A new type of single-photon spectrograph combining a tunable optical filter and a dispersive element is presented for measurement of the spectral properties of the two-photon state. In comparison with the previous single-photon spectrograph which is merely based on the dispersive Fourier transformation (DFT) technique, this scheme avoids the need for additional wavelength calibration and the electronic laser trigger for coincidence measurement; therefore, its application is extended to continuous wave (CW) pumped two-photon sources. The achievable precision of the spectrum measurement has also been discussed in theory and demonstrated experimentally with a CW pumped periodically poled lithium niobate (PPLN) waveguide-based spontaneous parametric down conversion photon source. Such a device is expected to be a versatile tool for the characterization of the frequency entangled two-photon state.
A new type of single-photon spectrograph combining a tunable optical filter and a dispersive element is presented for measurement of the spectral properties of the two-photon state. In comparison with the previous single-photon spectrograph which is merely based on the dispersive Fourier transformation (DFT) technique, this scheme avoids the need for additional wavelength calibration and the electronic laser trigger for coincidence measurement; therefore, its application is extended to continuous wave (CW) pumped two-photon sources. The achievable precision of the spectrum measurement has also been discussed in theory and demonstrated experimentally with a CW pumped periodically poled lithium niobate (PPLN) waveguide-based spontaneous parametric down conversion photon source. Such a device is expected to be a versatile tool for the characterization of the frequency entangled two-photon state.
In this Letter, a hybrid frequency-time spectrograph combining a tunable optical filter and a dispersive element is presented for measurement of the spectral properties of the two-photon state. In comparison with the previous single-photon spectrograph utilizing the dispersive Fourier transformation (DFT) technique, this method is advanced since it avoids the need for additional wavelength calibration and the electronic laser trigger for coincidence measurement; therefore, its application is extended to continuous wave (CW) pumped two-photon sources. The achievable precision of the spectrum measurement has also been discussed in theory and demonstrated experimentally with a CW pumped periodically poled lithium niobate (PPLN) waveguide-based spontaneous parametric down-conversion photon source. Such a device is expected to be a versatile tool for the characterization of the frequency entangled two-photon state.
An experimental detection scheme is demonstrated for directly measuring the spectrum of a parametric down-conversion (PDC) source by the use of a highly dispersive fibre. A temporally narrow pulse envelope is spectrally stretched in time by chromatic group velocity dispersion. In order to maximise this effect the high group velocity dispersion of a commercially available dispersion compensating fibre is employed. If the pulse contains only a single photon on average, detection with a conventional APD yields precise information about the arrival time of the photon at the end of the fiber. When carefully calibrated one can accurately reconstruct the original spectral distribution from the measured temporal distribution. Harnessing a single mode fiber for this type of spectrometer brings the benefits of low loss due to efficient guidance, simplicity of alignment and the necessity of only a single detector to scan the spectrum. A fibre assisted single photon spectrograph is employed to measure the correlation spectrum of a bi-photonic state. State generation is achieved by a type-II parametric downconversion process at 1550 nm in a KTP waveguide structure.
The frequency entangled biphoton source generated via spontaneous parametric down-conversion (SPDC) process has found important applications in the fields of quantum clock synchronization, quantum communication, quantum information processing, etc. As quantum technologies evolve, quantitative characterization of the frequency entanglement becomes necessary and has been implemented by measuring the spectral properties of the biphoton state. However, due to the high dark rate and low quantum efficiency of the InGaAs single-photon detectors, direct measurement of the spectral properties of the biphoton state at optical communication wavelength is hard to implement. In this paper, we report the measurement of the spectral properties of a biphoton state at optical communication wavelength which is generated from periodically poled potassium titanyl phosphate (PPKTP) pumped by an ultra-short pulsed optical source at 787 nm. Based on the coincidence measurement setup together with two infrared spectrometers, the spectra of the signal and idler photons are obtained with their center wavelengths being 1574.4 nm and 1574.9 nm, while their 3-dB bandwidths being 35.3 nm and 37.6 nm respectively. The joint spectrum of the photon pair is observed as well and shows a coincident-frequency entanglement and a joint spectrum bandwidth of 3 nm. According to the ratio of the single-photon spectral bandwidth to the joint spectral bandwidth of the photon pairs, the degree of frequency entanglement is quantified to be 12, denoting a relatively high quality of the entanglement.
