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Photonics Global Conference

Photon frequency up-conversion with pump-induced second-harmonic noise suppression

Maotong Liu, Han Chuen Lim, Akio Yoshizawa, Hidemi Tsuchida

Published Jan 1, 2010• 0 citations
Publisher

Abstract

A numerical simulation study is carried out to understand the optimal design of single photon frequency up-converter that achieves a large suppression of background noise without compromising photon frequency up-conversion efficiency. We show that by introducing a controlled amount of phase mismatch to the parasitic process of second-harmonic generation of the pump, background noise photons due to this process can be reduced effectively.

DOI
10.1109/pgc.2010.5706138

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•Aug 3, 2016•0 citations
A numerical simulation study is carried out to understand the optimal design of single photon frequency up-converter that achieves a large suppression of background noise without compromising photon frequency up-conversion efficiency. We show that by introducing a controlled amount of phase mismatch to the parasitic process of second-harmonic generation of the pump, background noise photons due to this process can be reduced effectively.
We demonstrated few-photon-level mid-infrared imaging at 3.39 μm by efficient frequency up-conversion. The quantum frequency up-conversion efficiency of mid-infrared photons was high to 32.9%, while mid-infrared images were up-converted with the quantum efficiency of 12.5%. The mid-infrared imaging sensitivity was improved by counting the frequency up-conversion photons temporally confined within the time-window of the pump pulse. The frequency up-conversion system induced a background noise of 1.1 × 103 counts per second.
Suppression of noise photons from unwanted nonlinear processes is a major hurdle in generating frequency-degenerate correlated photon pairs for quantum information, e.g. continuous-variable quantum computing [1]. On-chip, telecom-pumped and frequency-degenerate photon pairs have been produced either using second harmonic generation (SHG) followed by spontaneous parametric down conversion (SPDC) in thin film lithium niobate (TFLN) waveguides [2], or by dual-pump spontaneous four-wave mixing (DP-SFWM) in microresonators [3], [4]. However, the former needs careful filtering of the SHG pump, while the latter suffers from single-pump (SP-) SFWM contributions, which decrease the purity of the generated quantum states. SP-SFWM can be suppressed by linearly coupling resonators at the expense of reduced bandwidth [3] or efficiency [4].
Quantum noise in a model of singly resonant frequency doubling, including phase mismatch and driving in the harmonic mode, is analyzed. The use of a nonlinear normalization allows us to disentangle in the spectra the squeezing induced by the system dynamics from the deleterious effect of the noise coming from the various inputs. The physical insight gained permits the elaboration of general criteria to optimize noise-suppression performance. The subsequent application to the specific system here addressed reveals excellent squeezing behavior. In particular, unlimited degrees of squeezing in the harmonic mode are possible by means of an adequate phase mismatch or driving in the harmonic mode. This is in contrast with the standard phase-matched second-harmonic generation in which the squeezing is limited to 1/9. The applicability of the model, as well as possible experimental implementations, is extensively discussed.
Spatial-mode-selective frequency conversion is potentially useful for both classical and quantum communication applications. By a judicious choice of the quasi-phase-matching period in a Kai(2) multimode waveguide, such conversion can be achieved with high efficiency (close to 100%) and with low crosstalk (< -20 dB). For space-division multiplexing application with classical signals, where each spatial mode represents a separate signal channel, the selective conversion of a spatial mode without disrupting other signal modes can be used for reconfigurable spatial-mode de-multiplexing. This classical de-multiplexing capability can be also extended to the quantum regime, where the quantum state of the signal is preserved during frequency conversion, owing to the unitary nature of the sum-frequency generation (SFG) process. Building upon our previous experimental demonstration of the classical spatial-mode-selective frequency up-conversion in a two-mode PPLN waveguide, here we report the extension of this work into the single-photon-level regime. The signal (1540 nm) in either a single mode (TM00 or TM01) or a superimposition mode (TM00+TM01, TM00+iTM01) of the waveguide is selectively up-converted into TM01 SFG mode, by interacting with an appropriate pump mode (1560 nm). An accurate measurement of the single-photon-level SFG signals requires thorough filtering of the unwanted photons contributed by the second harmonic of the pump, residual pump noise extending to the signal band, and the Raman noise generated in the waveguide. We have investigated these unwanted photon sources and suppressed them by a combination of thin-film-interference and volume-Bragg-grating filters. Resulting single-photon-counting measurements show >70% internal conversion efficiency, better than -12dB crosstalk, and >100 ratio of the signal to background photon counts for all selected modes and mode superpositions.
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.