University Quantum Mechanics I · unit UQMI-04
Measurement and the Uncertainty Principle
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.
Extra challenging questions
A selection from this pathway. Open a topic below for its complete practice and challenge set.
Observables, Hermitian operators, and real eigenvalues: infer an input with uncertainty
An observable is represented by a Hermitian operator Q̂: one for which ∫ ψ*(Q̂ψ) dx is real for every allowed state ψ, which is the same as ∫ f*(Q̂g) dx = ∫ (Q̂f)* g dx for every allowed pair f and g. Use the supplied topic model Y_out = sqrt(max(-1-b*c,0)), where Y_out is IM PART OF EIGENVALUES, ± (q₀). The output symbol Y_out is distinct from all input symbols. ^ denotes a power; π = 3.141592653589793; sqrt(x) means √x; max(a,b) selects the larger value. c is Reverse coupling Q₂₁, measured in q₀. Keep b (Coupling Q₁₂) = 1 q₀; phi (Relative phase φ) = 90 ° fixed. For this exercise use only -1.575 q₀ ≤ c ≤ -1.05 q₀ 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 = 0.50472765 q₀, with a bounded reading uncertainty ±0.038034 q₀. Treat the model and all fixed inputs as exact for this exercise; the bound is not a standard deviation. First estimate the unknown c by straight-line interpolation between the two endpoint predictions. Then solve the actual model on the stated branch to an absolute input tolerance of 0.0000525 q₀, 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.
IM PART OF EIGENVALUES, ± against c
Predictions from the supplied Observables, Hermitian operators, and real eigenvalues 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): IM PART OF EIGENVALUES, ± / q₀
Horizontal axis (x): c: Reverse coupling Q₂₁ / q₀
Axes show the supplied calibration interval; the origin may lie outside the plot.
Read the exact data as a table
| c: Reverse coupling Q₂₁ / q₀ | IM PART OF EIGENVALUES, ± / q₀ |
|---|---|
| -1.575 | 0.758287544405155 |
| -1.44375 | 0.6661456297237115 |
| -1.3125 | 0.5590169943749475 |
| -1.18125 | 0.42573465914815994 |
| -1.05 | 0.22360679774997907 |
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(c) = 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(-1.575) = 0.758288 q₀ and Y_out(-1.05) = 0.223607 q₀. The target lies between them. On the selected branch the response is strictly decreasing.
- Endpoint interpolation gives c_line = -1.575 + (0.504728 − 0.758288)(-1.05 − -1.575)/(0.223607 − 0.758288) = -1.32603 q₀. 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 c = -1.25475 q₀; substituting back gives Y_out = 0.50472765 q₀.
- The interpolation error in the input is |c_line − c| = 0.071281 q₀. This is larger than the requested numerical tolerance.
- Invert both limits Y_out = 0.46669365 q₀ and 0.54276165 q₀. Sort the roots even if the response decreases: -1.29459 q₀ ≤ c ≤ -1.2178 q₀.
- The lower and upper input deviations are 0.0398402 q₀ and 0.036947 q₀. 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 c ≈ -1.25475 q₀; compatible interval [-1.29459, -1.2178] q₀. Endpoint interpolation gives -1.32603 q₀.
Compare the assumptions, method and units with your attempt. Explain any correction before trying another question.
Eigenvalue equations and determinate states: infer an input with uncertainty
A determinate state of an observable Q is a normalisable state in which every ideal measurement of Q returns the same value q; such states are exactly the allowed eigenfunctions of Q̂, and q is the eigenvalue. Use the supplied topic model Y_out = (1/r^2 + r^2)/4 + d^2/2, where Y_out is MEAN ⟨E⟩ (ħω). The output symbol Y_out is distinct from all input symbols. ^ denotes a power; π = 3.141592653589793. r is Trial width s, measured in b. Keep d (Centre offset x₀) = 0 b fixed. For this exercise use only 0.659375 b ≤ r ≤ 0.974375 b 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 = 0.5260562 ħω, with a bounded reading uncertainty ±0.0038073 ħω. Treat the model and all fixed inputs as exact for this exercise; the bound is not a standard deviation. First estimate the unknown r by straight-line interpolation between the two endpoint predictions. Then solve the actual model on the stated branch to an absolute input tolerance of 0.0000315 b, 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.
