We introduce a multi-step protocol for optical quantum state engineering that performs as deterministic "bright quantum scissors" (BQS), namely truncates an arbitrary input quantum state to have at least a certain number of photons. The protocol exploits single-photon pulses and is based on the effect of single-photon Raman interaction, which is implemented with a single three-level Λ system (e.g. a single atom) Purcell-enhanced by a single-sided cavity. A single step of the protocol realises the inverse of the bosonic annihilation operator. Multiple iterations of the protocol can be used to deterministically generate a chain of single-photons in a W state. Alternatively, upon appropriate heralding, the protocol can be used to generate Fock-state optical pulses. This protocol could serve as a useful and versatile building block for the generation of advanced optical quantum states that are vital for quantum communication, distributed quantum information processing, and all-optical quantum computing.
I. INTRODUCTIONThe field of quantum state engineering (QSE) aims at preparing arbitrary quantum states. Nonclassical states are highly sought after both as a means to test fundamental questions in quantum mechanics [1], as well as a source for various applications in quantum information [2,3], sensing and metrology [4]. Controlling and manipulating the quantum state of optical fields is of particular interest both for optical information processing [5,6] and for quantum communication [7] since optical photons are the ideal carriers of information over long distances. There are two main approaches to engineer the quantum state of an optical field [8]: first, by choosing the Hamiltonian correctly, one can utilise its time evolution to unitarily transform an initial state into the desired final state (e.g. generation of squeezed states and entangeled photon pairs by parametric down-conversion). Second, by introducing entanglement between the system of interest and an auxilary system folloed by appropriate measurements on the auxilary system, one can collapse the system of interest to the target state. This approach was used for example for the generation and entanglement of single photons in the DLCZ protocol for long-distance quantum communication [9], and in the recent generation of entangled atom-light Schrödinger cat states [10]. The two approaches may of course be combined for instance in the generation of optical Schrödinger cat states from squeezed vacuum, which is conditioned on the measurement of a subtracted photon diverted to an auxilary mode [11]. QSE of optical fields was discussed by Vogel et al. [12] in a paper proposing a recipe for generating an arbitrary quantum state in the field of * barak.dayan@weizmann.ac.il arXiv:1904.08197v1 [quant-ph]
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