Spatially dependent atom-photon entanglement

Physical Review A

Experimental validation and control of quantum traits for an open quantum system are important for any quantum information purpose. We consider a traveling atom qubit as a quantum memory with adjustable velocity inside a leaky cavity, adopting a quantum witness as a figure of merit for quantumness assessment. We show that this model constitutes an inherent physical instance where the quantum witness does not work properly if not suitably optimized. We then supply the optimal intermediate blind measurements which make the quantum witness a faithful tester of quantum coherence. We thus find that larger velocities protect quantumness against noise, leading to lifetime extension of hybrid qubit-photon entanglement and to higher phase estimation precision. Control of qubit motion thus reveals itself as a quantum enhancer.

Scientific Reports

the microwave-induced orbital angular momentum (oAM) transfer from a Laguerre-Gaussian (LG) beam to a weak plane-wave is studied in a closed-loop four-level ladder-type atomic system. the analytical investigation shows that the generated fourth field is an LG beam with the same OAM of the applied LG field. Moreover, the microwave-induced subluminal generated pulse can be switched to the superluminal one only by changing the relative phase of applied fields. It is shown that the OAM transfer in subluminal regime is accompanied by a slightly absorption, however, it switches to the slightly gain in superluminal regime. the transfer of light's oAM and control of the group velocity of the generated pulse can prepare a high-dimensional Hilbert space which has a major role in quantum communication and information processing. In the past three decades, much attention has been paid to the study of the optical phenomena using Laguerre-Gaussian (LG) laser fields which are derived by solving the Helmholtz equation in the cylindrical coordinates. The intensity profile of the LG beam has a doughnut-shaped pattern whose origin is the phase singularity and the azimuthal component of the beam's Poynting vector. It is well known that a light beam may have both the orbital angular momentum (OAM) due to its helical wavefront and the spin angular momentum associated with the circular polarization 1,2. Allen et al., for the first time, introduced a well-defined OAM for an LG laser light by measuring the induced mechanical torque on the suspended cylindrical lenses due to the OAM transfer 3. The LG modes can be practically generated in several common methods such as forked diffraction gratings 4 , computer-generated holograms 5 , cylindrical lens mode converters 6 , spiral phase plates 7 , and spatial light modulators (SLMs) 8. Light beams with OAM were strongly used in fundamental physical subjects like nonlinear optical phenomena, angular momentum conservation 9 , quantum effects 10 , nano optics 11 , imaging 12 , and quantum communications 13. In recent years, the LG beams and their features have been extensively investigated in many aspects. The spatially dependent EIT has been demonstrated by probing cold rubidium atoms with optical vortex light 14. Generation of the optical vortex beams from a non-vortex beam occurred by a wave mixing process in a nonlinear photonic crystal 15. Several nonlinear optical processes have been studied in different quantum systems using LG beams 16 such as the second-harmonic generation 9,17 , sum frequency generation 18 , and four-wave mixing (FWM) 19-22. The spatially structured optical transparency is theoretically investigated in a five-level combined tripod and Λ atom-light coupling scheme 23. Mahmoudi et al. have reported the spatially dependent atom-photon entanglement using LG laser beams. It was shown that the atom-photon entanglement can be controlled by OAM of light in closed-loop atomic systems 24. The group velocity of the LG beams in the free space has been also studied. Recently, Boyd et al. have shown that the group velocity of a twisted light depends on the distance across the light propagation direction in vacuum 25. In another work, the subluminal group velocity of the LG beam was reported and it was shown that an optical vortex beam is dispersed even in the free space 26. On the other hand, the OAM transfer from a pump field to both probe and generated fields at the FWM frequency was suggested in a two-level system 27. In the following, it was shown that the OAM transfer from two vortex control fields to a probe field occurs in both four-level tripod-type and double tripod-type atomic systems 28,29. Recently, the transfer of the optical vortex in four-level double-Λ EIT scheme, through a FWM process, has been reported only by one vortex control field 22. In this paper, we are going to introduce a model in which the microwave field induces an OAM transfer from strong coupling LG field to the generated fourth field (GFF). We consider a closed-loop four-level ladder-type atomic system and investigate the effect of the different parameters on the OAM transfer, as well as the intensity and phase profiles. In addition, we investigate the spatially dependent dispersion and absorption of the GFF and obtain an analytical expression to explain the group velocity behavior of the GFF. It is shown that a weak microwave probe field transfers the OAM of a strong coupling LG field to the GFF. The OAM transfer has been

Journal of the Optical Society of America B

We study the orbital angular momentum (OAM) transfer from a weak Laguerre-Gaussian (LG) field to a weak plane-wave in two closed-loop three-level V-type atomic systems. In the first scheme, the atomic system has two non-degenerate upper levels which the corresponding transition is excited by a microwave plane-wave. It is analytically shown that the microwave field induces an OAM transfer from an LG field to a generated third field. In the second scheme, we consider a three-level V-type atomic system with two near-degenerate excited states and study the effect of the quantum interference due to the spontaneous emission on the OAM transfer. It is found that the spontaneously generated coherence (SGC) induces the OAM transfer from the LG field to the weak planar field, while the OAM transfer does not occur in the absence of the SGC. The suggested models prepare a rather simple method for the OAM transfer which can be used in quantum information processing and data storage.

European Physical Journal Plus, 2021

We conduct a study into the impact of frequency modulation on the dynamic behavior of qubit-photon entanglement inside a leaky cavity and subsequently on the entanglement swapping between such two qubits. Furthermore, we investigate how frequency modulation can affect the entropic uncertainty and its lower bound. It is revealed that there are optimal modulation parameters for which the system experiences a long-lasting qubit-photon entanglement as well as a robust swapped entanglement. Moreover, the obtained result indicates that for the optimal modulation parameters, the entropic uncertainty can be maintained small for a long time.