are then taken to two corners of the universe. They can choose their basis (x for Alice and y for Bob) as shown in the figure below and announce their result depending on whether they see the particle oriented along their basis vector or orthogonal to it. In, all cases except when \(x=y=1\) their basis vectors are oriented at an angle of \( \frac{\pi}{8} \), and only when \(x=y=1\) there basis vectors are orientated at an angle of \( \frac{3\pi}{8} \). In both cases the probability of winning is \( \cos^2\frac{\pi}{8} \approx 0.85 \), much better than can be obtained classically.
So, as usual, we have two players Alice and Bob who can initially agree on a strategy and are then separated by a huge distance over which we will assume that they cannot communicate. Now, the game is that both Alice and Bob receive a binary input and produce a binary input. They win when they generate different inputs only when both their inputs are 1, otherwise they should produce the same output. Classically, we can see that the best that Alice and Bob can do is win with a probability of 0.75.
However, if they can share an entangled Bell state then they can achieve a better probability of success. An entangled state of two particles is one which cannot be expressed as a product of the states of the two particles. So, as an example,
\[ \lvert \Psi \rangle = \frac{1}{\sqrt{2}} \left( \lvert 00 \rangle + \lvert 11 \rangle \right) \]
is an entangled state. It is instructive to see that this state cannot be written as a tensor product of the individual states of the two particles. Now, the next fun thing to note is that the representation of this state when written in any other orthonormal basis \(u, u^{\perp} \) does not change (again, expand out the tensor products to see this). So, our state above can be expressed as
\[ \lvert \Psi \rangle = \frac{1}{\sqrt{2}} \left( \lvert uu \rangle + \lvert u^{\perp}u^{\perp} \rangle \right) \]
Now, let Alice and Bob share the entangled state with one particle each before separation and are then taken to two corners of the universe. They can choose their basis (x for Alice and y for Bob) as shown in the figure below and announce their result depending on whether they see the particle oriented along their basis vector or orthogonal to it. In, all cases except when \(x=y=1\) their basis vectors are oriented at an angle of \( \frac{\pi}{8} \), and only when \(x=y=1\) there basis vectors are orientated at an angle of \( \frac{3\pi}{8} \). In both cases the probability of winning is \( \cos^2\frac{\pi}{8} \approx 0.85 \), much better than can be obtained classically.
A nice article discussing the questions still surrounding the experimental results with Bell's states appeared here recently.
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