Entangled multiphoton states are a critical resource for the emerging applications of quantum science. Generation of reliable sources of such quantum states of light poses both theoretical and practical challenges, particularly in the small scale. We propose a deterministic source of entangled multiphoton states based on spontaneous emission from a ladder of cascaded quantum potential wells. The special ingredient of this scheme compared with a simple ensemble of emitters is the coupling between the two-level systems, creating sublevels in the energy ladder. This enables, via an indistinguishable multipath evolution, the creation of highly entangled multiphoton photon-number combination states in three modes, namely, tripartite entanglement in the frequency degree of freedom. Due to the intricate energy ladder of our proposed structure, we can apply control and switch the entanglement between two multiphoton modes by measuring the third. We further discuss an application as a qubit-pair source with an error-detection ancilla and comment on possible implementations.
Emitters based on semiconductor quantum dots are promising sources for both single photons and pairs of polarization-entangled photons. Typically, single photons are generated through a photon emission event bringing the electronic system from a single-exciton state back to the ground state. In the case of entangled photons two photons are generated through a cascaded emission from the biexciton to either one of the single-exciton states and then back to the ground state. Alternatively, emission of two photons can also be achieved through a higher-order two-photon process bringing the quantum dot directly from the biexciton state (through a virtual intermediate state) back to the ground state. Here we discuss in particular how this two-photon process can lead to certain conceptual advantages in both the generation of polarization-entangled photon pairs and generation of single photons.
The generation of entanglement between three identical coupled cavities, each containing a single three-level atom, is studied when the cavities exchange two coherent photons and are in the N=2, 4, and 6 manifolds, where
N represents the maximum number of photons possible in any one cavity. The combined states of the atom and the photon in a cavity is given by a qutrit for N=2, a five-dimensional qudit for N=4, and a seven-dimensional qudit for N=6. The conservation of the operator
N^ for the interacting three-cavity system limits the total number of tripartite states to only 6, 18, and 38, rather than the usual
33=27,
53=125, and
73=343 states for N=2, 4, and 6, respectively. The deterministic generation of entanglement from general initially unentangled tripartite states is studied in the limit of large hopping strength, where all the solutions are analytic and given in terms of exponential functions. Several types of resulting tripartite entanglement are analyzed in order to obtain maximally entangled states.