University Quantum Mechanics I · topic UQMI-10.09
The position representation: psi(x) = <x|psi>
Connect the ideas, make your working visible, and test the result. Use the hints when you need a starting point; open the full solution after your attempt.
Further practice
Work through a fresh case before tackling the extension.
The position representation: ψ(x) = ⟨x|ψ⟩: predict a finite change
The position representation expands a state in the eigenkets |x⟩ of the position operator, a basis with a continuous label normalised as ⟨x|x′⟩ = δ(x − x′), so that the wavefunction ψ(x) = ⟨x|ψ⟩ is the component of |ψ⟩ along |x⟩. Use the supplied topic model Y_out = sqrt(2)*(cosd(theta)*sin(pi*xp)+sind(theta)*sin(2*pi*xp)), where Y_out is POSITION COMPONENT ψ(x₀) (nm^(−1/2)). The output symbol Y_out is distinct from all input symbols. ^ denotes a power; π = 3.141592653589793; sqrt(x) means √x; sind/cosd/tand use degrees; unmarked trigonometric functions use radians. theta is Mixing angle θ of |1⟩ and |2⟩, measured in °. Keep nb (Number of bins N) = 8; xp (Probe position x₀) = 0.3 nm fixed. For this exercise use only 52.2 ° ≤ theta ≤ 160.875 ° on this continuous branch. A grid of test settings is an input restriction for this exercise, not by itself a claim of physical quantisation. This calculation retains the topic model's stated assumptions; it does not extend them to a different physical setup. Calculate Y_out at theta = 76.1085 ° and 91.323 °. Find the signed output change ΔY_out and the average sensitivity ΔY_out/Δtheta across this interval. State whether the response increases or decreases; distinguish this interval average from a derivative at one point.
POSITION COMPONENT ψ(x₀) against theta
Predictions from the supplied The position representation: ψ(x) = ⟨x|ψ⟩ model, with all other inputs fixed as stated in the question. 5 calibration points support a trend comparison; they are not measured data or a straight-line law.
Vertical axis (y): POSITION COMPONENT ψ(x₀) / nm^(−1/2)
Horizontal axis (x): theta: Mixing angle θ of |1⟩ and |2⟩ / °
Axes show the supplied calibration interval; the origin may lie outside the plot.
Read the exact data as a table
| theta: Mixing angle θ of |1⟩ and |2⟩ / ° | POSITION COMPONENT ψ(x₀) / nm^(−1/2) |
|---|---|
| 52.199999999999996 | 1.7639970779061793 |
| 79.36875 | 1.5329862567686225 |
| 106.5375 | 0.9636930422825609 |
| 133.70625 | 0.18174274273963525 |
| 160.875 | -0.6403125282054984 |
These are the supplied values, not a worked answer. No fitted trend is drawn.
Show a hint
Substitute each value of theta separately while keeping every other input fixed, then subtract in the same order.
Reveal the worked solution
- At theta = 76.1085 °, substitute into Y_out = sqrt(2)*(cosd(theta)*sin(pi*xp)+sind(theta)*sin(2*pi*xp)): Y₀ = 1.58034 nm^(−1/2).
- At theta = 91.323 °, the same fixed-parameter model gives Y₁ = 1.31822 nm^(−1/2).
- Signed change: ΔY_out = Y₁ − Y₀ = -0.262122 nm^(−1/2); the input change is Δtheta = 15.2145 °.
- Average sensitivity is ΔY_out/Δtheta = -0.0172284 nm^(−1/2) per °. Its negative sign means that this response decreases over the specified interval.
- This is a secant slope between two settings. It equals a point derivative only for a locally linear relation or in a justified small-interval limit.
Answer Y₀ = 1.58034 nm^(−1/2), Y₁ = 1.31822 nm^(−1/2); ΔY_out = -0.262122 nm^(−1/2); average sensitivity = -0.0172284 nm^(−1/2) per °.
Compare the assumptions, method and units with your attempt. Explain any correction before trying another question.
Extra challenging questions
Work through the linked stages and explain what each result means.
