Quantum Mechanics and Spin-1/2 Particles
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Questions and Answers

What results are obtained when measuring the spin of a spin-1/2 particle along any chosen direction?

  • Any continuous value
  • +~/2 only
  • −~/2 only
  • +~/2 or −~/2 (correct)
  • What happens when a measurement of spin is made along the z-direction?

  • The measurement collapses the wave function entirely.
  • Results will vary randomly in a continuous manner.
  • It yields a consistent value for subsequent measurements along the same direction. (correct)
  • The result is always ambiguous.
  • If the spin is known along the z-direction, what is the probability of obtaining the same or opposite spin result when measuring along a perpendicular direction?

  • 100% probability for the same result
  • 50% probability for each of ±~/2 (correct)
  • 0% probability for each direction
  • 25% probability for +~/2 and 75% for −~/2
  • What is the total number of quantized spin projections available for a spin-n particle?

    <p>2n + 1</p> Signup and view all the answers

    What does a measurement along the x-direction imply for subsequent measurements along the z-direction?

    <p>They become ambiguous with 50% probability for either result.</p> Signup and view all the answers

    What characterizes the mathematical representation of a spin-1/2 particle's state?

    <p>It is defined with two eigenvalues and eigenvectors.</p> Signup and view all the answers

    Why were silver atoms specifically chosen for the Stern Gerlach experiment?

    <p>They have spin but are charge neutral.</p> Signup and view all the answers

    What is the nature of quantum mechanics as described in relation to spin measurements?

    <p>It is the simplest explanation for behavior at the smallest scales.</p> Signup and view all the answers

    What does the equation $E = \hbar \omega$ represent?

    <p>Einstein's relation relating energy to angular frequency</p> Signup and view all the answers

    In the context of scattering, what does the potential $V(x)$ represent for $x < 0$?

    <p>A free particle state with zero potential energy</p> Signup and view all the answers

    The relationship $p = \hbar k$ signifies what in wave mechanics?

    <p>The de Broglie relation linking momentum to wave number</p> Signup and view all the answers

    What does the term $\psi(x, t) = \phi(x) T(t)$ indicate in quantum mechanics?

    <p>A combined wave function of spatial and temporal components</p> Signup and view all the answers

    What is the physical interpretation of the term $V_0$ in the potential step?

    <p>The height of the potential barrier encountered by the particle</p> Signup and view all the answers

    When solving the TISE in different regions, what boundary condition must be applied?

    <p>The wave function and its derivative must be continuous</p> Signup and view all the answers

    In the equation for kinetic energy $E = \frac{p^2}{2m}$, what does $p$ represent?

    <p>The particle's momentum</p> Signup and view all the answers

    For a plane wave described by $ ext{ψ(x, t)}$, what does the term $ ext{e}^{-iωt}$ signify?

    <p>The wave's temporal evolution</p> Signup and view all the answers

    What is the probability of measuring the state | ↑z i when the system is in the state | ↑x i?

    <p>1/2</p> Signup and view all the answers

    Which expression correctly represents the amplitude for measuring the state | ↑z i from the state | ↑x i?

    <p>h↑z | ↑x i = √ (1 + 0)/2</p> Signup and view all the answers

    What is the general formula for the probability of measuring state |ϕi when state |ψi is prepared?

    <p>|hϕ|ψi|^2</p> Signup and view all the answers

    In the context of repeated measurements, what happens when one output path of the Stern Gerlach apparatus is blocked?

    <p>It selects spins with a chosen orientation along a chosen direction.</p> Signup and view all the answers

    What does the expression |h↑z | ↑x i| evaluate to?

    <p>1/2</p> Signup and view all the answers

    When a state | ↑x i is expressed in the z-basis, what does the expression | ↑x i = √(1/2)(| ↑z i + | ↓z i) represent?

    <p>The state is equally likely to be measured as | ↑z i or | ↓z i.</p> Signup and view all the answers

    What is the meaning of the notation h↑z | ↑x i in the context of quantum states?

