Visualizing the correlation function

Oct 13, 2025 by wbrum.org 8 views

Introduction

The CHSH (Clauser-Horne-Shimony-Holt) inequality is a fundamental concept in quantum mechanics that tests the validity of local realism in a quantum system. It is a crucial component of the Ekert's E91 protocol, a quantum key distribution (QKD) protocol that relies on entanglement to generate secure keys. In this article, we will explore the implementation of the CHSH inequality using Qiskit, a popular open-source quantum development environment. We will discuss the inconsistent values of $S$ obtained in the CHSH inequality and provide a detailed analysis of the issue.

Background

The CHSH inequality is a mathematical statement that describes the maximum value of a correlation function in a local realistic theory. It is given by the following expression:

S = E(a, b) + E(a, d) + E(c, b) - E(c, d)

where $E(a, b)$ represents the expectation value of the product of two measurement outcomes, $a$ and $b$ , in a particular basis. The CHSH inequality states that $S \leq 2$ , and any value greater than 2 would indicate non-locality, which is a fundamental feature of quantum mechanics.

Qiskit Implementation

To implement the CHSH inequality using Qiskit, we need to create a quantum circuit that generates entangled states and measures the correlation function. The following code snippet demonstrates how to create a simple CHSH circuit using Qiskit:

from qiskit import QuantumCircuit, execute, Aer
from qiskit.quantum_info import Statevector
qc = QuantumCircuit(4)
qc.h(0)
qc.cx(0, 1)
qc.h(2)
qc.cx(2, 3)
qc.measure([0, 1, 2, 3], [0, 1, 2, 3])
backend = Aer.get_backend('qasm_simulator')
job = execute(qc, backend, shots=1024)
result = job.result()
counts = result.get_counts(qc)
correlation_function = bit counts[bit] / 1024 for bit in counts
S = correlation_function['00'] + correlation_function['11'] + correlation_function['10'] - correlation_function['01']
print(S)

Inconsistent Values of $S$

When running the above code, we may observe inconsistent values of $S$ that deviate from the expected value of 2. This can be attributed to several factors, including:

Quantum noise: Quantum circuits are prone to noise, which can cause errors in the measurement outcomes. This can lead to inconsistent values of $S$ .
Measurement errors: The measurement process can also introduce errors, which can affect the correlation function and, subsequently, the $S$ value.
Circuit implementation: The implementation of the CHSH circuit can also introduce errors, such as incorrect gate placement or measurement basis.

Analysis

To analyze the inconsistent values of $S$ , we need to examine the correlation function and identify the sources of error. We can do this by:

Visualizing the correlation function: We can plot the correlation function to visualize the distribution of measurement outcomes.
Analyzing the measurement outcomes: We can examine the individual measurement outcomes to identify any patterns or correlations.
Comparing with theoretical expectations: We can compare the obtained $S$ value with the theoretical expectation of 2 to identify any deviations.

Conclusion

In this article, we discussed the implementation of the CHSH inequality using Qiskit and the inconsistent values of $S$ obtained in the CHSH inequality. We analyzed the possible sources of error, including quantum noise, measurement errors, and circuit implementation errors. By visualizing the correlation function, analyzing the measurement outcomes, and comparing with theoretical expectations, we can identify the sources of error and improve the accuracy of the $S$ value.

Future Work

To improve the accuracy of the $S$ value, we can:

Optimize the CHSH circuit: We can optimize the CHSH circuit to reduce quantum noise and measurement errors.
Implement error correction: We can implement error correction techniques, such as quantum error correction codes, to mitigate the effects of quantum noise.
Use more robust measurement methods: We can use more robust measurement methods, such as machine learning-based methods, to improve the accuracy of the correlation function.

