Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)
Continuously Tunable Frequency Conversion in Germanium Doped Photonic Crystal Fiber Pumped Near Degeneracy
Leah R. Murphy, Mateusz J. Olszewski, Petros Androvitsaneas, Miguel Alvarez Perez, Will A. M. Smith, Anthony J. Bennett, Peter J. Mosley, Alex O. C. Davis
Large-scale quantum networks comprise various components such as photon sources and transmission lines, which operate at diverse wavelengths. The optimal transmission window through solid silica optical fibre is around 1.55 μm while efficient single-photon sources based on InAs quantum dots (QDs) operate in the range of 900–950 nm [1]. High indistinguishability between photons is also required to realise many quantum information technologies but any two dots seldom emit at the same exact wavelength [2] and sending photons through a quantum network may distort their spectra. Parametric nonlinear processes, such as Bragg-Scattering four wave mixing (BS-FWM), enable frequency conversion of photons while preserving their quantum properties [3],[4].
Large-scale quantum networks comprise various components such as photon sources and transmission lines, which operate at diverse wavelengths. The optimal transmission window through solid silica optical fibre is around 1.55 μm while efficient single-photon sources based on InAs quantum dots (QDs) operate in the range of 900–950 nm [1]. High indistinguishability between photons is also required to realise many quantum information technologies but any two dots seldom emit at the same exact wavelength [2] and sending photons through a quantum network may distort their spectra. Parametric nonlinear processes, such as Bragg-Scattering four wave mixing (BS-FWM), enable frequency conversion of photons while preserving their quantum properties [3],[4].
Future quantum networks will rely on the ability to coherently transfer optically encoded quantum information between different wavelength bands. Bragg-scattering four-wave mixing in optical fiber is a promising route to achieving this, but requires fibers with precise dispersion control and broadband transmission at signal, target, and pump wavelengths. Here, we introduce a photonic crystal fiber with a germanium-doped core featuring group velocity matching at 1550 nm, the telecoms C-band, and 920 nm, within the emission range of efficient single photon sources based on InAs quantum dots. With low chromatic walk-off and good optical guidance even at long wavelengths, large lengths of this fiber are used to achieve nanometer-scale frequency shifts between wavelengths around 920 nm with up to 79.4% internal conversion efficiency, allowing dissimilar InAs dots to be interfaced. We also show how cascading this frequency conversion can be used to generate a frequency comb away from telecoms wavelengths. Finally, we use the fiber to demonstrate tunable frequency conversion of weak classical signals around 918 nm to the telecoms C-band.
Future quantum communication networks will rely on the ability to coherently transfer quantum information between different wavelength bands. Coherent frequency conversion in optical fibre by Bragg-scattering fourwave mixing is a promising route to achieving this, but requires fibres with precise dispersion control and broadband transmission at signal, target and pump wavelengths. Here we introduce a photonic crystal fibre with a germania-doped core, for which the group velocities at 1550 nm and 920 nm are approximately matched, with good optical guidance even at long wavelengths. With low chromatic walk-off, large lengths of this fibre can be used to achieve frequency conversion between two wavelengths several nanometers apart in the GaAs quantum dot emission band. We demonstrate this with up to 78% internal conversion efficiency by pumping with two nearby C-band wavelengths. By cascading this interaction, we also show the generation of a frequency comb. We further demonstrate the potential of this fibre for downconversion between the GaAs dot band and telecoms C-band.
Abstract Low-noise frequency conversion of single photons is a critical tool in establishing fibre-based quantum networks. We show that a single photonic crystal fibre can achieve frequency conversion by Bragg-scattering four-wave mixing of source photons from an ultra-broad wavelength range by engineering a symmetric group velocity profile. Furthermore, we discuss how pump tuning can mitigate realistic discrepancies in device fabrication. This enables a single highly adaptable frequency conversion interface to link disparate nodes in a quantum network via the telecoms band.
