diff --git a/src/QuantumOpticsBase.jl b/src/QuantumOpticsBase.jl
index 14608f2e..dc835eee 100644
--- a/src/QuantumOpticsBase.jl
+++ b/src/QuantumOpticsBase.jl
@@ -3,7 +3,7 @@ module QuantumOpticsBase
 using SparseArrays, LinearAlgebra, LRUCache, Strided, UnsafeArrays, FillArrays
 import LinearAlgebra: mul!, rmul!
 
-import QuantumInterface: dagger, directsum, ⊕, dm, embed, expect, identityoperator, identitysuperoperator,
+import QuantumInterface: dagger, directsum, ⊕, dm, embed, nsubsystems, expect, identityoperator, identitysuperoperator,
         permutesystems, projector, ptrace, reduced, tensor, ⊗, variance, apply!, basis, AbstractSuperOperator
 
 # index helpers
diff --git a/src/apply.jl b/src/apply.jl
index 24e2d7f7..2336cb6d 100644
--- a/src/apply.jl
+++ b/src/apply.jl
@@ -1,11 +1,3 @@
-nsubsystems(s::Ket) = nsubsystems(s.basis)
-function nsubsystems(s::Operator)
-    s.basis_l == s.basis_r || throw(ArgumentError("`nsubsystem(::Operator)` is well defined only if the left and right bases are the same"))
-    nsubsystems(s.basis_l)
-end
-nsubsystems(b::CompositeBasis) = length(b.bases)
-nsubsystems(b::Basis) = 1
-
 function is_apply_shortcircuit(state, indices, operation)
     if nsubsystems(state) == 1
         basis(state)==basis(operation) || throw(ArgumentError("`apply!` failed due to incompatible bases of the state and the operation attempted to be applied on it"))