The position representation: ψ(x) = ⟨x|ψ⟩: infer an input with uncertainty
The position representation expands a state in the eigenkets |x⟩ of the position operator, a basis with a continuous label normalised as ⟨x|x′⟩ = δ(x − x′), so that the wavefunction ψ(x) = ⟨x|ψ⟩ is the component of |ψ⟩ along |x⟩. Use the supplied topic model Y_out = sqrt(2)*(cosd(theta)*sin(pi*xp)+sind(theta)*sin(2*pi*xp)), where Y_out is POSITION COMPONENT ψ(x₀) (nm^(−1/2)). The output symbol Y_out is distinct from all input symbols. ^ denotes a power; π = 3.141592653589793; sqrt(x) means √x; sind/cosd/tand use degrees; unmarked trigonometric functions use radians. theta is Mixing angle θ of |1⟩ and |2⟩, measured in °. Keep nb (Number of bins N) = 8; xp (Probe position x₀) = 0.3 nm fixed. For this exercise use only 52.2 ° ≤ theta ≤ 160.875 ° on this continuous branch. A grid of test settings is an input restriction for this exercise, not by itself a claim of physical quantisation. This calculation retains the topic model's stated assumptions; it does not extend them to a different physical setup. A reading gives Y_out = 0.63631536 nm^(−1/2), with a bounded reading uncertainty ±0.16915 nm^(−1/2). Treat the model and all fixed inputs as exact for this exercise; the bound is not a standard deviation. First estimate the unknown theta by straight-line interpolation between the two endpoint predictions. Then solve the actual model on the stated branch to an absolute input tolerance of 0.0108675 °, using a numerical bracket if required. Find the full input interval consistent with the bounded reading uncertainty and compare it with a symmetric input-error assumption. Do not assume that the supplied calibration points prove monotonicity outside this branch.
POSITION COMPONENT ψ(x₀) against theta
Predictions from the supplied The position representation: ψ(x) = ⟨x|ψ⟩ model, with all other inputs fixed as stated in the question. 5 calibration points support a trend comparison; they are not measured data or a straight-line law.
Vertical axis (y): POSITION COMPONENT ψ(x₀) / nm^(−1/2)
Horizontal axis (x): theta: Mixing angle θ of |1⟩ and |2⟩ / °
Axes show the supplied calibration interval; the origin may lie outside the plot.
Read the exact data as a table
| theta: Mixing angle θ of |1⟩ and |2⟩ / ° | POSITION COMPONENT ψ(x₀) / nm^(−1/2) |
|---|---|
| 52.199999999999996 | 1.7639970779061793 |
| 79.36875 | 1.5329862567686225 |
| 106.5375 | 0.9636930422825609 |
| 133.70625 | 0.18174274273963525 |
| 160.875 | -0.6403125282054984 |
These are the supplied values, not a worked answer. No fitted trend is drawn.
Show a hint
The model's endpoint predictions bracket the reading. Solve Y_out(theta) = Y_measured and then solve again at Y_measured − u and Y_measured + u; a nonlinear inverse can give unequal input deviations.
Reveal the worked solution
- Endpoint predictions are Y_out(52.2) = 1.764 nm^(−1/2) and Y_out(160.875) = -0.640313 nm^(−1/2). The target lies between them. On the selected branch the response is strictly decreasing.
- Endpoint interpolation gives theta_line = 52.2 + (0.636315 − 1.764)(160.875 − 52.2)/(-0.640313 − 1.764) = 103.171 °. This estimates the inverse of the endpoint chord, rather than assuming that chord is the physical model.
- Use the actual equation, not the chord, and bracket its root. Evaluating the midpoint and retaining the sign-changing half-interval gives theta = 118.492 °; substituting back gives Y_out = 0.63631536 nm^(−1/2).
- The interpolation error in the input is |theta_line − theta| = 15.3204 °. This is larger than the requested numerical tolerance.
- Invert both limits Y_out = 0.46716536 nm^(−1/2) and 0.80546536 nm^(−1/2). Sort the roots even if the response decreases: 112.475 ° ≤ theta ≤ 124.273 °.
- The lower and upper input deviations are 6.0168 ° and 5.78104 °. They differ because the inverse response is nonlinear; a symmetric error bar would only be a local approximation. This interval includes reading uncertainty only, not model error or uncertainty in fixed inputs.
Answer theta ≈ 118.492 °; compatible interval [112.475, 124.273] °. Endpoint interpolation gives 103.171 °.
Compare the assumptions, method and units with your attempt. Explain any correction before trying another question.