qkd

A QKD protocol simulator with the hardware in the loop, over a native Rust core.

Documentation: https://plutoniumm.github.io/qkd/

Excess noise ξ is an output: qkd.budget assembles it from hardware parameters. On a Gaussian-modulation q.Link, pilot-assisted DSP runs over simulated symbols, and its residual phase error is a ξ term. Once frames are estimated, the key rate comes from Alice and Bob's estimates, not from the simulated channel.

Install

pip install qkd

• Python 3.11 or later. numpy is a runtime dependency.
• Wheels: macOS arm64 and x86_64, Linux manylinux_2_28 aarch64 and x86_64, Windows x86_64. Other platforms install from the sdist, which needs a Rust toolchain.
• From source: Getting Started.

Example

Neither T nor ξ is stated. The pilot DSP runs over 200 000 simulated symbols; its residual phase error is the only ξ row.

import qkd as q

link = q.Link(
    modulation=q.GaussianModulation(v_a=4.0),
    channel=q.Fiber(length=15.0, alpha=0.2),
    alice=q.Alice(
        laser=q.Laser(linewidth=10e3),
        pilots=q.Pilots(power_db=12.0, freq=180e6),
        symbol_rate=100e6,
    ),
    bob=q.Bob(
        detector=q.Heterodyne(eta=0.6, v_el=0.1, trusted=True),
        lo=q.LocalLO(linewidth=10e3),
    ),
    dsp=q.DSP(phase=q.PilotPhase(), block=32),
    security=q.FiniteSize(beta=0.95, n=1e9),
)
res = link.run(symbols=200_000, seed=7)

res.dsp.v_err       # 0.0075742    rad^2, what the pilot estimator left
res.budget.total    # 0.0304118    SNU at the channel input, derived from it
res.est.xi          # 0.0142987    what Alice and Bob estimate from the samples
res.key_rate        # 0.0285553    bit/symbol

One block per protocol family and per layer: Usage by example.

Coverage

• Continuous variable: Gaussian modulation (homodyne, heterodyne), M-PSK discrete modulation, CV-MDI.
• Discrete variable: BB84 with weak coherent pulses and decoy states, six-state, SARG04, B92, DPS, RRDPS, COW, COW′, MDI-BB84, mode pairing, BBM92, E91, loss-tolerant source flaws.
• Beside the protocols: trusted-node networks (q.Network), Gaussian and truncated-Fock states (qkd.gaussian, qkd.fock), implementation attacks (qkd.attacks), reconciliation and privacy amplification (qkd.reconcile), composable ε accounting (qkd.security).
• Entry point and security analysis per family: Protocol coverage.
• Excluded by decision: twin-field, phase-matching and sending-or-not-sending; satellite and free-space channels; qudit protocols.

Before quoting a number

• qkd.gaussian and qkd.fock use ħ = 1, vacuum variance 1/2. qkd.budget, q.Link and q.Swap use shot-noise units, vacuum variance 1. Conventions
• thermal_loss and a nonzero q.Channel(xi=…) require ref="input" or ref="output". ξ at Bob is T times ξ at the channel input.
• q.Homodyne and q.Heterodyne default to trusted=True: eta and v_el are Bob's, not Eve's. Security
• A mismatched q.Link tree constructs, then raises on the first run(), measure(), claim() or explain(), naming the restriction. Refusals
• Most q.Link families are asymptotic. Finite-key routes per family: Security.
• Agreement with a paper is graded on two axes: reproducing what it computed, and reproducing what it measured at a stated number of fitted parameters. Validation
• The pilot DSP chain simulates heterodyne detection only.
• Every q.Link run is CPU. backend_info() reports the backend that resolved, not the one a run used. GPU kernels

License

MIT