Polarisation-preserving photon frequency conversion from a trapped-ion-compatible wavelength to the telecom C-band

Abstract

Given the great success in encoding, manipulating, storing and reading-out quantum information in their electronic states, trapped atomic ions represent a powerful platform with which to build, or integrate into, the nodes of quantum networks [5,6]. Indeed, an elementary quantum network consisting of ions in two traps a few meters apart, has been entangled via travelling ultraviolet photons [7]. A challenge is that most readily-accessible photonic transitions in trapped ions lie at wavelengths that suffer significant absorption loss in materials for manipulating and guiding light, thereby limiting the internode networking distance. Another challenge is that ionic transitions are fixed and narrowband, such that, except in rare cases [8], they cannot be interfaced with other examples of quantum matter to enable new ion-hybrid quantum systems [9]. Note that frequency-distinguishable quantum systems can be linked via their photons, though at the cost of reducing the efficiency of making that link [10].The aforementioned challenges could be overcome using quantum frequency conversion (QFC) [11,12]; a nonlinear optical process in which a photon of one frequency is converted to another, whilst preserving all the quantum and classical photon properties. QFC of single photons has recently been studied in a variety of contexts [13][14][15][16][17][18] and is typically achieved using three-wave mixing in a secondorder non-linear ( 2 ) crystal. It has been shown that QFC can preserve a broad range of photon properties, including first-and second-order coherence, and pre-existing photonphoton entanglement [12,19,20]. QFC could, therefore, act as a quantum photonic adapter for trapped ions, allowing their high-energy photonic transitions to be interfaced with the lower-energy photons better suited for long-distance travel through optical fibers, or with other forms of quantum matter.Interfacing trapped ions with the telecom wavelengths of 1310 nm (O band) or 1550 nm (C band) is particularly Abstract We demonstrate polarisation-preserving frequency conversion of single-photon-level light at 854 nm, resonant with a trapped-ion transition and qubit, to the 1550-nm telecom C band. A total photon in / fiber-coupled photon out efficiency of ∼30% is achieved, for a free-running photon noise rate of ∼60 Hz. This performance would enable telecom conversion of 854 nm polarisation qubits, produced in existing trapped-ion systems, with a signal-to-noise ratio greater than 1. In combination with near-future trappedion systems, our converter would enable the observation of entanglement between an ion and a photon that has travelled more than 100 km in optical fiber: three orders of magnitude further than the state-of-the-art.

Light-matter entanglement over 50 km of optical fibre

“…The overall efficiency of the frequency-conversion setup, including spectral filtering, is 0.25±0.02, measured with classical 854 nm light. For a detailed description see [13]. A short overview of the contributing photon losses are summarized in table I. Multiplying all the transmissions together leads to a total expected probability of detecting the photon after 50 km of (6.5 ± 1.5) × 10 −4 , which is consistent to within one standard deviation with the measured value of 5.3 × 10 −4 .…”

“…Photons from solid-state memories [14], cold gas memories [15,16], quantum dots and nitrogen-vacancy centres [17] have been converted to telecom wavelengths. Frequency conversion of photons from ions has recently been performed, including to the telecom C band (without entanglement) [18], to the telecom * ben.lanyon@uibk.ac.at, † These authors contributed equally O band with entanglement over 80 m [19] and directly to an atomic Rubidium line at 780 nm [20].We inject single-mode fibre-coupled photons from the ion into a polarisation-preserving photon conversion system (previously characterised using classical light [13]). In summary, a χ 2 optical nonlinearity is used to realise difference frequency generation, whereby the energy of the 854 nm photon is reduced by that of a pump-laser photon at 1902 nm, yielding 1550 nm.…”

“…A Raman laser pulse triggers emission of an 854 nm photon into the cavity, which exits to the right. The photon, polarisation-entangled with two electronic qubit states of the ion (two Zeeman states of the D J=5/2 manifold, not shown), is then wavelength-converted to 1550 nm using difference frequency generation involving ridge-waveguide-integrated periodically-poled lithium niobate (PPLN) chips and a strong (∼ 1 W) pump laser at 1902 nm [13]. The photon then passes through a 50 km single-mode fibre spool, is filtered with a 250 MHz bandwidth etalon (SF) to reduce noise from the conversion stage [13], and is polarisation-analysed using waveplates, a polarising beam splitter (PBS) and a solid-state single photon counting module (SPCM, InGaAs ID230 from IDQuantique).…”

“…In a third experiment, the 854 nm entangled state is reconstructed right out of the vacuum chamber (without conversion), yielding F m =0.967 ± 0.006. The observed drop in fidelity through the conversion stage alone is dominated by added photon noise (caused by Anti-Stokes Raman scattering of the pump laser [13]). The infidelity in the 854 nm state is consistent with that achieved in [11].The total probability that a Raman pulse led to the detection of a photon after 50 km was P = 5.3 × 10 −4 , which given an attempt rate of 2.2 kHz yielded a click rate of ≈ 1 cps.…”

Quantum optical frequency up-conversion for polarisation entangled qubits: towards interconnected quantum information devices

“…To ensure that the wavelength conversion process does not deteriorate the polarisation state between the input and output photon, two requirements need to be satisfied [26][27][28][29][30]. First, the conversion efficiency in both arms needs to be equalised; second, the optical phase difference between upper and lower arms needs to be zero to avoid polarisation state rotation.…”