Quantum Measurement

Table of Contents

1. Introduction
2. Classical vs Quantum Measurement
3. The Postulates of Quantum Measurement
4. Measurement in Dirac Notation
5. Projective Measurements
6. Measurement Basis and Observable Operators
7. Measurement Probabilities and the Born Rule
8. Collapse of the Wavefunction
9. Eigenstates and Eigenvalues
10. Examples of Projective Measurements
11. Two-Qubit Measurement
12. Measurement in Entangled Systems
13. Measurement Operators (POVMs)
14. Generalized Measurement and Kraus Operators
15. Non-Orthogonal States and Measurement Limits
16. Quantum No-Cloning and Measurement
17. Quantum Zeno Effect
18. Weak Measurement and Continuous Observation
19. Quantum Measurement in Circuits
20. Measurement-Based Quantum Computing
21. Decoherence and Measurement
22. Quantum Measurement in Quantum Error Correction
23. Role in Quantum Teleportation
24. Measurement Challenges in Physical Implementations
25. Conclusion

1. Introduction

Measurement is one of the most profound and subtle aspects of quantum mechanics. Unlike classical systems, where measurement simply reveals a pre-existing value, quantum measurement fundamentally affects the system being measured. It collapses the wavefunction and turns probabilistic amplitudes into definite outcomes.

2. Classical vs Quantum Measurement

In classical physics:

• Measurement reveals the state of a system.
• It does not disturb the system.

In quantum mechanics:

• Measurement changes the state of the system.
• It probabilistically projects the state onto an eigenvector of the observable.

3. The Postulates of Quantum Measurement

Measurement postulates state that:

1. Every observable corresponds to a Hermitian operator \( \hat{A} \).
2. Outcomes are eigenvalues \( a_i \) of \( \hat{A} \).
3. Probability of outcome \( a_i \):
   \[
   P(a_i) = |\langle a_i | \psi \rangle|^2
   \]
4. Post-measurement state becomes:
   \[
   |\psi’\rangle = \frac{P_i |\psi\rangle}{|P_i |\psi\rangle|}
   \]
   where \( P_i = |a_i\rangle \langle a_i| \) is the projector onto eigenstate.

4. Measurement in Dirac Notation

In Dirac notation, measuring \( |\psi\rangle \) in basis \( \{|a_i\rangle\} \) yields outcome \( a_i \) with probability:

\[
|\langle a_i | \psi \rangle|^2
\]

The wavefunction collapses to \( |a_i\rangle \).

5. Projective Measurements

Projective measurements use a complete set of orthonormal eigenstates:

\[
\sum_i |a_i\rangle \langle a_i| = \mathbb{I}
\]

Each outcome corresponds to projection \( P_i = |a_i\rangle \langle a_i| \).

6. Measurement Basis and Observable Operators

Measurement depends on the basis or observable chosen. For example:

• Measuring in the computational basis uses the operator:
  \[
  \hat{Z} = |0\rangle \langle 0| – |1\rangle \langle 1|
  \]
• Measuring in the Hadamard basis uses \( \hat{X} \), the Pauli-X operator.

7. Measurement Probabilities and the Born Rule

The Born rule is foundational:

• Probability of measuring state \( |\phi\rangle \) in \( |\psi\rangle \):
  \[
  P = |\langle \phi | \psi \rangle|^2
  \]

It connects the abstract wavefunction to experimental outcomes.

8. Collapse of the Wavefunction

Before measurement:
\[
|\psi\rangle = \alpha |0\rangle + \beta |1\rangle
\]

After measurement:

• Result 0 with probability \( |\alpha|^2 \) ⇒ state becomes \( |0\rangle \)
• Result 1 with probability \( |\beta|^2 \) ⇒ state becomes \( |1\rangle \)

9. Eigenstates and Eigenvalues

If \( \hat{A} |a_i\rangle = a_i |a_i\rangle \), then:

• \( a_i \) is a possible measurement result
• \( |a_i\rangle \) is the state after the measurement if outcome is \( a_i \)

10. Examples of Projective Measurements

Measuring:
\[
|\psi\rangle = \frac{1}{\sqrt{2}}(|0\rangle + |1\rangle)
\]
in \( \{|0\rangle, |1\rangle\} \) basis gives:

• 0 with probability \( \frac{1}{2} \)
• 1 with probability \( \frac{1}{2} \)

11. Two-Qubit Measurement

For entangled state:
\[
|\Phi^+\rangle = \frac{1}{\sqrt{2}}(|00\rangle + |11\rangle)
\]

Measuring one qubit collapses both due to entanglement.

12. Measurement in Entangled Systems

Measuring part of an entangled system affects the entire state. The result on one qubit instantaneously determines the state of the other in a correlated basis.

13. Measurement Operators (POVMs)

POVMs (Positive Operator-Valued Measures) generalize projective measurements:

• Each outcome associated with operator \( E_i \), where \( \sum_i E_i = \mathbb{I} \)
• More flexible; used in open systems and noisy environments

14. Generalized Measurement and Kraus Operators

Described by Kraus operators \( \{M_i\} \):

• Probability of outcome \( i \):
  \[
  P(i) = \langle \psi | M_i^\dagger M_i | \psi \rangle
  \]
• Post-measurement state:
  \[
  |\psi’\rangle = \frac{M_i |\psi\rangle}{\sqrt{P(i)}}
  \]

15. Non-Orthogonal States and Measurement Limits

Quantum mechanics forbids perfect discrimination of non-orthogonal states:
\[
|\psi\rangle = |0\rangle, \quad |\phi\rangle = \frac{1}{\sqrt{2}}(|0\rangle + |1\rangle)
\]

cannot be perfectly distinguished by measurement.

16. Quantum No-Cloning and Measurement

The no-cloning theorem states one cannot duplicate an arbitrary unknown quantum state. This restricts how measurements and copying interact.

17. Quantum Zeno Effect

Frequent measurements can freeze quantum evolution. Measuring a decaying system rapidly can suppress its transition, effectively preventing change.

18. Weak Measurement and Continuous Observation

Weak measurements only partially disturb the system. Used in:

• Quantum control
• Real-time tracking of state evolution
• Quantum feedback systems

19. Quantum Measurement in Circuits

In circuit diagrams, measurement is denoted with a meter symbol or classical bit output. Often at the end of algorithms to read results.

20. Measurement-Based Quantum Computing

Uses cluster states and entanglement as a resource. Computation proceeds via measurements instead of gate-based evolution.

21. Decoherence and Measurement

Measurement is tied to decoherence, where a system entangles with the environment, effectively performing a measurement and collapsing the state.

22. Quantum Measurement in Quantum Error Correction

Syndrome measurements detect errors without collapsing the encoded logical state. Measurement outcomes guide corrective operations.

23. Role in Quantum Teleportation

In teleportation:

1. Alice measures her qubits in a Bell basis.
2. Sends classical results to Bob.
3. Bob applies a correction based on measurement.

Measurement is essential to transferring quantum information.

24. Measurement Challenges in Physical Implementations

• Fidelity: accuracy of measurement outcome
• Speed: must be faster than decoherence
• Nondestructive measurement: desired in some systems
• Crosstalk: interference between qubits

25. Conclusion

Quantum measurement lies at the boundary between the quantum and classical worlds. It not only reveals outcomes but defines them. Mastery of quantum measurement is essential for building, understanding, and operating quantum computers, and it continues to provoke deep philosophical questions about the nature of reality.