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Towards a single photon frequency conversion

Oscar A. Jimenez Gordillo

Published May 1, 2015• 0 citations
OpenAlex

Abstract

Quantum frequency up-conversion (QFC) of non-classical states of light allows the\nintegration of different quantum systems working at different energies. This process takes\nadvantage of telecommunication wavelengths photons for optical fiber transmission of\nquantum information, and near visible wavelengths for data manipulation and storage.\nThe key objective of QFC is to guarantee that the input photon number probability\ndistribution is maintained after the conversion process. With this, we will be able to\nefficiently study the single-photon emission properties of epitaxial InAs/GaAs quantum\ndots embedded in a photonic crystal nanocavities without the problems that inefficient IR\nwavelengths detectors imply. The up-conversion process consists on combining two\noptical fields, in a nonlinear medium, to generate a third field that is equal to the two inputs\nfrequency sum. To fulfill the momentum conservation, required by this process, we need\nto compensate the wave vector mismatch between the output and input beams. This is\nachieved by using a grating in the nonlinear medium, a process named quasi-phase\nmatching (QPM). In this work we use a Zinc doped periodically poled LiNbO3 (Zn:PPLN\nWG) waveguide as the nonlinear material. To achieve the quantum frequency conversion\nof a semiconductor quantum dot (QD) single photons emission, embedded in a photonic\ncrystal nanocavity, we started by characterizing the performance of the PPLN-WG in the\noptical power macro-regime. Signal photons, produced by an 1175nm laser, simulating\nour QD emission line, are combined with pump photons, produced by a C-band laser,\nwith a dichroic mirror before entering the PPLN waveguide. In order to achieve the phase\nmatched wavelength in the PPLN, its temperature must be finely tuned. The signal\ncoming out of the PPLN waveguide is filtered and analyzed.

