diff --git a/CHANGELOG.md b/CHANGELOG.md
index 4c30de4..52ab459 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,11 @@
 # News
 
+## v0.4.0 - 2024-11-26
+
+- Add `OperatorBasis` and `SuperOperatorBasis` abstract types along with corresponding `fullbasis` function to obtain these from instances of subtypes of `AbstractOperator` and `AbstractSuperOperator`.
+- Change type parameters for `StateVector`, `AbstractKet` `AbstractBra` `AbstractOperator` `AbstractSuperOperator` to elimitate all type parameters.
+
+
 ## v0.3.6 - 2024-09-08
 
 - Add `coherentstate`, `thermalstate`, `displace`, `squeeze`, `wigner`, previously from QuantumOptics.
diff --git a/Project.toml b/Project.toml
index d8b70c0..9f743ed 100644
--- a/Project.toml
+++ b/Project.toml
@@ -1,7 +1,7 @@
 name = "QuantumInterface"
 uuid = "5717a53b-5d69-4fa3-b976-0bf2f97ca1e5"
 authors = ["QuantumInterface.jl contributors"]
-version = "0.3.6"
+version = "0.4.0"
 
 [deps]
 LinearAlgebra = "37e2e46d-f89d-539d-b4ee-838fcccc9c8e"
diff --git a/src/QuantumInterface.jl b/src/QuantumInterface.jl
index dd23d4c..c23e652 100644
--- a/src/QuantumInterface.jl
+++ b/src/QuantumInterface.jl
@@ -9,8 +9,8 @@ module QuantumInterface
 
 Return the basis of an object.
 
-If it's ambiguous, e.g. if an operator has a different left and right basis,
-an [`IncompatibleBases`](@ref) error is thrown.
+If it's ambiguous, e.g. if an operator or superoperator has a different
+left and right basis, an [`IncompatibleBases`](@ref) error is thrown.
 """
 function basis end
 
diff --git a/src/abstract_types.jl b/src/abstract_types.jl
index 0650290..70a2ce6 100644
--- a/src/abstract_types.jl
+++ b/src/abstract_types.jl
@@ -21,9 +21,9 @@ in respect to a certain basis. These coefficients are stored in the
 `data` field and the basis is defined in the `basis`
 field.
 """
-abstract type StateVector{B,T} end
-abstract type AbstractKet{B,T} <: StateVector{B,T} end
-abstract type AbstractBra{B,T} <: StateVector{B,T} end
+abstract type StateVector end
+abstract type AbstractKet <: StateVector end
+abstract type AbstractBra <: StateVector end
 
 """
 Abstract base class for all operators.
@@ -36,7 +36,7 @@ For fast time evolution also at least the function
 implemented. Many other generic multiplication functions can be defined in
 terms of this function and are provided automatically.
 """
-abstract type AbstractOperator{BL,BR} end
+abstract type AbstractOperator end
 
 """
 Base class for all super operator classes.
@@ -52,4 +52,4 @@ A_{bl_1,bl_2} = S_{(bl_1,bl_2) ↔ (br_1,br_2)} B_{br_1,br_2}
 A_{br_1,br_2} = B_{bl_1,bl_2} S_{(bl_1,bl_2) ↔ (br_1,br_2)}
 ```
 """
-abstract type AbstractSuperOperator{B1,B2} end
+abstract type AbstractSuperOperator end