A photonic process named as quantum state joining has been recently experimentally demonstrated [C. Vitelli et al., Nature Photon. 7, 521 (2013)] that corresponds to the transfer of the internal two-dimensional quantum states of two input photons, i.
e., two photonic qubits, into the four-dimensional quantum state of a single photon, i.e., a photonic ququart. A scheme for the inverse process, namely quantum state splitting, has also been theoretically proposed. Both processes can be iterated in a cascaded layout, to obtain the joining and/or splitting of more than two qubits, thus leading to a general scheme for varying the number of photons in the system while preserving its total quantum state, or quantum information content. Here, we revisit these processes from a theoretical point of view. After casting the theory of the joining and splitting processes in the more general photon occupation number notation, we introduce some modified schemes that are in principle unitary (not considering the implementation of the CNOT gates) and do not require projection and feed-forward steps. This can be particularly important in the quantum state splitting case, to obtain a scheme that does not rely on postselection. Moreover, we formally prove that the quantum joining of two photon states with linear optics requires the use of at least one ancilla photon. This is somewhat unexpected, given that the demonstrated joining scheme involves the sequential application of two CNOT quantum gates, for which a linear optical scheme with just two photons and postselection is known to exist. Finally we explore the relationship between the joining scheme and the generation of clusters of multi-particle entangled states involving more than one qubit per particle.
Recent experimental results demonstrated the generation of a quantum superpositon (MQS), involving a number of photons in excess of 5x10^4, which showed a high resilience to losses. In order to perform a complete analysis on the effects of de-coheren
ce on this multiphoton fields, obtained through the Quantum Injected Optical Parametric Amplifier (QIOPA), we invesigate theoretically the evolution of the Wigner functions associated to these states in lossy conditions. Recognizing the presence of negative regions in the W-representation as an evidence of non-classicality, we focus our analysis on this feature. A close comparison with the MQS based on coherent states allows to identify differences and analogies.
We present the realization of a ultra fast shutter for optical fields, which allows to preserve a generic polarization state, based on a self-stabilized interferometer. It exhibits high (or low) transmittivity when turned on (or inactive), while the
fidelity of the polarization state is high. The shutter is realized through two beam displacing prisms and a longitudinal Pockels cell. This can represent a useful tool for controlling light-atom interfaces in quantum information processing.
NOON states are path entangled states which can be exploited to enhance phase resolution in interferometric measurements. In the present paper we analyze the quantum states obtained by optical parametric amplification of polarization NOON states. Fir
st we study, theoretically and experimentally, the amplification of a 2-photon state by a collinear Quantum Injected Optical Parametric Amplifier (QIOPA). We compared the stimulated emission regime with the spontaneous one, studied by Sciarrino et al. (PRA 77, 012324), finding comparable visibilities between the two cases but an enhancement of the signal in the stimulated case. As a second step, we show that the collinear amplifier cannot be successfully used for amplifying N-photon states with N>2 due to the intrinsic lambda/4 oscillation pattern of the crystal. To overcome this limitation, we propose to adopt a scheme for the amplification of a generic state based on a non-collinear QIOPA and we show that the state obtained by the amplification process preserves lambda/N feature and exhibits a high resilience to losses. Furthermore, an asymptotic unity visibility can be obtained when correlation functions with sufficiently high order M are analyzed.
