SARG04 Protocol: PNS-Resistant Quantum Key Distribution

Channel Controls

Emission rate 4.0 /s

Channel loss 10%

Live Statistics

Photons sent

Detected

Sifted key bits

Sifting efficiency

0.0%

Quantum bit error rate (QBER)

0.0%

How it works

SARG04 (Scarani–Acín–Ribordy–Gisin, 2004) reuses the same four BB84 polarization states — |0⟩, |1⟩ (rectilinear basis Z) and |+⟩, |−⟩ (diagonal basis X) — but changes what Alice reveals during sifting.

|+⟩ = (|0⟩+|1⟩)/√2   |−⟩ = (|0⟩−|1⟩)/√2
Pairs (both members share one logical bit):
  bit 0 → {|0⟩,|+⟩}  or  {|0⟩,|−⟩}
  bit 1 → {|1⟩,|+⟩}  or  {|1⟩,|−⟩}

Alice sends one state at random. Instead of announcing her basis (BB84), she announces one of the two non-orthogonal pairs that contains her state. Bob's random measurement basis always matches exactly one pair member:

If that member's basis equals Bob's and his outcome is the state orthogonal to it, he has ruled that member out — by elimination the sent state was the other member, so he reads off its bit unambiguously.
Otherwise the round is inconclusive and discarded (sifting efficiency ≈ 25%, same order as BB84's own 50% × 50%).

Because Eve never learns which pair Alice will announce, and the two states in a pair are only 45° apart on the Bloch sphere (Born-rule overlap |⟨+|0⟩|² = 0.5), an intercept-resend attack disturbs the state before the pair is even known — this is what makes SARG04 more resistant than plain BB84 to photon-number-splitting attacks on weak coherent-pulse sources, at the cost of a lower raw key rate. Toggle the eavesdropper above to watch the QBER rise as Eve's guessed basis mismatches Alice's ~50% of the time.

Z-basis photon (|0⟩/|1⟩) X-basis photon (|+⟩/|−⟩) Sifted — kept Discarded / lost

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