Quantum information systems are commonly operated in conventional communication bands (1310 and 1550 nm) over an optical fiber to take advantage of low transmission loss. However, the detection and spectral measurement of single photons in these communication bands are limited due to high noise and low sensitivity of single photon detectors in the wavelength ranges. To demonstrate high efficiency detection and high sensitivity spectral measurement, we have implemented a single photon detector and a spectrometer based on frequency up-conversion technology. This detector and spectrometer uses a 5-cm periodically poled lithium niobate (PPLN) waveguide and a tunable pump laser around 1550 nm, to convert signal photons around 1310 to 710 nm. The converted photons are then detected by a silicon-based avalanche photodiode (APD). The overall detection efficiency of the single photon detector is as high as 32%, which is three times higher than commercial InGaAs APDs. The sensitivity of the spectrometer is measured to be −126 dBm, which is at least three orders-of-magnitude better than any commercial optical spectrum analyzer in this wavelength range.
We report the generation and characterization of a frequency anti-correlated entangled biphoton source at the wavelength of 1560nm via a continuous-wave laser pumped spontaneous parametric down-conversion process. The spectra of the signal and idler photons are measured to have their center wavelength being 1559.7nm and 1559.8nm while their 3-dB bandwidths being 3.2nm and 3.15nm respectively. The joint spectrum of the photon pair is observed to be frequency anti-correlated and have a spectral bandwidth of 0.5nm. According to the ratio of the single-photon spectral bandwidth to the joint spectral bandwidth of the photon pairs, the degree of frequency entanglement is quantified to be 6.4. By investigating the relative overlap between the achieved two-photon spectral intensity function and its transpose, the spectral indisinguishablity of the photon pairs is expected to be 90%. Based on a HOM interferometric coincidence measurement setup, a visibility of 89.8% is demonstrated, which shows a good agreement with the expectation.
The frequency entangled photon pairs generated by spontaneous parametric down-conversion (SPDC) possess wide applications in quantum optics and relevant fields.To facilitate the practical quantum information technologies,particularly in optical fiber links,a frequency entangled source at telecommunication wavelength with features of compactness,portability,high efficiency and miniaturization is highly desired.In this paper,we report the experimental generation of a miniaturized frequency entangled source in telecommunication band from a 10 mm-long type-Ⅱ periodically poled lithium niobate (PPLN) waveguide pumped by a 780 nm distributed Bragg reflector (DBR) laser diode.The frequency entangled photon pairs generated by SPDC possess wide applications in quantum optics and relevant fields.When the DBR laser diode is driven by a current of 170 mA at a temperature controlled to 20℃,the output power is measured to be 70.4 mW with a central wavelength of 780.585 nm.Under this pump,the orthogonally-polarized photon pairs are generated and output from the PPLN waveguide.After filtering out the remaining pump by three high-performance long-pass filters mounted on an adjustable U-type fiber bench,the photon-pair generation rate,spectral and temporal properties of the generated frequency entangled source are measured.The results show that the generation rate of the photon pairs,after being corrected for the detection efficiencies of the single photon detectors and the optical losses,is achieved to be 1.86×107 s-1 at a pump power of 44.9 mW (coupled into the waveguide).Optimizing the working temperature of the waveguide and fixing it at 46.5℃,the frequency degeneracy of the SPDC generated photon pairs is realized.Based on the coincidence measurement setup together with two infrared spectrometers,the spectra of the signal and idler photons are obtained with their center wavelengths of 1561.43 nm and 1561.45 nm,and their 3-dB bandwidths of 3.62 nm and 3.60 nm respectively.The joint spectrum of the photon pair is observed,showing a joint spectrum bandwidth of 3.18 nm.The degree of frequency entanglement is quantified to be 1.13 according to the bandwidth ratio between the single photon spectrum and the joint spectrum.Furthermore,based on the Hong-Ou-Mandel (HOM) interferometric coincidence measurement setup,a visibility of about 96.1% is observed,which indicates the very good frequency indistinguishibility of the down-converted biphotons.The measured 3-dB width of the HOM dip is 1.47 ps and shows good agreement with the measured single-photon spectral bandwidth.The experimental results lay a solid foundation for developing portable,miniaturized frequency entangled sources at telecommunication band for the further applications in quantum information areas,such as quantum time synchronization.