diff --git a/docs/src/api.md b/docs/src/api.md
index 3113534a..a5eeee85 100644
--- a/docs/src/api.md
+++ b/docs/src/api.md
@@ -68,6 +68,7 @@ LinearAlgebra.normalize!
 unit
 LinearAlgebra.inv
 LinearAlgebra.diag
+proj
 ptrace
 purity
 tidyup
diff --git a/src/qobj/arithmetic_and_attributes.jl b/src/qobj/arithmetic_and_attributes.jl
index 9c1271d1..18a37a68 100644
--- a/src/qobj/arithmetic_and_attributes.jl
+++ b/src/qobj/arithmetic_and_attributes.jl
@@ -6,7 +6,7 @@ Arithmetic and Attributes for QuantumObject
 
 export trans, dag, dagger, matrix_element, unit
 export sqrtm, logm, expm, sinm, cosm
-export ptrace, purity
+export proj, ptrace, purity
 export tidyup, tidyup!
 export get_data, get_coherence
 
@@ -558,6 +558,14 @@ LinearAlgebra.diag(
     k::Int = 0,
 ) where {T,ObjType<:Union{OperatorQuantumObject,SuperOperatorQuantumObject}} = diag(A.data, k)
 
+@doc raw"""
+    proj(ψ::QuantumObject)
+
+Return the projector for a [`Ket`](@ref) or [`Bra`](@Ref) type of [`QuantumObject`](@ref)
+"""
+proj(ψ::QuantumObject{<:AbstractArray{T},KetQuantumObject}) where {T} = ψ * ψ'
+proj(ψ::QuantumObject{<:AbstractArray{T},BraQuantumObject}) where {T} = ψ' * ψ
+
 @doc raw"""
     ptrace(QO::QuantumObject, sel::Vector{Int})
 
diff --git a/test/quantum_objects.jl b/test/quantum_objects.jl
index f8e89aaa..19ebbc4b 100644
--- a/test/quantum_objects.jl
+++ b/test/quantum_objects.jl
@@ -123,7 +123,7 @@
 
     N = 10
     a = fock(N, 3)
-    @test sparse(ket2dm(a)) ≈ projection(N, 3, 3)
+    @test proj(a) ≈ proj(a') ≈ sparse(ket2dm(a)) ≈ projection(N, 3, 3)
     @test isket(a') == false
     @test isbra(a') == true
     @test size(a) == (N,)