In this tutorial, you will run an experiment on a quantum computer to demonstrate the violation of the CHSH inequality with the Estimator primitive.
The CHSH inequality, named after the authors Clauser, Horne, Shimony, and Holt, is used to experimentally prove Bell's theorem (1969). This theorem asserts that local hidden variable theories cannot account for some consequences of entanglement in quantum mechanics. The violation of the CHSH inequality is used to show that quantum mechanics is incompatible with local hidden-variable theories. This is an important experiment for understanding the foundation of quantum mechanics.
The 2022 Nobel Prize for Physics was awarded to Alain Aspect, John Clauser and Anton Zeilinger in part for their pioneering work in quantum information science, and in particular, for their experiments with entangled photons demonstrating violation of Bell’s inequalities.
For this experiment, we will create an entangled pair on which we measure each qubit on two different bases. We will label the bases for the first qubit and and the bases for the second qubit and . This allows us to compute the CHSH quantity :
Each observable is either or . Clearly, one of the terms must be , and the other must be . Therefore, . The average value of must satisfy the inequality:
Expanding in terms of , , , and results in:
You can define another CHSH quantity :
This leads to another inequality:
If quantum mechanics can be described by local hidden variable theories, the previous inequalities must hold true. However, as is demonstrated in this notebook, these inequalities can be violated in a quantum computer. Therefore, quantum mechanics is not compatible with local hidden variable theories.
If you want to learn more theory, explore Entanglement in Action with John Watrous.
You will create an entangled pair between two qubits in a quantum computer by creating the Bell state . Using the Estimator primitive, you can directly obtain the expectation values needed (, and ) to calculate the expectation values of the two CHSH quantities and . Before the introduction of the Estimator primitive, you would have to construct the expectation values from the measurement outcomes.
You will measure the second qubit in the and bases. The first qubit will be measured also in orthogonal bases, but with an angle with respect to the second qubit, which we are going to sweep between and . As you will see, the Estimator primitive makes running parameterized circuits very easy. Rather than creating a series of CHSH circuits, you only need to create one CHSH circuit with a parameter specifying the measurement angle and a series of phase values for the parameter.
Finally, you will analyze the results and plot them against the measurement angle. You will see that for certain range of measurement angles, the expectation values of CHSH quantities or , which demonstrates the violation of the CHSH inequality.
First, we write the circuit with the parameter , which we call
After creating the parameterized CHSH circuit, you will create a list of phase values to be assigned to the circuit in the next step. You can use the following code to create a list of 21 phase values range from to with equal spacing, that is, , , , ..., , .
Now we need observables from which to compute the expectation values. In our case we are looking at orthogonal bases for each qubit, letting the parameterized rotation for the first qubit sweep the measurement basis nearly continuously with respect to the second qubit basis. We will therefore choose the observables , , , and .
For this example, the circuit and the operators are simple, so no optimizations are needed.
In order to execute the entire experiment in one call to the
The individual expectation values can then be extracted from the batched results:
After running the circuits, we need to build the CHSH witness functions. We first build the quantities , , , and , by looking at the parity of the outcomes for the four families of circuits we built (two measurement bases for each of the two qubits). Then we use those quantities to build the witness functions as defined previously.
Note that now we do not have to build the observables ourselves because they are returned to us! The computed expectation values are given in
In the figure, the red dashed lines delimit the classical bounds () and the dash-dotted blue lines delimit the quantum bounds (). You can see that the experimental results resemble the general trend of the simulated results and there are regions where the CHSH witness quantities exceeds the classical bounds. Congratulations! You have successfully demonstrated the violation of CHSH inequality in a real quantum system!