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The quantum information community is entering a period in which scientists and engineers are transitioning from ‘understanding’ to ‘control’ by harnessing aspects of quantum mechanics to realize capabilities in computing, sensing, and communication that are not possible in the classical world. Although many of these quantum technologies are still in their very early stages, it is clear that our future world will include many quantum devices. The existence of multiple devices will, in turn, necessitate the transfer of quantum information between devices—if isolated devices are useful, it follows that connected devices will be even more useful. Therefore, it is imperative to develop the resources needed for connecting quantum devices at different locations. One such resource is quantum entanglement, a physical phenomenon in which two or more quantum systems cannot be described independently, even when separated by large distances. Entanglement is an important resource for quantum computing and is critical for overcoming loss constraints that limit the transfer of quantum states across large distances. The latter can be accomplished via ‘quantum repeaters,’ which propose to connect multiple shorter entangled photon links to form a longer entangled photon link. Much of our understanding of entanglement comes from experiments involving entangled photon pairs, which are still the best candidates for the exchange of quantum information across any appreciable distance. Since the 1990s, spontaneous parametric downconversion (SPDC) has been established in the academic and research community as a reliable source of entangled photon pairs. During this time, source brightness and stability has improved considerably over the first demonstrations. Yet, entangled photon sources are still built primarily by graduate students, and there is significant variability among sources. Moreover, SPDC entangled photon sources lack flexibility, typically designed to output a single type of entangled state.
In this paper we report on the efforts to develop a ‘user-programmable’ entangled photon source capable of producing any type of two-photon entangled state. The goal is a source capable of emitting photon pairs with user-defined polarization states. That is, the user will be able to adjust the probability amplitudes {𝛼,𝛽,𝛾,𝛿} in the general expression for a two-photon polarization state, |𝜓⟩=𝛼|𝐻𝐻⟩+𝛽|𝐻𝑉⟩+𝛾|𝑉𝐻⟩+𝛿|𝑉𝑉⟩, thereby accessing any point in the two-photon polarization entanglement Hilbert space. This requires a mapping between the experimentally controllable parameters and the probability amplitudes listed above. We will describe this mapping for our particular experimental setup and will share results showing the states that our system is able to access.
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