Efficient frequency conversion is crucial for interfacing photons with a variety of quantum systems ranging from matter qubits (color centers, quantum dots, atoms), optical fibers, detectors, that often operate at widely different wavelengths. Frequency conversion is also a resource to process quantum information.1, 2 Parametric processes such as sum frequency generation (SFG) and Bragg scattering four wave mixing (BS-FWM) offer a good versatility for such frequency conversion. It is well understood that those nonlinear processes have to be strong enough and satisfy phase matching to achieve a high conversion efficiency. While it is enough to consider those two aspects for moderate conversion efficiency up to 50%, the analysis is a bit more complicated when targeting efficiencies closely approaching unity. We are reviewing the pitfall that must be avoided to indeed reach near unity frequency transduction.
Photon pairs (biphotons) are ubiquitous in quantum technologies. In particular, in quantum imaging and spectroscopy they enable measurements in hard-to-access spectral ranges [1] and increased sensitivity [2] . Among different sources of biphotons, four-wave mixing (FWM) in optical fibers is most efficient due to the large nonlinear interaction lengths and easy integrability into optical networks. However, fiber sources are usually not tuneable except by tuning the pump wavelength. Hollow-core photonic crystals fibers (PCF) filled by noble gas solve this problem: their dispersion landscape can be tailored by simply changing the gas pressure [3] , [4] . Additionally, they are free from Raman scattering and have a higher damage threshold than solid-core fibers [4] .
Single-photon sources based on quantum dots have been shown to exhibit almost ideal properties such as high brightness and purity in terms of clear anti-bunching as well as high two-photon interference visibilities of the emitted photons, making them promising candidates for different quantum information applications such as quantum computing, quantum communication and quantum teleportation. However, as most single-photon sources also quantum dots typically emit light at wavelengths of electronic transitions within the visible or the near infrared range. In order to establish quantum networks with remote building blocks, low-loss single photons at telecom wavelengths are preferable, though. Despite recent progress on emitters of telecom-photons, the most efficient single-photon sources still work at shorter wavelengths. On that matter, quantum frequency down-conversion, being a nonlinear optical process, has been used in recent years to alter the wavelength of single photons to the telecom wavelength range while conserving their nonclassical properties. Characteristics such as lifetime, first-order coherence, anti-bunching and entanglement have been shown to be conserved or even improved due to background suppression during the conversion process, while the conservation of indistinguishability was yet to be shown. Here we present our experimental results on quantum frequency down-conversion of single photons emitted by an InAs/GaAs quantum dot at 903.6 nm following a pulsed excitation of a p-shell exciton at 884 nm. The emitted fluorescence photons are mixed with a strong pump-field at 2155 nm inside a periodically poled lithium niobate ridge waveguide and converted to 1557 nm. Common issues of a large background due to Raman-scattered pump-light photons spectrally overlapping with the converted single photons could largely be avoided, as the pump-wavelength was chosen to be fairly longer than the target wavelength. Additional narrowband spectral filtering at the telecom regime as a result of the small conversion bandwidth and using a high-performance fiber-Bragg-grating solely left the detector dark counts as the only noise source in our setup. Therefore, we could achieve conversion efficiencies of more than 20 %. In order to test the indistinguishability, sequentially emitted photons were fed into a Mach-Zehnder interferometer and spatially as well as temporally overlapped at the output beam splitter. Cross-correlation measurements between both output-ports of the beam splitter exhibit two-photon interference contrasts of more than 40 % prior to and after the down-conversion step. Accordingly, we demonstrate that the process of quantum frequency conversion preserves photon indistinguishability and can be used to establish a versatile source of indistinguishable single photons at the telecom C-Band. Furthermore our scheme allows for converting photons in a wavelength band from 900 nm to 910 nm to the same telecom target wavelength. This enables us to test indistinguishability of frequency-converted photons, originally stemming from different sources with dinstinguishable wavelengths.