MEAN ⟨E⟩ against r
Predictions from the supplied Eigenvalue equations and determinate states 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): MEAN ⟨E⟩ / ħω
Horizontal axis (x): r: Trial width s / b
Axes show the supplied calibration interval; the origin may lie outside the plot.
Read the exact data as a table
| r: Trial width s / b | MEAN ⟨E⟩ / ħω |
|---|---|
| 0.659375 | 0.6837033937131669 |
| 0.738125 | 0.5950671056334456 |
| 0.816875 | 0.5414738327951809 |
| 0.895625 | 0.512200719018691 |
| 0.974375 | 0.5006740230959315 |
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(r) = 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(0.659375) = 0.683703 ħω and Y_out(0.974375) = 0.500674 ħω. The target lies between them. On the selected branch the response is strictly decreasing.
- Endpoint interpolation gives r_line = 0.659375 + (0.526056 − 0.683703)(0.974375 − 0.659375)/(0.500674 − 0.683703) = 0.930691 b. 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 r = 0.851525 b; substituting back gives Y_out = 0.5260562 ħω.
- The interpolation error in the input is |r_line − r| = 0.0791664 b. This is larger than the requested numerical tolerance.
- Invert both limits Y_out = 0.5222489 ħω and 0.5298635 ħω. Sort the roots even if the response decreases: 0.842011 b ≤ r ≤ 0.861903 b.
- The lower and upper input deviations are 0.00951368 b and 0.0103776 b. 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 r ≈ 0.851525 b; compatible interval [0.842011, 0.861903] b. Endpoint interpolation gives 0.930691 b.
Compare the assumptions, method and units with your attempt. Explain any correction before trying another question.
Eigenbasis expansion and the probability of an eigenvalue: infer an input with uncertainty
The probability that a measurement of Q returns the eigenvalue qₙ is |cₙ|², where cₙ = ∫ ψₙ* ψ dx is the overlap of the prepared state ψ with the normalised eigenfunction ψₙ of Q̂. Use the supplied topic model Y_out = (1/r)*(sin(pi*((p0*r)/r-p0)+0.000001)/(pi*((p0*r)/r-p0)+0.000001))^2, where Y_out is |cₙ|² NEAREST p₀ (before display rounding) (dimensionless). The output symbol Y_out is distinct from all input symbols. ^ denotes a power; π = 3.141592653589793; unmarked trigonometric functions use radians. r is Ring circumference L, measured in w. Keep p0 (Mean momentum p₀) = 0.25 h/w fixed. For this exercise use only 2.6375 w ≤ r ≤ 7.3625 w 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 = 0.18116763, with a bounded reading uncertainty ±0.0068016. Treat the model and all fixed inputs as exact for this exercise; the bound is not a standard deviation. First estimate the unknown r by straight-line interpolation between the two endpoint predictions. Then solve the actual model on the stated branch to an absolute input tolerance of 0.0004725 w, 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.
|cₙ|² NEAREST p₀ (before display rounding) against r
Predictions from the supplied Eigenbasis expansion and the probability of an eigenvalue 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): |cₙ|² NEAREST p₀ (before display rounding)
Horizontal axis (x): r: Ring circumference L / w
Axes show the supplied calibration interval; the origin may lie outside the plot.