    <p>It represents the measurement of | ↑x i when prepared in the | ↑z i basis.</p> Signup and view all the answers

    What does the expression √(h↑z | ↑z i) yield?

    <p>1</p> Signup and view all the answers

    What happens when you divide both sides of the equation by φT?

    <p>You only have functions of time on one side and functions of position on the other side.</p> Signup and view all the answers

    Why must both sides of the equation be equal to the same constant?

    <p>Because the right side contains space-dependent functions only.</p> Signup and view all the answers

    What does the constant E represent in the context of the equation?

    <p>It signifies the energy of the system.</p> Signup and view all the answers

    What is the key result of solving the time-independent Schrödinger equation (TISE)?

    <p>It gives solutions to the time-dependent Schrödinger equation (TDSE).</p> Signup and view all the answers

    What does the expression ψ(x, t) represent?

    <p>It denotes the wavefunction at a specific time and location.</p> Signup and view all the answers

    Which statement about the time evolution of the wavefunction is true?

    <p>It is given by an exponential function involving energy.</p> Signup and view all the answers

    What implication does successfully solving the TISE have on the TDSE?

    <p>You can describe the wavefunction for all future and past times.</p> Signup and view all the answers

    What condition must be true for expressions involving φ(x) and ψ(x, t) to hold?

    <p>They must hold in the absence of measurement.</p> Signup and view all the answers

    What does the expression | ↑x ih↑x | + | ↓x ih↓x | represent in quantum mechanics?

    <p>A superposition of states with definite spin in the x-direction</p> Signup and view all the answers

    What is the result of measuring the spin in the x-direction according to the provided content?

    <p>It randomizes the result of the z-direction measurement</p> Signup and view all the answers

    What is the significance of the gap between the two possible paths followed by the particle?

    <p>It allows for macroscopic separation of quantum states</p> Signup and view all the answers

    Which aspect differentiates quantum superposition from classical probabilities as mentioned in the content?

    <p>Quantum superpositions have uncertain outcomes even when paths are defined</p> Signup and view all the answers

    What role does the Stern-Gerlach apparatus play in the described experiment?

    <p>It allows for the separation of particle states based on spin</p> Signup and view all the answers

    What happens to the z-direction spin information when x-direction measurement information is erased?

    <p>It is preserved despite previous measurements</p> Signup and view all the answers

    What is the overall implication of the quantum eraser experiment described?

    <p>It demonstrates the complex nature of quantum state measurements</p> Signup and view all the answers

    How does measuring along x affect the quantum state originally prepared in z?

    <p>It completely collapses the z-state</p> Signup and view all the answers

    What is a characteristic of the Schrödinger and Heisenberg pictures?

    <p>They are equivalent and yield the same amplitudes.</p> Signup and view all the answers

    What does the Heisenberg equation of motion express?

    <p>It states the relationship between an operator and the Hamiltonian.</p> Signup and view all the answers

    When is an observable quantity A associated with operator  considered conserved?

    <p>When the derivative of its expectation value with respect to time equals zero.</p> Signup and view all the answers

    What condition must be met for an operator  to be considered conserved in relation to the Hamiltonian Ĥ?

    <p>The commutator Â, Ĥ must equal zero.</p> Signup and view all the answers

    What does the equality in Eq. 226 represent?

    <p>The correlation between the Schrödinger and Heisenberg pictures.</p> Signup and view all the answers

    What is implied by the statement about amplitudes being the same in both pictures?

    <p>The underlying physics remains consistent regardless of the picture used.</p> Signup and view all the answers

    What does the term 'arbitrary choice' in Eq. 228 suggest?

    <p>The choice of state vector can vary without altering measurable outcomes.</p> Signup and view all the answers

    In the context of quantum mechanics, what is primarily affected by a time-dependent potential?

    <p>The explicit time dependence of the Schrödinger operators.</p> Signup and view all the answers

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