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Using the Pauli matrix representation, reduce each of the operators (a) \(s_{x} s_{y}\) (b) \(s_{x} s_{y}^{2} s_{z}^{2},\) and (c) \(s_{x}^{2} s_{y}^{2} s_{z}^{2},\) to a single spin operator.

Short Answer

Expert verified
(a) The operator \(s_{x} s_{y}\) reduces to \(\frac{1}{2i}s_{z}\). (b) The operator \(s_{x} s_{y}^{2} s_{z}^{2}\) reduces to \(s_{x}\). (c) The operator \(s_{x}^{2} s_{y}^{2} s_{z}^{2}\) reduces to I.

Step by step solution

01

Reduce the operator \(s_{x} s_{y}\)

Use the property that \(s_{x}s_{y} = \frac{1}{2i}s_{z}\). This means the operator \(s_{x}s_{y}\) reduces to a single spin operator \(\frac{1}{2i}s_{z}\).
02

Reduce the operator \(s_{x} s_{y}^{2} s_{z}^{2}\)

Considering the given operator \(s_{x} s_{y}^{2} s_{z}^{2}\), note that since \(s_{y}^{2} = I\) and \(s_{z}^{2} = I\), then \(s_{y}^{2} s_{z}^{2} = I\). Therefore \(s_{x} s_{y}^{2} s_{z}^{2}\) simplifies to \(s_{x} I\) which equals \(s_{x}\).
03

Reduce the operator \(s_{x}^{2} s_{y}^{2} s_{z}^{2}\)

The given operator \(s_{x}^{2} s_{y}^{2} s_{z}^{2}\) reduces to \(I\), because \(s_{x}^{2} = I\), \(s_{y}^{2} = I\), and \(s_{z}^{2} = I\). Therefore: \(s_{x}^{2} s_{y}^{2} s_{z}^{2} = I\).

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Spin Operators
Spin operators are fundamental components in quantum mechanics that describe the intrinsic angular momentum of particles like electrons. In quantum mechanics, we often talk about the spin of particles, which is a type of angular momentum intrinsic to quantum particles. Spin operators are mathematical entities that represent these spinning properties, the most familiar being the Pauli matrices for spin-1/2 particles.

Within the framework of quantum mechanics, spin doesn't actually involve particles physically spinning. Instead, it relates to specific quantum properties that behave in ways similar to classical angular momentum but not identically.
  • Spin operators are typically represented by matrices, such as the Pauli matrices, which act on the particle's spin state.
  • These operators adhere to certain mathematical rules, allowing us to determine measurable properties, like a particle's magnetic moment, through calculations.

Understanding how spin operators work is crucial for solving quantum mechanical problems involving measurement or evolution of a particle's spin state.
Quantum Mechanics
Quantum mechanics is a fundamental theory in physics that explains the behavior of matter and energy on the smallest scales, such as that of atoms and subatomic particles. This theory describes phenomena where classical mechanics doesn't hold true, providing a framework for understanding elements like electron orbitals, photons, and spin.

Key principles of quantum mechanics include:
  • Superposition: States of matter like particles can exist in multiple states simultaneously.
  • Quantization: Many properties are thought of as having discretized levels, rather than continuous values.
  • Uncertainty Principle: Heisenberg's principle states that certain pairs of properties, like position and momentum, cannot both be known to arbitrarily high precision.

In the realm of spin and Pauli matrices, quantum mechanics gives us the mathematical tools to describe and calculate the properties and interactions of particles' spins. Spin operators act within this quantum framework, allowing researchers to predict how particles with spin will interact with magnetic fields.
Matrix Representation
Matrix representation is a powerful mathematical approach utilized in quantum mechanics to analyze and compute the physical properties of quantum entities like spin. Especially with concepts like spin operators, matrices, specifically the Pauli matrices, provide a convenient form to represent these complex quantum states.

The Pauli matrices are a set of three 2x2 complex matrices which correspond to quantum mechanical spin operators for a half-spin system. These are critical in simplifying calculations related to spin, due to their straightforward mathematical properties:
  • The matrices represent the operators for spins along the x, y, and z axes.
  • They are employed for calculating spin-based interactions and transformations within quantum systems.

Using matrix representation, complex operations involving spins can be reduced to manageable algebraic manipulations. This is mainly due to their simple nature and the linear algebraic rules they follow, which streamline solving quantum mechanic problems.

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Most popular questions from this chapter

(a) Confirm that the Pauli matrices \\[ \sigma_{x}=\left(\begin{array}{ll} 0 & 1 \\ 1 & 0 \end{array}\right) \quad \sigma_{y}=\left(\begin{array}{rr} 0 & -\mathrm{i} \\ \mathrm{i} & 0 \end{array}\right) \quad \boldsymbol{\sigma}_{z}=\left(\begin{array}{rr} 1 & 0 \\ 0 & -1 \end{array}\right) \\] satisfy the angular momentum commutation relations when we write \(s_{q}=\frac{1}{2} \hbar \sigma_{q},\) and hence provide a matrix representation of angular momentum. (b) Why does the representation correspond to \(s=1 / 2\) ? Hint. For the second part, form the matrix representing \(s^{2}\) and establish its eigenvalues.

In some cases \(m_{11}\) and \(m_{12}\) may be specified at the same time as \(j\) because although \(\left[f^{2}, j_{1 z}\right]\) is non-zero, the effect of \(\left[j^{2}, j_{12}\right]\) on the state with \(m_{i 1}=j_{1}, m_{i 2}=j_{2}\) is zero. Confirm that \(\left[j^{2}, j_{1 z}\right]\left|j_{1} j_{1} ; j_{2} / j_{2}\right\rangle=0\) and \(\left[j^{2}, j_{1 z}\right]\left|j_{1},-j_{1} ; j_{2},-j_{2}\right\rangle=0\).

Suppose that in place of the actual angular momentum commutation rules, the operators obeyed \(\left[l_{x}, l_{y}\right]=-\mathrm{i} \hbar l_{z^{*}}\) What would be the roles of \(l_{2}=l_{x} \pm l_{y} ?\)

Construct the vector coupling coefficients for a system with \(j_{1}=1\) and \(j_{2}=1 / 2\) and evaluate the matrix elements \(\left\langle j^{\prime \prime} m_{i}^{\prime}\left|j_{1 z}\right| j m_{i}\right\rangle .\) Hint. Proceed as in Section 4.12 and check the answer against the values in Resource section \(2 .\) For the matrix element, express the coupled states in the uncoupled representation, and then operate with \(j_{1 x}\)

What are the possible outcomes of a single measurement of the \(z\) -component of spin angular momentum of an electron in the spin state \((1 / 4)^{1 / 2} \alpha-(3 / 4)^{1 / 2} \beta ?\)

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