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Quantum frequency up-conversion (QFC) of non-classical states of light allows the\nintegration of different quantum systems working at different energies. This process takes\nadvantage of telecommunication wavelengths photons for optical fiber transmission of\nquantum information, and near visible wavelengths for data manipulation and storage.\nThe key objective of QFC is to guarantee that the input photon number probability\ndistribution is maintained after the conversion process. With this, we will be able to\nefficiently study the single-photon emission properties of epitaxial InAs/GaAs quantum\ndots embedded in a photonic crystal nanocavities without the problems that inefficient IR\nwavelengths detectors imply. The up-conversion process consists on combining two\noptical fields, in a nonlinear medium, to generate a third field that is equal to the two inputs\nfrequency sum. To fulfill the momentum conservation, required by this process, we need\nto compensate the wave vector mismatch between the output and input beams. This is\nachieved by using a grating in the nonlinear medium, a process named quasi-phase\nmatching (QPM). In this work we use a Zinc doped periodically poled LiNbO3 (Zn:PPLN\nWG) waveguide as the nonlinear material. To achieve the quantum frequency conversion\nof a semiconductor quantum dot (QD) single photons emission, embedded in a photonic\ncrystal nanocavity, we started by characterizing the performance of the PPLN-WG in the\noptical power macro-regime. Signal photons, produced by an 1175nm laser, simulating\nour QD emission line, are combined with pump photons, produced by a C-band laser,\nwith a dichroic mirror before entering the PPLN waveguide. In order to achieve the phase\nmatched wavelength in the PPLN, its temperature must be finely tuned. The signal\ncoming out of the PPLN waveguide is filtered and analyzed.
The quantum nature of light at the single photon level allows for unique applications that classical physics neither predicts nor describes. Most notably, appropriate conditions may cause the states of photons with indistinguishable properties to become entangled, enabling novel approaches to quantum computation, secure communications, and metrology. \n \nAny operational quantum information network transporting entangled-photon states must establish a high-fidelity link between its distant nodes despite the inherent fragility of entangled states. However, the lack of a universal operating wavelength for all optical devices makes this a substantial challenge. Telecommunication fibers, entangled photon pair sources, quantum optical memories, and quantum repeaters all function in disparate spectral windows dictated by the properties of the materials used to fabricate them. Quantum frequency conversion (QFC), the single-photon limit of nonlinear parametric sum and difference frequency generation in optics, offers a bridge between spectral regions with full preservation of quantum state character, including entanglement. \n \nPeriodically poled optical waveguides in ferroelectric crystals are versatile tools in nonlinear optics. Confining the nonlinear interaction to a waveguide greatly enhances its efficiency compared to bulk optics. Periodic poling, the process of periodically inverting the domains of a ferroelectric medium using a strong electric field, enables a variety of quasi-phase matching configurations to engineer a desired nonlinear interaction. \n \nThis work concentrates on the design, development, fabrication, and characterization of a titanium-diffused periodically poled lithium niobate (Ti:PPLN) waveguide device. This device is designed to execute single-step quantum frequency conversion from 369.5nm to 1550nm in order to facilitate a quantum state transfer between standard fiber telecommunication wavelengths and an atomic quantum memory system employing trapped ytterbium ions. \n \nThe creation of phase matching conditions for such an extreme distance in frequency demands precise control of high magnitude electric fields on the single-micron scale. As the first demonstration of its kind in Ti:PPLN, the development of these devices included novel improvements to existing fabrication methods, improving the state-of-the-art of precision, quality, and yield for poling of ferroelectric crystals.
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.
We demonstrate background-free quantum frequency conversion of single photons from an epitaxially-grown InAs quantum dot. Single photons at \approx 980 nm are combined with a pump laser near 1550 nm inside a periodically-poled lithium niobate (PPLN) waveguide, generating single photons at \approx 600 nm. The large red-detuning between the pump and signal wavelengths ensures nearly background-free conversion, avoiding processes such as upconversion of anti-Stokes Raman scattered pump photons in the PPLN crystal. Second-order correlation measurements on the single photon stream are performed both before and after conversion, confirming the preservation of photon statistics during the frequency conversion process.
Near-infrared single photon sources in telecommunication bands, especially at 1550 nm, are required for long-distance quantum communication. Here a down-conversion quantum interface is implemented, where the single photons emitted from single InAs quantum dot at 864 nm is down converted to 1552 nm by using a fiber-coupled periodically poled lithium niobate (PPLN) waveguide and a 1.95 μmm pump laser, and the frequency conversion efficiency is ∼40%. The single-photon purity of quantum dot emission is preserved during the down-conversion process, i.e., g (2) (0), only 0.22 at 1552 nm. This present technique advances the III–V semiconductor quantum dots as a promising platform for long-distance quantum communication.
We show that quantum frequency conversion (QFC) can overcome the spectral distinguishability common to inhomogeneously broadened solid-state quantum emitters. QFC is implemented by combining single photons from an InAs/GaAs quantum dot (QD) at 980 nm with a 1550 nm pump laser in a periodically poled lithium niobate (PPLN) waveguide to generate photons at 600 nm with a signal-to-background ratio exceeding 100:1. Photon correlation and two-photon interference measurements confirm that both the single photon character and wave packet interference of individual QD states are preserved during frequency conversion. Finally, we convert two spectrally separate QD transitions to the same wavelength in a single PPLN waveguide and show that the resulting field exhibits nonclassical two-photon interference.
Quantum frequency converters are key enabling technologies in photonic quantum information science to bridge the gap between quantum emitters and telecom photons. Here, we report a coherent frequency converter scheme combining a fiber-coupled nonlinear optical Lithium Niobate waveguide with a fiber-pigtailed single-photon source based on semiconductor quantum dots. Single and indistinguishable photons are converted from 925.7 nm to the telecommunication C-band, with a 48.4% end-to-end efficiency and full preservation of single-photon purity and indistinguishability. The integration of the two fiber-based modules achieving top-level performance represents an important step toward the practical interconnection of future quantum information processing systems operating at different wavelengths.