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EntanglementConsumer, SwapperProt, EntanglementTracker, EntanglerProt might fail to correctly track entangled states across a repeater chain #303

@LBacciottini

Description

@LBacciottini

Describe the bug 🐞

Consider a repeater chain with two end nodes and more than one repeater. For example, consider repeater #2. Assume that the SwapperProt installed on that node just swapped slots 1 and 4 and saved two EntanglementHistory entries. Almost simultaneously, repeater #1 swapped slots 3 and 5.
Pre-swaps, slots 2.4 and 1.5 were entangled, so repeater #1 sends out an EntanglementUpdateX to repeater #2 regarding slot 2.4. Under normal circumstances, repeater #2 would map the EntanglementUpdateX message to the corresponding EntanglementHistory entry and correctly track the logical entangled state. However, the bug occurs if an EntanglerProt selects slot 2.2 and generates a new Bell pair in the time between the swap at repeater #2 and the delivery of EntanglementUpdate X to repeater #2. In this unfortunate case, the update will be mapped to the new EntanglementCounterpart entry available at repeater #2 regarding slot 2.2, and not to the EntanglementHistory entry, which would be the correct one. This creates a mismatch between the actual quantum state and the logical state of the simulation.

The runtime error manifests when the EntanglementConsumer tries to calculate the expectation value of observable X \otimes X on the end-to-end Bell pair. The expectation value returns nothing because the size of the quantum state of the two passed register slots is larger than a 2-qubit state (it's 4, because each of those slots is entangled to another slot somewhere in the network). Finally, when the EntanglementConsumer evaluates real(nothing), the error occurs.

Expected behavior

I would expect this mismatch not to happen: the simulator should be able to map every update to the actual Bell pair they refer to, or at least be able to detect the mismatch (which is usually very unlikely to happen).

Minimal Reproducible Example 👇

using QuantumSavory.ProtocolZoo
using QuantumSavory
using Graphs
import ConcurrentSim
using ConcurrentSim: @process, now, Process

function repeater_chain(nrepeaters::Int, nslots::Int; linklength::AbstractFloat=0.0, coherencetime::Union{AbstractFloat, Nothing}=nothing)

    speedOfLightFiber = 2e8 # km/s
    delay = linklength / speedOfLightFiber
    delay_us = delay * 1e6 # in microseconds

    graph = grid([nrepeaters + 2])
    if coherencetime === nothing
        noisemodel = nothing
        registers = vcat([Register(nslots*5)], [Register(nslots) for _ in 1:nrepeaters], [Register(nslots*5)])
    else
        noisemodel = Depolarization(coherencetime)
        registers = vcat([Register(nslots*5)], [Register(nslots, noisemodel) for _ in 1:nrepeaters], [Register(nslots*5)])
    end
    return RegisterNet(graph, registers; classical_delay=delay_us)
end



function get_entangler(sim, net, nodeA, nodeB, rate)
    return EntanglerProt(sim, net, nodeA, nodeB; rate=rate, rounds=-1, margin=1, retry_lock_time=nothing)
end

function get_swapper(sim, net, node)
    return SwapperProt(sim, net, node; nodeL= <(node), nodeH= >(node), retry_lock_time=nothing)
end


function get_tracker(sim, net, node)
    return EntanglementTracker(sim, net, node)
end


function get_consumer(sim, net, nodeA, nodeB)
    return EntanglementConsumer(sim, net, nodeA, nodeB, period=nothing)
end


function setup(nrepeaters::Int, nslots::Int, linkcapacity::AbstractFloat; linklength::AbstractFloat=0.0, coherencetime::Union{AbstractFloat, Nothing}=nothing, outfolder::String="./out/", outfile::Union{String, Nothing}=nothing, usetempfile::Bool=false)
    net = repeater_chain(nrepeaters, nslots; linklength=linklength, coherencetime=coherencetime)
    sim = get_time_tracker(net)

    # Setup an entanglement tracker at each node
    for node in 1:(nrepeaters+2)
        tracker = get_tracker(sim, net, node)
        @process tracker()
    end

    # Setup the entanglement swapping protocols at each repeater
    for node in 2:(nrepeaters+1)
        swapper = get_swapper(sim, net, node)
        @process swapper()
    end

    # Setup the entangler protocol at each link
    for node in 1:(nrepeaters+1)
        entangler = get_entangler(sim, net, node, node+1, linkcapacity)
        @process entangler()
    end

    # Setup consumers at the end nodes
    consumer = get_consumer(sim, net, 1, nrepeaters+2)
    @process consumer()

    return (sim, net)
end


sim, net = setup(2, 100, 10.0; linklength=20.0, coherencetime=1e6)
run(sim, 500)

Error & Stacktrace or other complete output produced by the MRE ⚠️

ERROR: MethodError: no method matching real(::Nothing)
The function `real` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  real(::Type{Union{}}, Any...)
   @ Base complex.jl:123
  real(::ChainRulesCore.ZeroTangent)
   @ ChainRulesCore ~/.julia/packages/ChainRulesCore/Vsbj9/src/tangent_arithmetic.jl:111
  real(::Nemo.ZZiRingElem)
   @ Nemo ~/.julia/packages/Nemo/SKc7w/src/gaussiannumbers/ZZi.jl:157
  ...

