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Register a new userNonlinear pi phase shift for single fiber-guided photons interacting with a single atom
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Realizing a strong interaction between individual optical photons is an important objective of research in quantum science and technology. Since photons do not interact directly, this goal requires, e.g., an optical medium in which the light experiences a phase shift that depends nonlinearly on the photon number. Once the additional phase shift for two photons reaches pi, such an ultra-strong nonlinearity could even enable the direct implementation of high-fidelity quantum logic operations. However, the nonlinear response of standard optical media is many orders of magnitude too weak for this task. Here, we demonstrate the realization of an optical fiber-based nonlinearity that leads to an additional two-photon phase shift close to the ideal value of pi. Our scheme employs a whispering-gallery-mode resonator, interfaced by an optical nanofiber, where the presence of a single rubidium atom in the resonator results in a strongly nonlinear response. We experimentally show that this results in entanglement of initially independent incident photons. The demonstration of this ultra-strong nonlinearity in a fiber-integrated system is a decisive step towards scalable quantum logics with optical photons.
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We report on a direct measurement of a phase shift on a weak coherent beam by a single Rb-87 atom in a Mach-Zehnder interferometer. A maximum phase shift of about 1 degree is observed experimentally.
We succeeded in measuring phase shift spectra of a microsphere cavity coupled with a tapered fiber using a weak coherent probe light at the single photon level. We utilized a tapered fiber with almost no depolarization and constructed a very stable phase shift measurement scheme based on polarization analysis using photon counting. Using a very weak probe light (bar{n} = 0:41), we succeeded in observing the transition in the phase shift spectrum between undercoupling and overcoupling (at gap distances of 500 and 100 nm, respectively).We also used quantum state tomography to obtain a purity spectrum. Even in the overcoupling regime, the average purity was 0.982 pm 0.024 (minimum purity: 0.892), suggesting that the coherence of the fiber-microsphere system was well preserved. Based on these results, we believe this system is applicable to quantum phase gates using single light emitters such as diamond nitrogen vacancy centers.
In analogy to transistors in classical electronic circuits, a quantum optical switch is an important element of quantum circuits and quantum networks. Operated at the fundamental limit where a single quantum of light or matter controls another field or material system, it may enable fascinating applications such as long-distance quantum communication, distributed quantum information processing and metrology, and the exploration of novel quantum states of matter. Here, by strongly coupling a photon to a single atom trapped in the near field of a nanoscale photonic crystal cavity, we realize a system where a single atom switches the phase of a photon, and a single photon modifies the atoms phase. We experimentally demonstrate an atom-induced optical phase shift that is nonlinear at the two-photon level, a photon number router that separates individual photons and photon pairs into different output modes, and a single-photon switch where a single gate photon controls the propagation of a subsequent probe field. These techniques pave the way towards integrated quantum nanophotonic networks involving multiple atomic nodes connected by guided light.
It has been suggested that second-order nonlinearities could be used for quantum logic at the single-photon level. Specifically, successive two-photon processes in principle could accomplish the phase shift (conditioned on the presence of two photons in the low frequency modes) $ |011 rangle longrightarrow i|100 rangle longrightarrow -|011 rangle $. We have analyzed a recent scheme proposed by Xia et al. to induce such a conditional phase shift between two single-photon pulses propagating at different speeds through a nonlinear medium with a nonlocal response. We present here an analytical solution for the most general case, i.e. for an arbitrary response function, initial state, and pulse velocity, which supports their numerical observation that a $pi$ phase shift with unit fidelity is possible, in principle, in an appropriate limit. We also discuss why this is possible in this system, despite the theoretical objections to the possibility of conditional phase shifts on single photons that were raised some time ago by Shapiro and by one of us.
A diamond nano-crystal hosting a single nitrogen vacancy (NV) center is optically selected with a confocal scanning microscope and positioned deterministically onto the subwavelength-diameter waist of a tapered optical fiber (TOF) with the help of an atomic force microscope. Based on this nano-manipulation technique we experimentally demonstrate the evanescent coupling of single fluorescence photons emitted by a single NV-center to the guided mode of the TOF. By comparing photon count rates of the fiber-guided and the free-space modes and with the help of numerical FDTD simulations we determine a lower and upper bound for the coupling efficiency of (9.5+/-0.6)% and (10.4+/-0.7)%, respectively. Our results are a promising starting point for future integration of single photon sources into photonic quantum networks and applications in quantum information science.
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