Read the exact data as a table
| r: Ring circumference L / w | |cₙ|² NEAREST p₀ (before display rounding) |
|---|---|
| 2.6375 | 0.37914691943115325 |
| 3.81875 | 0.26186579378060015 |
| 5 | 0.19999999999993334 |
| 6.18125 | 0.16177957532856083 |
| 7.3625 | 0.13582342954155066 |
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(r) = 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(2.6375) = 0.379147 and Y_out(7.3625) = 0.135823. The target lies between them. On the selected branch the response is strictly decreasing.
- Endpoint interpolation gives r_line = 2.6375 + (0.181168 − 0.379147)(7.3625 − 2.6375)/(0.135823 − 0.379147) = 6.48198 w. 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 r = 5.51975 w; substituting back gives Y_out = 0.18116763.
- The interpolation error in the input is |r_line − r| = 0.96223 w. This is larger than the requested numerical tolerance.
- Invert both limits Y_out = 0.17436603 and 0.18796923. Sort the roots even if the response decreases: 5.32002 w ≤ r ≤ 5.73506 w.
- The lower and upper input deviations are 0.19973 w and 0.215312 w. 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 r ≈ 5.51975 w; compatible interval [5.32002, 5.73506] w. Endpoint interpolation gives 6.48198 w.
Compare the assumptions, method and units with your attempt. Explain any correction before trying another question.
Collapse, repeatability, and the state after measurement: infer an input with uncertainty
The projection rule: a measurement that returns qₙ leaves the system in the matching eigenfunction ψₙ, or, when the result cannot tell several eigenfunctions apart, in the part of ψ it cannot exclude, renormalised. That state is where every later calculation starts. Use the supplied topic model Y_out = 1/(b-a-(sin(2*pi*b)-sin(2*pi*a))/(2*pi)), where Y_out is DENSITY RESCALE 1/P (dimensionless). The output symbol Y_out is distinct from all input symbols. ^ denotes a power; π = 3.141592653589793; unmarked trigonometric functions use radians. a is Band starts at a, measured in nm. Keep b (Band ends at b) = 0.75 nm fixed. For this exercise use only 0.0425 nm ≤ a ≤ 0.3575 nm 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 = 1.2005952, with a bounded reading uncertainty ±0.01501. Treat the model and all fixed inputs as exact for this exercise; the bound is not a standard deviation. First estimate the unknown a by straight-line interpolation between the two endpoint predictions. Then solve the actual model on the stated branch to an absolute input tolerance of 0.0000315 nm, 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.
DENSITY RESCALE 1/P against a
Predictions from the supplied Collapse, repeatability, and the state after measurement 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): DENSITY RESCALE 1/P
Horizontal axis (x): a: Band starts at a / nm
Axes show the supplied calibration interval; the origin may lie outside the plot.
Read the exact data as a table
| a: Band starts at a / nm | DENSITY RESCALE 1/P |
|---|---|
| 0.0425 | 1.1005317685018885 |
| 0.12125 | 1.1138811603477352 |
| 0.2 | 1.1620876497534212 |
| 0.27875 | 1.2706961182702252 |
| 0.3575 | 1.4795869522398626 |
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(a) = 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(0.0425) = 1.10053 and Y_out(0.3575) = 1.47959. The target lies between them. On the selected branch the response is strictly increasing.
- Endpoint interpolation gives a_line = 0.0425 + (1.2006 − 1.10053)(0.3575 − 0.0425)/(1.47959 − 1.10053) = 0.125654 nm. 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 a = 0.23465 nm; substituting back gives Y_out = 1.2005952.
- The interpolation error in the input is |a_line − a| = 0.108996 nm. This is larger than the requested numerical tolerance.
- Invert both limits Y_out = 1.1855852 and 1.2156052. Sort the roots even if the response decreases: 0.222469 nm ≤ a ≤ 0.245614 nm.
- The lower and upper input deviations are 0.0121807 nm and 0.0109639 nm. 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 a ≈ 0.23465 nm; compatible interval [0.222469, 0.245614] nm. Endpoint interpolation gives 0.125654 nm.
Compare the assumptions, method and units with your attempt. Explain any correction before trying another question.