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arXiv:2304.13459 · Physical Review Applied

Low-noise quantum frequency conversion in a monolithic cavity with bulk periodically poled potassium titanyl phosphate

Felix Mann, Helen M. Chrzanowski, Felipe Lucas Gewers, Marlon Placke, Sven Ramelow

Published Nov 3, 2023• 28 citations
arXivHTMLPDF

Abstract

Interfacing the different building blocks of a future large-scale quantum network will require efficient and noiseless frequency conversion of quantum light. Nitrogen-vacancy centers in diamond are a leading candidate to form the nodes of such a network. However, the performance of a suitable converter remains a bottleneck, with existing demonstrations severely limited by parasitic noise arising at the target telecommunication wavelength. Here we demonstrate a platform for efficient low-noise quantum frequency conversion based on a monolithic bulk periodically poled potassium titanyl phosphate cavity and show its suitability for the conversion of 637-nm single photons from nitrogen-vacancy centers in diamond to telecommunication wavelengths. By resonantly enhancing the power of an off-the-shelf pump laser, we achieve an internal conversion efficiency of $(72.3\ifmmode\pm\else\textpm\fi{}0.4)\mathrm{%}$ while generating noise of only $(110\ifmmode\pm\else\textpm\fi{}4)\phantom{\rule{0.2em}{0ex}}\mathrm{kHz}/\mathrm{nm}$ at the target wavelength without the need for any active stabilization. This constitutes a fivefold reduction in noise over existing state-of-the-art single-step converters at these wavelengths. We verify the almost-ideal preservation of nonclassical correlations by converting photons from a spontaneous-parametric-down-conversion source and moreover show the preservation of time-energy entanglement via Franson interferometry.