Stacktrace:
 [1] macro expansion
   @ ~/.julia/dev/QuantumSavory/src/ProtocolZoo/ProtocolZoo.jl:495 [inlined]
 [2] (::QuantumSavory.ProtocolZoo.var"##prot::EntanglementConsumer_FSMI#487"{})(_arg::Dict{…})
   @ QuantumSavory.ProtocolZoo ~/.julia/packages/ResumableFunctions/cprlE/src/macro.jl:461
 [3] execute(ev::ConcurrentSim.Operator, proc::Process)
   @ ConcurrentSim ~/.julia/packages/ConcurrentSim/V71SQ/src/processes.jl:32
 [4] (::ConcurrentSim.var"#append_callback##0#append_callback##1"{})()
   @ ConcurrentSim ~/.julia/packages/ConcurrentSim/V71SQ/src/base.jl:51
 [5] step(sim::ConcurrentSim.Simulation)
   @ ConcurrentSim ~/.julia/packages/ConcurrentSim/V71SQ/src/simulations.jl:39
 [6] run(env::ConcurrentSim.Simulation, until::ConcurrentSim.Timeout)
   @ ConcurrentSim ~/.julia/packages/ConcurrentSim/V71SQ/src/base.jl:91
 [7] run(env::ConcurrentSim.Simulation, until::Int64)
   @ ConcurrentSim ~/.julia/packages/ConcurrentSim/V71SQ/src/events.jl:29
 [8] top-level scope
   @ ~/JuliaProjects/QSavoryBug/src/setup.jl:78
Some type information was truncated. Use `show(err)` to see complete types.

Environment (please complete the following information):

  • Output of using Pkg; Pkg.status()
Project QSavoryBug v1.0.0-DEV
Status `~/JuliaProjects/QSavoryBug/Project.toml`
  [6ed1e86c] ConcurrentSim v1.5.0
⌃ [86223c79] Graphs v1.13.1
⌃ [7ed4a6bd] LinearSolve v3.49.0
  [2de2e421] QuantumSavory v0.6.0 `~/.julia/dev/QuantumSavory`
Info Packages marked with ⌃ have new versions available and may be upgradable.
  • Output of using Pkg; Pkg.status(; mode = PKGMODE_MANIFEST)
Project QSavoryBug v1.0.0-DEV
Status `~/JuliaProjects/QSavoryBug/Manifest.toml`
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  [b50d4dc9] QECCore v0.1.1
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  [6e0679c1] QuantumOptics v1.2.4
  [4f57444f] QuantumOpticsBase v0.5.9
  [2de2e421] QuantumSavory v0.6.0 `~/.julia/dev/QuantumSavory`
  [efa7fd63] QuantumSymbolics v0.4.11
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  [2efcf032] SymbolicIndexingInterface v0.3.46
⌃ [19f23fe9] SymbolicLimits v0.2.3
⌅ [d1185830] SymbolicUtils v3.32.0
⌅ [0c5d862f] Symbolics v6.58.0
  [3783bdb8] TableTraits v1.0.1
  [bd369af6] Tables v1.12.1
  [ed4db957] TaskLocalValues v0.1.3
  [8ea1fca8] TermInterface v2.0.0
  [8290d209] ThreadingUtilities v0.5.5
  [a759f4b9] TimerOutputs v0.5.29
  [3bb67fe8] TranscodingStreams v0.11.3
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  [9d95972d] TupleTools v1.6.0
  [a7c27f48] Unityper v0.1.6
  [c4a57d5a] UnsafeArrays v1.0.8
  [e17b2a0c] UnsafePointers v1.0.0
  [409d34a3] VectorInterface v0.5.0
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⌅ [68821587] Arpack_jll v3.5.1+1
  [6e34b625] Bzip2_jll v1.0.9+0
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  [4607b0f0] SuiteSparse
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  [8dfed614] Test v1.11.0
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Info Packages marked with ⌃ and ⌅ have new versions available. Those with ⌃ may be upgradable, but those with ⌅ are restricted by compatibility constraints from upgrading. To see why use `status --outdated -m`
  • Output of versioninfo()
Julia Version 1.12.1
Commit ba1e628ee49 (2025-10-17 13:02 UTC)
Build Info:
  Official https://julialang.org release
Platform Info:
  OS: macOS (arm64-apple-darwin24.0.0)
  CPU: 10 × Apple M4
  WORD_SIZE: 64
  LLVM: libLLVM-18.1.7 (ORCJIT, apple-m4)
  GC: Built with stock GC
Threads: 1 default, 1 interactive, 1 GC (on 4 virtual cores)
Environment:
  JULIA_EDITOR = code
  JULIA_VSCODE_REPL = 1

Additional context

I would suggest two possible approaches to fix this:

  1. Minimal Fix --> The entanglement consumer checks that the size of the quantum state is correct, and if not, discards the e2e Bell pair and maybe prints a warning
  2. Full Fix (this could solve more bugs that could span from the same issue) --> Implementing a uuid identification system for Bell pairs in the network, so that every update message can be inequivocably mapped to its associated entry. This would require modifications to protocols as well as message formats, possibly complicating the logic.

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