Prepare-and-measure QKD

BB84

Alice and Bob use random bases, discard mismatches, estimate QBER, then reconcile and privacy amplify the sifted key.

Core idea

Security intuition comes from nonorthogonal states: measurement in the wrong basis disturbs correlations and is visible as excess QBER under ideal assumptions.

Minimal mathematics

Sifted rate is approximately raw detections times the matching-basis probability. A common asymptotic teaching proxy is R >= q Q_mu [1 - f h2(E) - h2(E)].

Interactive demo

BB84 mini-demo

Eve/intercept fraction

Sifted bits

Demo QBER

0.0%

This deterministic teaching trace shows 16 transmissions, 12 sifted bits, and a demo QBER of 0.0%. It illustrates intercept-resend disturbance only and is not a finite-key or device-imperfection security proof.

Row-by-row BB84 sifting view showing Alice and Bob bases, kept bits, and whether the deterministic teaching trace produced an error.
i	Alice bit	Alice basis	Bob basis	Bob bit	Kept	Error
0	0	+	x	0	no	no
1	1	x	+	1	no	no
2	0	+	+	0	yes	no
3	1	x	x	1	yes	no
4	1	+	+	1	yes	no
5	0	x	x	0	yes	no
6	1	x	+	0	no	no
7	0	+	+	0	yes	no
8	1	x	x	1	yes	no
9	0	+	+	0	yes	no
10	0	x	x	0	yes	no
11	1	x	+	0	no	no
12	0	+	+	0	yes	no
13	1	x	x	1	yes	no
14	0	+	+	0	yes	no
15	0	x	x	0	yes	no

Guided lab

Worksheet mode captures observations from the BB84 mini-demo and exports them as reproducible notes.

Eve/intercept fraction

Intercepted pulses

2/16

Sifted key length

12 bits

Observed QBER

Step 1

Inspect Alice and Bob basis choices and mark which rows survive sifting.

Step 2

Compare the kept rows only and note which ones turn into visible bit errors.

Step 3

Explain how the observed QBER changes when Eve touches a larger fraction of pulses.

The worksheet table below records the same 16-bit teaching trace used to compute 2/16 sifted bits and a 0.0% QBER proxy. Use it to justify each worksheet answer from visible evidence rather than from generalized security claims.

BB84 worksheet trace with row-level intercept flags, sifted-bit retention, and error outcomes for the current teaching preset.
i	Alice bit	Alice basis	Bob basis	Bob bit	Eve touched	Kept	Error
0	0	+	x	0	yes	no	no
1	1	x	+	1	yes	no	no
2	0	+	+	0	no	yes	no
3	1	x	x	1	no	yes	no
4	1	+	+	1	no	yes	no
5	0	x	x	0	no	yes	no
6	1	x	+	0	no	no	no
7	0	+	+	0	no	yes	no
8	1	x	x	1	no	yes	no
9	0	+	+	0	no	yes	no
10	0	x	x	0	no	yes	no
11	1	x	+	0	no	no	no
12	0	+	+	0	no	yes	no
13	1	x	x	1	no	yes	no
14	0	+	+	0	no	yes	no
15	0	x	x	0	no	yes	no

Use this worksheet to build intuition about disturbance and sifting. It does not certify secure key generation or model real QKD hardware in detail.

Worksheet mode

Capture observations, then export the answers as reproducible JSON or Markdown for lab handoff.

Question 1

Which rows survive basis sifting, and why are the others discarded before key generation?

Expected observation: Exactly 12 rows remain because only matching-basis rows contribute to the sifted key in this teaching run.

Your answer for question 1Keep the answer evidence-based and tied to the displayed metrics or table rather than claiming certified security.

Question 2

What evidence of disturbance appears after sifting when Eve intercepts part of the traffic?

Expected observation: The kept rows show about 0% QBER, so disturbance becomes visible only after Alice and Bob compare a sample of their sifted outcomes.

Your answer for question 2Keep the answer evidence-based and tied to the displayed metrics or table rather than claiming certified security.

Question 3

Why is this lab still only an educational model rather than a certified BB84 security analysis?

Expected observation: The worksheet ignores finite-key proof terms, detector side channels, authentication costs, and calibrated hardware imperfections beyond the simple disturbance story.

Your answer for question 3Keep the answer evidence-based and tied to the displayed metrics or table rather than claiming certified security.

Typical assumptions

Authenticated public discussion
Calibrated sources and detectors
Finite-key terms are available in the dedicated finite-key BB84 teaching estimator and omitted from the asymptotic MVP pages

Common failure modes

High QBER
Detector side channels
Weak coherent pulse multi-photon risk without decoys
Authentication failure

References

Bennett and Brassard, 1984
ETSI ISG QKD publications

Learn

Protocol explainers and guided context

Loading protocol summaries, references, and related workbench links.

Preparing learning view

Loading workbench view

Preparing calibrated controls, default presets, and explanatory notes.

