Quantum Frequency Conversion between Infrared and Ultraviolet

Abstract

We report on the implementation of quantum frequency conversion (QFC) between infrared (IR) and ultraviolet (UV) wavelengths by using single-stage upconversion in a periodically poled KTP waveguide. Due to the monolithic waveguide design, we manage to transfer a telecommunication band input photon to the wavelength of the ionic dipole transition of Yb + at 369.5 nm. The external (internal) conversion efficiency is around 5% (10%). The high energy pump used in this converter introduces a spontaneous parametric downconversion (SPDC) process, which is a cause for noise in the UV mode. Using this SPDC process, we show that the converter preserves non-classical correlations in the upconversion process, rendering this miniaturized interface a source for quantum states of light in the UV.Quantum networks and long distance quantum communication rely on the faithful transfer and manipulation of quantum states. Because a single quantum system does not necessarily incorporate all the benefits needed, a hybrid system [1, 2] with different nodes operating at dissimilar frequencies may be used to perform each task at its optimal frequency. The process of quantum frequency conversion (QFC) [3,4] has been established as a means to bridge the gap between differing frequencies while keeping the quantum correlations intact. On the one side of that gap, telecommunications bands in the infrared spectral region have consensually been identified as the optimal wavelengths for quantum state transfer because of low loss in optical fibers and a multitude of experimental studies have shown QFC from [4][5][6][7][8][9][10][11] and to [12][13][14][15][16][17] the telecommunications bands, c.f. Fig. 1 (a). Looking at the other side of the gap, one finds that QFC experiments have so far mainly focused on convenient laser wavelengths or transitions in the red/near-infrared spectral region. However, high fidelity photonic state manipulation strongly benefits from high energy transitions and indeed, most of the beneficial ionic transitions are situated at ultraviolet (UV) wavelengths. Most prominently, the Ytterbium-(Yb + ) transition at 369.5 nm constitutes an almost ideal 2-level quantum system due to the Yb + -ion's specific electronic structure [18]. It has been shown that the S 1/2 → P 1/2 transition can act as a photonic interface for efficient and long lived storage of quantum bits [19], and the Yb-ion proves to be useful for quantum computing [18,20] and fundamental studies of light-matter interactions [21]. While this ion is thus an ideal system for the manipulation of quantum states, its application for quantum networks and long distance quantum communication in a hybrid system is conditioned on the possibility to connect it to the optimal fiber transmission window (c.f. fiber attenuation in Fig. 1 (c)). This is a challenging venture and involves significant engineering efforts because of the large energy gap between in-and output, as well as the properties of nonlinear materials at UV wavelengths. In contrast to a [3-10, 12-17, 24-29]...

Fabrication limits of waveguides in nonlinear crystals and their impact on quantum optics applications

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