As in many other systems, vortices can exist in BECs. Vortices can be created, for example, by "stirring" the condensate with lasers, rotating the confining trap, or by rapid cooling across the phase transition. The vortex created will be a quantum vortex with core shape determined by the interactions. Fluid circulation around any point is quantized due to the single-valued nature of the order BEC order parameter or wavefunction, that can be written in the form ψ ψ ( r → → ) = ϕ ϕ ( ρ ρ , z ) e i ℓ ℓ θ θ where ρ ρ , z and θ θ are as in the cylindrical coordinate system, and ℓ ℓ is the angular quantum number (a.k.a. the "charge" of the vortex). Since the energy of a vortex is proportional to the square of its angular momentum, in trivial topology only ℓ ℓ = 1 vortices can exist in the steady state ; Higher-charge vortices will have a tendency to split into ℓ ℓ = 1 vortices, if allowed by the topology of the geometry.