In a new paper published as the July 2014 cover article in Nature Photonics, physicists Hyunseok Jeong, et al., at institutions in South Korea, Italy, and Australia, have devised and experimentally demonstrated a novel scheme to generate entanglement between quantum and classical (or "particle-like" and "wave-like") states of light. This study marks the first time that physicists have generated entanglement between a single photon and a coherent wave-like state of light.
According to the scientists, this hybrid entanglement can be considered as the closest analogy of Schrödinger's Gedankenexperiment realized so far. It has practical applications, too, as it provides a new type of qubit (a hybrid qubit) that can be used for efficient quantum computation.
In previous attempts to generate entanglement between a single photon and coherent wave-like state of light, the major obstacle was the requirement of a strong and noiseless nonlinear interaction between photons; this was extremely difficult because photons seldom interact with each other.
In the new study, the researchers introduced a new scheme to generate hybrid entanglement that is experimentally accessible. The new scheme is based on superposition. Similar to how Schrödinger's cat is both alive and dead at the same time before anyone looks, the photons also occupy two states at the same time.
Rather than being dead and alive, however, the states correspond to the long and short arms of the interferometer through which each photon travels. It's as if a photon traveled through both arms at the same time. By performing a procedure that erases the information regarding which arm the photons travelled through, the physicists could entangle a photon with a wave-like state that they call a photon-added coherent state.
Because the new state is essentially a superposition including both particle and wave components, it can be viewed as a new type of qubit. Usually, a qubit consists of a particle that is in a superposition of two quantum states. The new idea here is that a qubit can consist of a quantum (or particle-like) state and a classical (or wave-like) one, which could effectively combine the intrinsic advantages of both. In this way, a hybrid qubit could offer unique advantages for quantum computing and quantum teleportation.