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noncliff: add copy, == for GeneralizedStabilizer, UnitaryPauliChannel #419

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Nov 5, 2024
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noncliff: add copy, == for GeneralizedStabilizer, UnitaryPauliChannel #419

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noncliff: add copy, == for GeneralizedStabilizer, UnitaryPauliChannel
Fe-r-oz Nov 3, 2024
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9 changes: 8 additions & 1 deletion src/nonclifford.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -64,6 +64,8 @@ function GeneralizedStabilizer(state)
end

GeneralizedStabilizer(s::GeneralizedStabilizer) = s
Base.copy(sm::GeneralizedStabilizer) = GeneralizedStabilizer(copy(sm.stab),copy(sm.destabweights))
Base.:(==)(sm₁::GeneralizedStabilizer, sm₂::GeneralizedStabilizer) = sm₁.stab==sm₂.stab && sm₁.destabweights==sm₂.destabweights

function Base.show(io::IO, s::GeneralizedStabilizer)
println(io, "A mixture ∑ ϕᵢⱼ Pᵢ ρ Pⱼ† where ρ is")
Expand Down Expand Up @@ -145,8 +147,11 @@ with ϕᵢⱼ | Pᵢ | Pⱼ:
0.853553+0.0im | + _ | + _
0.146447+0.0im | + Z | + Z

julia> expect(P"-X", sm)
julia> χ′ = expect(P"-X", sm)
0.7071067811865475 + 0.0im

julia> prob = (real(χ′)+1)/2
0.8535533905932737
```

"""
Expand Down Expand Up @@ -342,6 +347,8 @@ struct UnitaryPauliChannel{T,S,P} <: AbstractPauliChannel
end

PauliChannel(p::UnitaryPauliChannel) = p.paulichannel
Base.copy(p::UnitaryPauliChannel) = UnitaryPauliChannel(map(copy, p.paulis), map(copy, p.weights))
Base.:(==)(p₁::UnitaryPauliChannel, p₂::UnitaryPauliChannel) = p₁.paulis==p₂.paulis && p₁.weights==p₂.weights
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The tuples might be immutable, but the objects they contain are not-necessarily immutable. E.g. if I do something stupid like mul_left!(p.paulis, some_other_pauli) on a copied p, the original p would also be modified. I am stupid to do that to begin with, but nonetheless I would be right to be surprised that modifying the copy modified the original as well.

More generally, let the compiler worry about immutability unless you know for a fact that the compiler is doing a bad job. The compiler can figure out when immutability guarantees let it reuse memory:

Lastly, you are probably doing this because Tuple does not have a copy method. That is a frustration I have with julia -- the story behind it can be found starting in this discussion: https://discourse.julialang.org/t/why-can-i-deepcopy-nothing-but-not-copy-nothing/112429 (in particular see the links by stevengj).

Anyway, to make sure this is an actual copy, the clunky but necessary map(copy, p.paulis) can be used.

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Thank you for these insights! I avoided using copy(p.paulis) so it wouldn’t look stupid.


function Base.show(io::IO, pc::UnitaryPauliChannel)
println(io, "A unitary Pauli channel P = ∑ ϕᵢ Pᵢ with the following branches:")
Expand Down
13 changes: 13 additions & 0 deletions test/test_nonclifford_quantumoptics.jl
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Expand Up @@ -50,3 +50,16 @@ qo_tgate.data[2,2] = exp(im*pi/4)
end
end
end

@testset "copy and ==" begin
for n in 1:10
s = random_stabilizer(n)
sm = GeneralizedStabilizer(s)
i = rand(1:n)
apply!(sm, embed(n, i, pcT))
smcopy = copy(sm)
@test smcopy == sm
nc = embed(n, rand(1:n), pcT)
@test copy(nc) == nc
end
end
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