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11.2.1 Bell Rotations
The first step in the measurement consists of applying a rotation operation to
each qubit that rotates the qubits’ states along the Bloch sphere in such a manner
that the axis along which the measurement is to be done is moved onto the Z-axis.
As described before, this is achieved with a microwave pulse of the right phase
and amplitude as indicated in Figure 11.2b. For different experimental runs, i.e.
different pairs from the ensemble, the rotation is adjusted to correspond to the
axes a, a ′ , b, and b ′ as defined in the previous chapter.
11.2.2 Tunneling Measurement
In the second step, the usual tunneling measurement is used to measure the
state along the Z-axis. Each experimental run will then yield one of the four states
| 00 〉, | 01 〉, | 10 〉 or | 11 〉. Their occurrences are counted for the different rotations
a, a ′ , b, and b ′ and their relative rates for each axes combination are interpreted
as the probabilities P | 00 〉 (a, b), P | 00 〉 (a ′ , b), P | 00 〉 (a, b ′ ), P | 00 〉 (a ′ , b ′ ), P | 01 〉 (a, b), etc.
These probabilities are combined into the correlation measures E xy according to
Equation 10.2, which in turn give the value for S (Equation 10.3).
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