Allow single-mode homodyne and generaldyne measurements

CHANGELOG.md

@@ -1,5 +1,9 @@
 # News
 
+## v1.3.6 - 2026-03-19
+
+- Allow `homodyne` and `rand(Homodyne, ...)` to measure single-mode Gaussian states while keeping `ptrace` on all modes unsupported.
+
 ## v1.3.5 - 2026-03-19
 
 - `express(gaussian_state, ::QuantumOpticsRepr)` is now implemented, letting Gabs.jl pure Gaussian states to be converted to Kets from QuantumOptics.jl

Project.toml

@@ -1,7 +1,7 @@
 name = "Gabs"
 uuid = "0eb812ee-a11f-4f5e-b8d4-bb8a44f06f50"
 authors = ["Andrew Kille"]
-version = "1.3.5"
+version = "1.3.6"
 
 [deps]
 LinearAlgebra = "37e2e46d-f89d-539d-b4ee-838fcccc9c8e"

src/homodyne.jl

@@ -85,7 +85,7 @@ function homodyne(
     basis = state.basis
     nmodes = basis.nmodes
     indlength = length(indices)
-    indlength < nmodes || throw(ArgumentError(Gabs.INDEX_ERROR))
+    indlength <= nmodes || throw(ArgumentError(Gabs.INDEX_ERROR))
     indlength == length(angles) || throw(ArgumentError(Gabs.GENERALDYNE_ERROR))
     # perform conditional mapping of Gaussian quantum state
     result′, a, A = _homodyne_filter(rng, state, indices, angles; squeeze)
@@ -124,7 +124,7 @@ function homodyne(
     basis = state.basis
     nmodes = basis.nmodes
     indlength = length(indices)
-    indlength < nmodes || throw(ArgumentError(Gabs.INDEX_ERROR))
+    indlength <= nmodes || throw(ArgumentError(Gabs.INDEX_ERROR))
     indlength == length(angles) || throw(ArgumentError(Gabs.GENERALDYNE_ERROR))
     # perform conditional mapping of Gaussian quantum state
     result′, a, A = _homodyne_filter(rng, state, indices, angles; squeeze)
@@ -203,7 +203,7 @@ function Base.rand(
 ) where {Tm,Tc,R,G}
     basis = state.basis
     indlength = length(indices)
-    indlength < basis.nmodes || throw(ArgumentError(Gabs.INDEX_ERROR))
+    indlength <= basis.nmodes || throw(ArgumentError(Gabs.INDEX_ERROR))
     indlength == length(angles) || throw(ArgumentError(Gabs.GENERALDYNE_ERROR))
     nmodes′ = basis.nmodes - indlength
     mean, covar = state.mean, state.covar
@@ -266,7 +266,7 @@ function Base.rand(
     basis = state.basis
     nmodes = basis.nmodes
     indlength = length(indices)
-    indlength < nmodes || throw(ArgumentError(Gabs.INDEX_ERROR))
+    indlength <= nmodes || throw(ArgumentError(Gabs.INDEX_ERROR))
     indlength == length(angles) || throw(ArgumentError(Gabs.GENERALDYNE_ERROR))
     nmodes′ = nmodes - indlength
     mean, covar = state.mean, state.covar
@@ -385,4 +385,4 @@ function _homodyne_filter(
     # promote output array type to ensure it matches the input array type
     result′ = Gabs._promote_output_vector(Tm, resultmean, 2*indlength)
     return result′, a, A
-end
+end

src/states.jl

@@ -840,4 +840,4 @@ function _sympspectrum(M::AbstractMatrix{<:Number}, select::Function; pre::Union
     M = isnothing(post) ? M : M * post
     M = isnothing(invscale) ? imag.(eigvals(M)) : imag.(eigvals(M)) ./ invscale
     return filter(x -> select(x), M)
-end
+end

test/test_measurements.jl

@@ -96,6 +96,22 @@
                 @test isapprox(state.mean[1:2], zeros(2), atol=1e-12)
                 @test isapprox(state.covar[1:2, 1:2], Matrix{Float64}(I,2,2), atol=1e-12)
             end
+
+            for basis in (qpairbasis, qblockbasis)
+                seed = 1234
+                st = coherentstate(basis, 0.3 + 0.2im)
+                h_int = homodyne(MersenneTwister(seed), st, 1, [0.0]; squeeze)
+                h_vec = homodyne(MersenneTwister(seed), st, [1], [0.0]; squeeze)
+                @test h_int.result == h_vec.result
+                @test h_int.state == h_vec.state
+                @test isapprox(h_int.state, vacuumstate(basis), atol = 1e-12)
+                @test_throws ArgumentError ptrace(h_int.state, 1)
+
+                samples_int = rand(MersenneTwister(seed), Homodyne, st, 1, [0.0]; shots = 2, squeeze)
+                samples_vec = rand(MersenneTwister(seed), Homodyne, st, [1], [0.0]; shots = 2, squeeze)
+                @test samples_int == samples_vec
+                @test size(samples_int) == (2, 2)
+            end
 
             st = squeezedstate(QuadPairBasis(4), 0.5, π/2)
             st_block = changebasis(QuadBlockBasis, st)
@@ -172,6 +188,6 @@
 
             @test_throws ArgumentError rand(Homodyne, rs_qpair, collect(1:5), [0.0])
             @test_throws ArgumentError rand(Homodyne, rs_qblock, collect(1:5), [π/2])
-        end        
+        end
     end
-end
+end

test/test_states.jl

@@ -108,6 +108,9 @@
             @test ptrace(state, [2, 3]) == s1
             @test_throws ArgumentError ptrace(state, [1, 2, 3, 4])
 
+            single_mode = coherentstate(basis, alpha)
+            @test_throws ArgumentError ptrace(single_mode, 1)
+
             sstatic = coherentstate(SVector{2}, SMatrix{2,2}, basis, alpha)
             tpstatic = sstatic ⊗ sstatic ⊗ sstatic
             @test ptrace(tpstatic, 1) == sstatic ⊗ sstatic
@@ -169,4 +172,4 @@
         @test isapprox(det(s_qpair.covar), prod(abs2, spec_qpair), atol=1e-3)
         @test isapprox(det(s_qblock.covar), prod(abs2, spec_qblock), atol=1e-3)
     end
-end
+end