1. A process comprising: exciting a first system using an excitation that transforms a first state of the first system to an excited state of the first system; exciting a second system using an excitation that transforms a first state of the second system to an excited state of the second system; performing a first measurement of photonic modes corresponding to transitions from the excited states respectively to the first states, wherein the first measurement is unable to distinguish whether photons originated from the first system or the second system; performing operations on the first system and the second system, wherein each of the operations transforms the first state of the system to a second state of the system and transforms the second state of the system to the first state of the system; after performing the operations, exciting the first system and the second system using the excitations that transform the first states to the excited states respectively associated with the first system and the second system; and performing a second measurement of the photonic modes, wherein the second measurement is unable to distinguish whether photons originated from the first system or the second system, and measurement results of the first and second measurements indicate whether the process successfully entangled quantum states of the first system and the second system.