BB84 mini-demo

Eve/intercept fraction

Sifted bits

Demo QBER

0.0%

Row-by-row BB84 sifting view showing Alice and Bob bases, kept bits, and whether the deterministic teaching trace produced an error.
i	Alice bit	Alice basis	Bob basis	Bob bit	Kept	Error
0	0	+	x	0	no	no
1	1	x	+	1	no	no
2	0	+	+	0	yes	no
3	1	x	x	1	yes	no
4	1	+	+	1	yes	no
5	0	x	x	0	yes	no
6	1	x	+	0	no	no
7	0	+	+	0	yes	no
8	1	x	x	1	yes	no
9	0	+	+	0	yes	no
10	0	x	x	0	yes	no
11	1	x	+	0	no	no
12	0	+	+	0	yes	no
13	1	x	x	1	yes	no
14	0	+	+	0	yes	no
15	0	x	x	0	yes	no

BB84 worksheet trace with row-level intercept flags, sifted-bit retention, and error outcomes for the current teaching preset.

Alice bit

Alice basis

Bob basis

Bob bit

Eve touched

Kept

Error

yes

BB84

Core idea

Minimal mathematics

Interactive demo

BB84 mini-demo

Guided lab

Worksheet mode

Typical assumptions

Common failure modes

Related tools

References

Protocol explainers and guided context

Preparing learning view

BB84

Core idea

Minimal mathematics

Interactive demo

BB84 mini-demo

Guided lab

Worksheet mode

Typical assumptions

Common failure modes

Related tools

References

i	Alice bit	Alice basis	Bob basis	Bob bit	Kept	Error
0	0	+	x	0	no	no
1	1	x	+	1	no	no
2	0	+	+	0	yes	no
3	1	x	x	1	yes	no
4	1	+	+	1	yes	no
5	0	x	x	0	yes	no
6	1	x	+	0	no	no
7	0	+	+	0	yes	no
8	1	x	x	1	yes	no
9	0	+	+	0	yes	no
10	0	x	x	0	yes	no
11	1	x	+	0	no	no
12	0	+	+	0	yes	no
13	1	x	x	1	yes	no
14	0	+	+	0	yes	no
15	0	x	x	0	yes	no

i	Alice bit	Alice basis	Bob basis	Bob bit	Eve touched	Kept	Error
0	0	+	x	0	yes	no	no
1	1	x	+	1	yes	no	no
2	0	+	+	0	no	yes	no
3	1	x	x	1	no	yes	no
4	1	+	+	1	no	yes	no
5	0	x	x	0	no	yes	no
6	1	x	+	0	no	no	no
7	0	+	+	0	no	yes	no
8	1	x	x	1	no	yes	no
9	0	+	+	0	no	yes	no
10	0	x	x	0	no	yes	no
11	1	x	+	0	no	no	no
12	0	+	+	0	no	yes	no
13	1	x	x	1	no	yes	no
14	0	+	+	0	no	yes	no
15	0	x	x	0	no	yes	no

i	Alice bit	Alice basis	Bob basis	Bob bit	Kept	Error
0	0	+	x	0	no	no
1	1	x	+	1	no	no
2	0	+	+	0	yes	no
3	1	x	x	1	yes	no
4	1	+	+	1	yes	no
5	0	x	x	0	yes	no
6	1	x	+	0	no	no
7	0	+	+	0	yes	no
8	1	x	x	1	yes	no
9	0	+	+	0	yes	no
10	0	x	x	0	yes	no
11	1	x	+	0	no	no
12	0	+	+	0	yes	no
13	1	x	x	1	yes	no
14	0	+	+	0	yes	no
15	0	x	x	0	yes	no

i	Alice bit	Alice basis	Bob basis	Bob bit	Eve touched	Kept	Error
0	0	+	x	0	yes	no	no
1	1	x	+	1	yes	no	no
2	0	+	+	0	no	yes	no
3	1	x	x	1	no	yes	no
4	1	+	+	1	no	yes	no
5	0	x	x	0	no	yes	no
6	1	x	+	0	no	no	no
7	0	+	+	0	no	yes	no
8	1	x	x	1	no	yes	no
9	0	+	+	0	no	yes	no
10	0	x	x	0	no	yes	no
11	1	x	+	0	no	no	no
12	0	+	+	0	no	yes	no
13	1	x	x	1	no	yes	no
14	0	+	+	0	no	yes	no
15	0	x	x	0	no	yes	no

i	Alice bit	Alice basis	Bob basis	Bob bit	Kept	Error
0	0	+	x	0	no	no
1	1	x	+	1	no	no
2	0	+	+	0	yes	no
3	1	x	x	1	yes	no
4	1	+	+	1	yes	no
5	0	x	x	0	yes	no
6	1	x	+	0	no	no
7	0	+	+	0	yes	no
8	1	x	x	1	yes	no
9	0	+	+	0	yes	no
10	0	x	x	0	yes	no
11	1	x	+	0	no	no
12	0	+	+	0	yes	no
13	1	x	x	1	yes	no
14	0	+	+	0	yes	no
15	0	x	x	0	yes	no

