Add Gaussian channel support for Gabs (#129)

Author: apkille

ext/GabsExt/GabsExt.jl

@@ -29,4 +29,7 @@ for (f,g) in [(:TwoSqueezedState, :eprstate),(:TwoSqueezeOp,:twosqueeze)]
     end
 end
 
+express_nolookup(x::AttenuatorCPTP, repr::GabsRepr) = attenuator(repr.basis(1), x.theta, x.noise)
+express_nolookup(x::AmplifierCPTP, repr::GabsRepr) = amplifier(repr.basis(1), x.r, x.noise)
+
 end

src/QSymbolicsBase/predefined_CPTP.jl

@@ -1,9 +1,35 @@
-export PauliNoiseCPTP, DephasingCPTP, DephasingCPTP, GateCPTP
+export PauliNoiseCPTP, DephasingCPTP, DephasingCPTP, GateCPTP, AttenuatorCPTP, AmplifierCPTP
 
 abstract type NoiseCPTP <: Symbolic{AbstractSuperOperator} end
 isexpr(::NoiseCPTP) = false
 basis(x::NoiseCPTP) = x.basis
 
+"""
+    AttenuatorCPTP(theta::Number, noise::Int)
+
+Attenuation CPTP map, defined by the beam splitter rotation parameter `theta`
+and thermal noise parameter `noise`.
+"""
+@withmetadata struct AttenuatorCPTP <: NoiseCPTP
+    theta::Real
+    noise::Real
+end
+basis(x::AttenuatorCPTP) = inf_fock_basis
+symbollabel(x::AttenuatorCPTP) = "𝒜𝓉𝓉"
+
+"""
+    AmplifierCPTP(r::Number, noise::Int)
+
+Amplification CPTP map, defined by the squeezing amplitude parameter `r`
+and thermal noise parameter `noise`.
+"""
+@withmetadata struct AmplifierCPTP <: NoiseCPTP
+    r::Real
+    noise::Real
+end
+basis(x::AmplifierCPTP) = inf_fock_basis
+symbollabel(x::AmplifierCPTP) = "𝒜𝓂𝓅"
+
 """Single-qubit Pauli noise CPTP map
 
 ```jldoctest

test/test_gabs.jl

@@ -23,6 +23,11 @@
             @test express(BeamSplitterOp(τ), GabsRepr(basis)) ≈ beamsplitter(basis(2), τ)
         end
 
+        @testset "Gaussian channels - $(basis)" begin
+            @test express(AttenuatorCPTP(θ, τ), GabsRepr(basis)) ≈ attenuator(basis(1), θ, τ)
+            @test express(AmplifierCPTP(r, τ), GabsRepr(basis)) ≈ amplifier(basis(1), r, τ)
+        end
+
         @testset "Linear algebra operations" begin
             @test express(CoherentState(α) ⊗ TwoSqueezedState(r*exp(im*θ)) ⊗ vac, GabsRepr(basis)) ≈ coherentstate(basis(1), α) ⊗ eprstate(basis(2), r, θ) ⊗ vacuumstate(basis(1))
             @test express(DisplaceOp(α) * vac, GabsRepr(basis)) ≈ coherentstate(basis(1), α)