References

[1] Clauser, J. F., Horne, M. A., Shimony, A., & Holt, R. A. (1969). Proposed experiment to test local hidden-variable theories. Physical Review Letters, 23(15), 880-884.
[2] Ekert, A. K. (1991). Quantum cryptography based on Bell's theorem. Physical Review Letters, 67(6), 661-663.
[3] Qiskit documentation. (n.d.). Retrieved from https://qiskit.org/documentation/

Code

The code used in this article is available on GitHub: https://github.com/username/qiskit-chsh-inequality

Acknowledgments

Introduction

In our previous article, we discussed the implementation of the CHSH inequality using Qiskit and the inconsistent values of $S$ obtained in the CHSH inequality. In this article, we will address some of the frequently asked questions (FAQs) related to this topic.

Q: What is the CHSH inequality?

A: The CHSH inequality is a mathematical statement that describes the maximum value of a correlation function in a local realistic theory. It is given by the following expression:

S = E(a, b) + E(a, d) + E(c, b) - E(c, d)

where $E(a, b)$ represents the expectation value of the product of two measurement outcomes, $a$ and $b$ , in a particular basis.

Q: What is the significance of the CHSH inequality?

A: The CHSH inequality is significant because it tests the validity of local realism in a quantum system. If the CHSH inequality is violated, it indicates non-locality, which is a fundamental feature of quantum mechanics.

Q: What are the possible sources of error in the CHSH inequality?

A: The possible sources of error in the CHSH inequality include:

Quantum noise: Quantum circuits are prone to noise, which can cause errors in the measurement outcomes.
Measurement errors: The measurement process can also introduce errors, which can affect the correlation function and, subsequently, the $S$ value.
Circuit implementation: The implementation of the CHSH circuit can also introduce errors, such as incorrect gate placement or measurement basis.

Q: How can I optimize the CHSH circuit to reduce quantum noise and measurement errors?

A: To optimize the CHSH circuit, you can try the following:

Use error correction techniques: Implement error correction techniques, such as quantum error correction codes, to mitigate the effects of quantum noise.
Use more robust measurement methods: Use more robust measurement methods, such as machine learning-based methods, to improve the accuracy of the correlation function.
Optimize the circuit layout: Optimize the circuit layout to reduce the effects of quantum noise and measurement errors.

Q: How can I implement error correction in the CHSH circuit?

A: To implement error correction in the CHSH circuit, you can use quantum error correction codes, such as the surface code or the Shor code. These codes can detect and correct errors in the measurement outcomes.

Q: What are some common pitfalls to avoid when implementing the CHSH inequality?

A: Some common pitfalls to avoid when implementing the CHSH inequality include:

Incorrect gate placement: Make sure to place the gates correctly in the circuit to avoid errors.
Incorrect measurement basis: Make sure to use the correct measurement basis to avoid errors.
Insufficient error correction: Make sure to implement sufficient error correction techniques to mitigate the effects of quantum noise and measurement errors.

Q: How can I troubleshoot issues with the CHSH inequality?

A: To troubleshoot issues with the CHSH inequality, you can try the following:

Visualize the correlation function: Plot the correlation function to visualize the distribution of measurement outcomes.
Analyze the measurement outcomes: Examine the individual measurement outcomes to identify any patterns or correlations.
Compare with theoretical expectations: Compare the obtained $S$ value with the theoretical expectation of 2 to identify any deviations.

Conclusion

In this article, we addressed some of the frequently asked questions (FAQs) related to the CHSH inequality and its implementation using Qiskit. We hope that this article has provided valuable insights and guidance for researchers and practitioners working on quantum information processing and quantum cryptography.

References

[1] Clauser, J. F., Horne, M. A., Shimony, A., & Holt, R. A. (1969). Proposed experiment to test local hidden-variable theories. Physical Review Letters, 23(15), 880-884.
[2] Ekert, A. K. (1991). Quantum cryptography based on Bell's theorem. Physical Review Letters, 67(6), 661-663.
[3] Qiskit documentation. (n.d.). Retrieved from https://qiskit.org/documentation/

Code

The code used in this article is available on GitHub: https://github.com/username/qiskit-chsh-inequality

Acknowledgments

This work was supported by the [funding agency]. We would like to thank [researcher name] for their valuable feedback and suggestions.