University Physics V · topic UPV-20.08
Quantum cryptography: BB84, E91, and their assumptions
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.
Quantum cryptography: BB84 and E91: predict a finite change
Quantum key distribution grows a shared secret key between two parties over a public quantum channel, because an eavesdropper who measures the non-orthogonal signal states necessarily raises an error rate the parties can detect and bound. Use the supplied topic model Y_out = 1-(1+c)*(-(max((2*q+f*(50-q))/200,1e-4)*ln(max((2*q+f*(50-q))/200,1e-4))+(1-max((2*q+f*(50-q))/200,1e-4))*ln(1-max((2*q+f*(50-q))/200,1e-4)))/ln(2)), where Y_out is KEY RATE r (BB84) (per sifted bit). The output symbol Y_out is distinct from all input symbols. ^ denotes a power; π = 3.141592653589793; ln(x) means the natural logarithm; max(a,b) selects the larger value. f is Fraction intercepted by Eve, dimensionless. Keep q (Channel's own error rate) = 2 %; c (Error-correction cost c) = 1 × h(Q) fixed. For this exercise use only 0.10625 ≤ f ≤ 0.89375 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 f = 0.2795 and 0.38975. Find the signed output change ΔY_out and the average sensitivity ΔY_out/Δf across this interval. State whether the response increases or decreases; distinguish this interval average from a derivative at one point.
KEY RATE r (BB84) against f
Predictions from the supplied Quantum cryptography: BB84 and E91 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): KEY RATE r (BB84) / per sifted bit
Horizontal axis (x): f: Fraction intercepted by Eve
Axes show the supplied calibration interval; the origin may lie outside the plot.
Read the exact data as a table
| f: Fraction intercepted by Eve | KEY RATE r (BB84) / per sifted bit |
|---|---|
| 0.10625 | 0.4660707914880242 |
| 0.303125 | 0.10883409465323679 |
| 0.5 | -0.16847762328571192 |
| 0.6968749999999999 | -0.3913661895641982 |
| 0.8937499999999999 | -0.5715474137122871 |
These are the supplied values, not a worked answer. No fitted trend is drawn.
Show a hint
Substitute each value of f separately while keeping every other input fixed, then subtract in the same order.
Reveal the worked solution
- At f = 0.2795, substitute into Y_out = 1-(1+c)*(-(max((2*q+f*(50-q))/200,1e-4)*ln(max((2*q+f*(50-q))/200,1e-4))+(1-max((2*q+f*(50-q))/200,1e-4))*ln(1-max((2*q+f*(50-q))/200,1e-4)))/ln(2)): Y₀ = 0.146705 per sifted bit.
- At f = 0.38975, the same fixed-parameter model gives Y₁ = -0.0210044 per sifted bit.
- Signed change: ΔY_out = Y₁ − Y₀ = -0.16771 per sifted bit; the input change is Δf = 0.11025.
- Average sensitivity is ΔY_out/Δf = -1.52118 per sifted bit per unit input. 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 = 0.146705 per sifted bit, Y₁ = -0.0210044 per sifted bit; ΔY_out = -0.16771 per sifted bit; average sensitivity = -1.52118 per sifted bit per unit input.
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.
Quantum cryptography: BB84 and E91: infer an input with uncertainty
Quantum key distribution grows a shared secret key between two parties over a public quantum channel, because an eavesdropper who measures the non-orthogonal signal states necessarily raises an error rate the parties can detect and bound. Use the supplied topic model Y_out = 1-(1+c)*(-(max((2*q+f*(50-q))/200,1e-4)*ln(max((2*q+f*(50-q))/200,1e-4))+(1-max((2*q+f*(50-q))/200,1e-4))*ln(1-max((2*q+f*(50-q))/200,1e-4)))/ln(2)), where Y_out is KEY RATE r (BB84) (per sifted bit). The output symbol Y_out is distinct from all input symbols. ^ denotes a power; π = 3.141592653589793; ln(x) means the natural logarithm; max(a,b) selects the larger value. f is Fraction intercepted by Eve, dimensionless. Keep q (Channel's own error rate) = 2 %; c (Error-correction cost c) = 1 × h(Q) fixed. For this exercise use only 0.10625 ≤ f ≤ 0.89375 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 = -0.27239228 per sifted bit, with a bounded reading uncertainty ±0.044873 per sifted bit. Treat the model and all fixed inputs as exact for this exercise; the bound is not a standard deviation. First estimate the unknown f by straight-line interpolation between the two endpoint predictions. Then solve the actual model on the stated branch to an absolute input tolerance of 0.00007875, 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.
KEY RATE r (BB84) against f
Predictions from the supplied Quantum cryptography: BB84 and E91 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): KEY RATE r (BB84) / per sifted bit
Horizontal axis (x): f: Fraction intercepted by Eve
Axes show the supplied calibration interval; the origin may lie outside the plot.
Read the exact data as a table
| f: Fraction intercepted by Eve | KEY RATE r (BB84) / per sifted bit |
|---|---|
| 0.10625 | 0.4660707914880242 |
| 0.303125 | 0.10883409465323679 |
| 0.5 | -0.16847762328571192 |
| 0.6968749999999999 | -0.3913661895641982 |
| 0.8937499999999999 | -0.5715474137122871 |
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(f) = 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(0.10625) = 0.466071 per sifted bit and Y_out(0.89375) = -0.571547 per sifted bit. The target lies between them. On the selected branch the response is strictly decreasing.
- Endpoint interpolation gives f_line = 0.10625 + (-0.272392 − 0.466071)(0.89375 − 0.10625)/(-0.571547 − 0.466071) = 0.666706. 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 f = 0.586625; substituting back gives Y_out = -0.27239228 per sifted bit.
- The interpolation error in the input is |f_line − f| = 0.0800813. This is larger than the requested numerical tolerance.
- Invert both limits Y_out = -0.31726528 per sifted bit and -0.22751928 per sifted bit. Sort the roots even if the response decreases: 0.548216 ≤ f ≤ 0.626692.
- The lower and upper input deviations are 0.0384094 and 0.0400666. 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 f ≈ 0.586625; compatible interval [0.548216, 0.626692]. Endpoint interpolation gives 0.666706.
Compare the assumptions, method and units with your attempt. Explain any correction before trying another question.