i	Alice bit	Alice basis	Bob basis	Bob bit	Eve touched	Kept	Error
0	0	+	x	0	yes	no	no
1	1	x	+	1	yes	no	no
2	0	+	+	0	no	yes	no
3	1	x	x	1	no	yes	no
4	1	+	+	1	no	yes	no
5	0	x	x	0	no	yes	no
6	1	x	+	0	no	no	no
7	0	+	+	0	no	yes	no
8	1	x	x	1	no	yes	no
9	0	+	+	0	no	yes	no
10	0	x	x	0	no	yes	no
11	1	x	+	0	no	no	no
12	0	+	+	0	no	yes	no
13	1	x	x	1	no	yes	no
14	0	+	+	0	no	yes	no
15	0	x	x	0	no	yes	no

i	Alice bit	Alice basis	Bob basis	Bob bit	Kept	Error
0	0	+	x	0	no	no
1	1	x	+	1	no	no
2	0	+	+	0	yes	no
3	1	x	x	1	yes	no
4	1	+	+	1	yes	no
5	0	x	x	0	yes	no
6	1	x	+	0	no	no
7	0	+	+	0	yes	no
8	1	x	x	1	yes	no
9	0	+	+	0	yes	no
10	0	x	x	0	yes	no
11	1	x	+	0	no	no
12	0	+	+	0	yes	no
13	1	x	x	1	yes	no
14	0	+	+	0	yes	no
15	0	x	x	0	yes	no

i	Alice bit	Alice basis	Bob basis	Bob bit	Eve touched	Kept	Error
0	0	+	x	0	yes	no	no
1	1	x	+	1	yes	no	no
2	0	+	+	0	no	yes	no
3	1	x	x	1	no	yes	no
4	1	+	+	1	no	yes	no
5	0	x	x	0	no	yes	no
6	1	x	+	0	no	no	no
7	0	+	+	0	no	yes	no
8	1	x	x	1	no	yes	no
9	0	+	+	0	no	yes	no
10	0	x	x	0	no	yes	no
11	1	x	+	0	no	no	no
12	0	+	+	0	no	yes	no
13	1	x	x	1	no	yes	no
14	0	+	+	0	no	yes	no
15	0	x	x	0	no	yes	no

i	Alice bit	Alice basis	Bob basis	Bob bit	Kept	Error
0	0	+	x	0	no	no
1	1	x	+	1	no	no
2	0	+	+	0	yes	no
3	1	x	x	1	yes	no
4	1	+	+	1	yes	no
5	0	x	x	0	yes	no
6	1	x	+	0	no	no
7	0	+	+	0	yes	no
8	1	x	x	1	yes	no
9	0	+	+	0	yes	no
10	0	x	x	0	yes	no
11	1	x	+	0	no	no
12	0	+	+	0	yes	no
13	1	x	x	1	yes	no
14	0	+	+	0	yes	no
15	0	x	x	0	yes	no

i	Alice bit	Alice basis	Bob basis	Bob bit	Eve touched	Kept	Error
0	0	+	x	0	yes	no	no
1	1	x	+	1	yes	no	no
2	0	+	+	0	no	yes	no
3	1	x	x	1	no	yes	no
4	1	+	+	1	no	yes	no
5	0	x	x	0	no	yes	no
6	1	x	+	0	no	no	no
7	0	+	+	0	no	yes	no
8	1	x	x	1	no	yes	no
9	0	+	+	0	no	yes	no
10	0	x	x	0	no	yes	no
11	1	x	+	0	no	no	no
12	0	+	+	0	no	yes	no
13	1	x	x	1	no	yes	no
14	0	+	+	0	no	yes	no
15	0	x	x	0	no	yes	no

i	Alice bit	Alice basis	Bob basis	Bob bit	Kept	Error
0	0	+	x	0	no	no
1	1	x	+	1	no	no
2	0	+	+	0	yes	no
3	1	x	x	1	yes	no
4	1	+	+	1	yes	no
5	0	x	x	0	yes	no
6	1	x	+	0	no	no
7	0	+	+	0	yes	no
8	1	x	x	1	yes	no
9	0	+	+	0	yes	no
10	0	x	x	0	yes	no
11	1	x	+	0	no	no
12	0	+	+	0	yes	no
13	1	x	x	1	yes	no
14	0	+	+	0	yes	no
15	0	x	x	0	yes	no

i	Alice bit	Alice basis	Bob basis	Bob bit	Eve touched	Kept	Error
0	0	+	x	0	yes	no	no
1	1	x	+	1	yes	no	no
2	0	+	+	0	no	yes	no
3	1	x	x	1	no	yes	no
4	1	+	+	1	no	yes	no
5	0	x	x	0	no	yes	no
6	1	x	+	0	no	no	no
7	0	+	+	0	no	yes	no
8	1	x	x	1	no	yes	no
9	0	+	+	0	no	yes	no
10	0	x	x	0	no	yes	no
11	1	x	+	0	no	no	no
12	0	+	+	0	no	yes	no
13	1	x	x	1	no	yes	no
14	0	+	+	0	no	yes	no
15	0	x	x	0	no	yes	no