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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Spiral Bandwidth of Four-Wave Mixing in Rubidium Vapour
摘要： Atomic vapours are an experimentally simple and ef?cient system in which to study nonlinear wave mixing. We study a four-wave mixing (FWM) process in rubidium vapour which ef?ciently converts 780 and 776 nm light to 420 nm and 5.2 μm ?elds [1]. In particular, we quantitatively investigate the transfer of orbital angular momentum (OAM) between the FWM ?elds. OAM, and structured light more generally, is an important research tool for optical manipulation, imaging and communication. Phase-matched nonlinear processes are both longitudinally and transversely phase coherent, and therefore OAM, which is associated with spiral phase fronts, must be conserved between the pump and generated ?elds [2]. This makes wave mixing an ideal tool for frequency conversion and generation of a variety of OAM states for use in both classical and quantum communication [3]. In our FWM process, conservation of OAM determines the total OAM carried by the 420 nm and 5.2 μm ?elds - but not how it is distributed between them. Fig. 1 (a) shows the intensity pro?le and interferogram (formed by interfering the beam with its mirror image) of the 420 nm ?eld when the pump beams each carry (cid:2)ˉh of OAM ((cid:2) = (cid:2)780 = (cid:2)776). We perform quantitative analysis of each interferogram to obtain the full Laguerre-Gauss mode decomposition of the 420 nm ?eld (Fig. 1 (b)). For small values of pump OAM, the 420 nm light is generated predominantly in one mode [4], but as the pump OAM increases the OAM spectrum broadens [5]. In order for OAM to be conserved, this indicates that the 420 nm and 5.2 μm light is generated in an OAM-entangled state. From the 420 nm mode decomposition we infer the spiral bandwidth (a measure of the number of entangled modes), Δ(cid:2)B, as well as the entanglement entropy, S, of this state. Both the experimental and theoretical results show that the generated state is strongly pump mode-dependent, with the spiral bandwidth increasing with increasing pump OAM. These results indicate that this system is likely to be an ef?cient source of OAM-entangled photon pairs with widely-disparate wavelengths.
关键词： entanglement entropy,orbital angular momentum,four-wave mixing,rubidium vapour,spiral bandwidth
更新于2025-09-16 10:30:52
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Spin-orbit-coupled quantum memory of a double quantum dot
摘要： The concept of quantum memory plays an incisive role in the quantum information theory. As confirmed by the several recent rigorous mathematical studies, the quantum memory inmate in the bipartite system ρAB can reduce the uncertainty about part B, after measurements done on part A. In the present work, we extend this concept to systems with a spin-orbit coupling and introduce the notion of spin-orbit quantum memory. We self-consistently explore the Uhlmann fidelity, the pre- and the post-measurement entanglement entropy, and the post-measurement conditional quantum entropy of the system with spin-orbit coupling and show that measurement performed on the spin subsystem decreases the uncertainty of the orbital part. The uncovered effect enhances with the strength of the spin-orbit coupling. We study the concept of macroscopic realism introduced by Leggett and Garg [Phys. Rev. Lett. 54, 857 (1985)] and observe that POVM measurements done on the system under the particular protocol are noninvasive. For the extended system, we perform quantum Monte Carlo calculations and consider the reshuffling of the electron densities due to an external electric field.
关键词： quantum Monte Carlo,spin-orbit coupling,entanglement entropy,POVM measurements,conditional quantum entropy,quantum memory,Uhlmann fidelity
更新于2025-09-12 10:27:22