diff --git a/CHANGELOG.md b/CHANGELOG.md
index fa9af19..462f775 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,9 @@
 # News
 
+## v0.4.3 - 2026-06-14
+
+- Add a `lazy::Bool=false` option to `QuantumOpticsRepr`. When set, downstream `express` conversions (in `QuantumSymbolics.jl`) translate symbolic operator sums, products, and tensor products into `QuantumOpticsBase` lazy operators. The existing `QuantumOpticsRepr()` and `QuantumOpticsRepr(cutoff)` constructors are preserved.
+
 ## v0.4.2 - 2025-11-29
 
 - Define `commutator` and `anticommutator`.
diff --git a/Project.toml b/Project.toml
index 2029043..4b0e668 100644
--- a/Project.toml
+++ b/Project.toml
@@ -1,7 +1,7 @@
 name = "QuantumInterface"
 uuid = "5717a53b-5d69-4fa3-b976-0bf2f97ca1e5"
 authors = ["QuantumInterface.jl contributors"]
-version = "0.4.2"
+version = "0.4.3"
 
 [deps]
 LinearAlgebra = "37e2e46d-f89d-539d-b4ee-838fcccc9c8e"
diff --git a/src/express.jl b/src/express.jl
index c088c27..fb8670a 100644
--- a/src/express.jl
+++ b/src/express.jl
@@ -22,10 +22,16 @@ express(s, repr::AbstractRepresentation) = express(s, repr, UseAsState())
 # Commonly used representations -- interfaces for each one defined in separate packages
 ##
 
-"""Representation using kets, bras, density matrices, and superoperators governed by `QuantumOptics.jl`."""
-Base.@kwdef struct QuantumOpticsRepr <: AbstractRepresentation 
+"""Representation using kets, bras, density matrices, and superoperators governed by `QuantumOptics.jl`.
+
+With `lazy=true`, downstream `express` conversions build the lazy operator types of
+`QuantumOpticsBase` (`LazySum`, `LazyProduct`, `LazyTensor`) for symbolic sums, products,
+and tensor products instead of materializing dense matrices."""
+Base.@kwdef struct QuantumOpticsRepr <: AbstractRepresentation
     cutoff::Int = 2
+    lazy::Bool = false
 end
+QuantumOpticsRepr(cutoff::Int) = QuantumOpticsRepr(; cutoff)
 """Similar to `QuantumOpticsRepr`, but using trajectories instead of superoperators."""
 struct QuantumMCRepr <: AbstractRepresentation end
 """Representation using tableaux governed by `QuantumClifford.jl`"""
diff --git a/test/runtests.jl b/test/runtests.jl
index 826fe33..3c2359d 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -27,6 +27,7 @@ println("Starting tests with $(Threads.nthreads()) threads out of `Sys.CPU_THREA
 
 @doset "sortedindices"
 @doset "bases"
+@doset "express"
 #VERSION >= v"1.9" && @doset "doctests"
 get(ENV,"JET_TEST","")=="true" && @doset "jet"
 VERSION >= v"1.9" && @doset "aqua"
diff --git a/test/test_express.jl b/test/test_express.jl
new file mode 100644
index 0000000..45aefbe
--- /dev/null
+++ b/test/test_express.jl
@@ -0,0 +1,28 @@
+using Test
+using QuantumInterface: QuantumOpticsRepr
+
+@testset "QuantumOpticsRepr constructors" begin
+    # the default stays eager
+    @test QuantumOpticsRepr() == QuantumOpticsRepr(2, false)
+    @test QuantumOpticsRepr().cutoff == 2
+    @test QuantumOpticsRepr().lazy == false
+
+    # the positional `cutoff` constructor is preserved
+    @test QuantumOpticsRepr(4) == QuantumOpticsRepr(cutoff=4)
+    @test QuantumOpticsRepr(4).cutoff == 4
+    @test QuantumOpticsRepr(4).lazy == false
+
+    # the new `lazy` option is opt-in via keyword
+    @test QuantumOpticsRepr(lazy=true).lazy == true
+    @test QuantumOpticsRepr(cutoff=4, lazy=true) == QuantumOpticsRepr(4, true)
+end
+
+@testset "QuantumOpticsRepr cache-key equality" begin
+    # `express` caches conversions in a `Dict` keyed on the representation value, so an
+    # eager and a lazy representation of the same object must compare unequal (and hash
+    # differently) or the cache would hand back the wrong type.
+    @test QuantumOpticsRepr(4) == QuantumOpticsRepr(cutoff=4, lazy=false)
+    @test QuantumOpticsRepr(4) != QuantumOpticsRepr(4, true)
+    @test hash(QuantumOpticsRepr(4)) == hash(QuantumOpticsRepr(cutoff=4, lazy=false))
+    @test hash(QuantumOpticsRepr(2)) != hash(QuantumOpticsRepr(2, true))
+end