DOI
10.1103/physrevapplied.20.054010

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Interfacing the different building blocks of a future large-scale quantum network will require efficient and noiseless frequency conversion of quantum light. Nitrogen-vacancy centers in diamond are a leading candidate to form the nodes of such a network. However, the performance of a suitable converter remains a bottleneck, with existing demonstrations severely limited by parasitic noise arising at the target telecommunication wavelength. Here we demonstrate a platform for efficient low-noise quantum frequency conversion based on a monolithic bulk periodically poled potassium titanyl phosphate cavity and show its suitability for the conversion of 637-nm single photons from nitrogen-vacancy centers in diamond to telecommunication wavelengths. By resonantly enhancing the power of an off-the-shelf pump laser, we achieve an internal conversion efficiency of (72.3\ifmmode\pm\else\textpm\fi{}0.4)\mathrm{%} while generating noise of only (110\ifmmode±\else\textpm\fi4)kHz/nm(110\ifmmode\pm\else\textpm\fi{}4)\phantom{\rule{0.2em}{0ex}}\mathrm{kHz}/\mathrm{nm}(110\ifmmode±\else\textpm\fi4)kHz/nm at the target wavelength without the need for any active stabilization. This constitutes a fivefold reduction in noise over existing state-of-the-art single-step converters at these wavelengths. We verify the almost-ideal preservation of nonclassical correlations by converting photons from a spontaneous-parametric-down-conversion source and moreover show the preservation of time-energy entanglement via Franson interferometry.
Diamond tin-vacancy (SnV) centers are promising candidates for building quantum network nodes. However, their native photon emission at 619 nm is incompatible with metropolitan-scale networks operating at low-loss telecom wavelengths. To address this, we demonstrate highly efficient, low-noise quantum frequency conversion (QFC) of 619 nm photons to the telecom S-band at 1480 nm. The conversion process combines 619 nm photons with 1064 nm pump light in an actively stabilized cavity containing a bulk monocrystalline potassium titanyl arsenate (KTA) crystal. We achieve an internal (external) conversion efficiency of (48 ± 3) % ((28 ± 2) %) and a noise photon rate per wavelength of (2.2 ± 0.9) cts/s/pm, which is spectrally flat in the investigated frequency range of 40 GHz. Furthermore, we demonstrate that the efficiency remains above 80% of its maximum over a frequency range of 70 GHz. Finally, we generate a string of photons from a single waveguide-embedded SnV − center using a train of excitation pulses and send these through the QFC. After the QFC, we observe telecom light displaying the SnV lifetime, confirming successful conversion. These results represent a critical step towards metropolitan-scale fiber-based quantum networks using SnV − centers.
Diamond tin-vacancy (SnV) centers are promising candidates for building quantum network nodes. However, their native photon emission at 619 nm is incompatible with metropolitan-scale networks operating at low-loss telecom wavelengths. To address this, we demonstrate highly efficient, low-noise quantum frequency conversion (QFC) of 619 nm photons to the telecom S-band at 1480 nm. The conversion process combines 619 nm photons with 1064 nm pump light in an actively stabilized cavity containing a bulk monocrystalline potassium titanyl arsenate (KTA) crystal. We achieve an internal (external) conversion efficiency of (48 +/- 3)% ((28 +/- 2)%) and a noise photon rate per wavelength of 2.2 +/- 0.9 cts/s/pm, which is spectrally flat in the investigated frequency range of 40 GHz. Furthermore, we demonstrate that the efficiency remains above 80% of its maximum over a frequency range of 70 GHz. Finally, we generate a string of photons from a single waveguide-embedded SnV center using a train of excitation pulses and send these through the QFC. After the QFC, we observe a string of telecom photons displaying the SnV lifetime, confirming successful conversion. These results represent a critical step towards metropolitan-scale fiber-based quantum networks using SnV centers.
We present a novel device for the efficient, low-noise down-conversion of low-power light from the red visible spectrum. It can be applied to interface electronic spin-qubits in nitrogen-vacancy centers in diamond to the telecommunication wavelengths and thus offers the possibility to implement large-scale quantum networks in existing fiber infrastructure. In comparison to state-of-the-art frequency converters for this application, the device presented here generates an overall noise rate, which is one order of magnitude better than current record values. Simultaneously, an internal conversion efficiency of 50 % was measured. Furthermore, the converter serves as a feasibility demonstration of a general concept for the down-conversion of light from the sub-micron spectral range to telecommunication wavelengths.
Quantum frequency conversion (QFC) will be an indispensable ingredient in future quantum technologies. For example, large-scale fibre-based quantum networks will require QFC to interconnect heterogeneous building blocks like emitters, channels, memories and detectors. The performance of existing QFC devices - typically realised in periodically-poled nonlinear crystals - is often severely limited by parasitic noise that arises when the pump wavelength lies between the inter-converted wavelengths. Here we comprehensively investigate the noise spectrum of a QFC device pumped by a CW 1064 nm laser. The converter was realised as a bulk periodically-poled potassium titanyl phosphate (ppKTP) crystal quasi-phase-matched for conversion between 637 nm and 1587 nm, which was also polished and coated to resonantly enhance the pump field by a factor of 50. While Raman scattering dominates the noise background from 1140 nm to 1330 nm, at larger energy shifts (beyond 60 THz), parasitic spontaneous parametric down-conversion (SPDC) is the strongest noise source. Further, the noise spectrum was contaminated by a regular succession of narrow-band peaks, which we attribute to a heretofore unidentified higher-order counter-propagating SPDC processes - with quasi-phase-matching orders up to 44 evident in our measurements. This work provides a comprehensive overview of the limiting noise sources in QFC devices that use quasi-phase-matched crystals and will prove an invaluable resource in guiding their future development.
Interconnected quantum devices are the building blocks of quantum networks, where state transduction plays a central role. The frequency conversion of photons into the telecommunication C-band is decisive in taking advantage of current low-loss transmission lines. Here, we report the difference frequency conversion of 637.2 nm fluorescent light from a cluster of NV centers in diamond to tunable wavelengths between 1559.0 nm and 1564.9 nm. In order to avoid detrimental noise from spontaneous emissions, we use a two-step conversion device based on a single-pumped periodically poled lithium niobate waveguide. We observed a total external (internal) conversion efficiency of 3.0 ± 0.1 (20.5 ± 0.8) % with a noise rate of 2.4 ± 0.8 (16 ± 5) cps/GHz.