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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - A Ps-Pulse Laser for Ultrafast Entanglement Generation at 42.66 Ghz Repetition Rate
摘要： Entangled-photon sources are a key element for most quantum-information applications, ranging from quantum communication systems to quantum simulators and full photonic quantum computers. Spontaneous parametric downconversion (SPDC), requiring a pump laser and a nonlinear medium, is up-to-date still the preferred technique to generate such photonic entanglement. To scale the transmission rates or gate operations of the aforementioned applications, high clock rates of the pump lasers are needed. Moreover, photonic quantum-processing algorithms require photons with not only high entanglement visibility but also high spectral indistinguishability and purity. This can either be achieved by narrow spectral filtering, costly in rate, or by suitable choice of the pump pulse length (typically in the ps range). Attempts have been made to increase the rate of conventional fs-lasers by adding multiple spatial-paths [1] but this method cannot be used to go beyond GHz repetition rates. This results in the quest to produce pump lasers with very high repetition rate, ps-pulse width and high enough power to drive the weak SPDC process. We report here on an implementation of a compact and fast ps-pump laser setup which can generate entangled photon pairs at repetition rates up to 42.66 GHz. Our entangled-photon source can be operated at these high generation rates and is basically only limited by the jitter of the detectors. In addition, we achieve a high spectral purity without narrow bandpass-filtering by mutually matching the length of the PDC crystal and the pump laser‘s pulse length [2], yielding polarisation-entangled photon pairs with minimal spectral correlations. The pump laser consists of a monolithically integrated mode locked DBR laser emitting pulses with a fixed repetition rate of 42.66 GHz at 1554 nm with 1.8 ps length. A pulse picker, made of an intensity modulator and driven by the same controller as the pump laser, allows adjusting the repetition rate used in the experiment, as depicted in Fig. 1. The power of the laser pulses is boosted with a series of optical amplifiers to 25 dBm. Conversion to the proper wavelength for the PDC process takes place in a second-harmonic-generation module, which finally generates output pulses of (cid:79)=777 nm to pump the entangled-photon source. Our entanglement source is based on a Sagnac-interferometer, where a ppKTP crystal is pumped by two pulsed laser beams from opposed directions. The generated photon pair at 1554 nm is split into different spatial modes by a polarizing beamsplitter, collected in optical fibers and detected with InGaAs detectors (10% efficiency). An experimental analysis of the entangled polarisation state yields an average visibility of 0.95 ± 0.01, corresponding to a CHSH parameter of S=2.685 which is in good agreement with theoretical calculations. Two-photon interference based on the Hong-Ou-Mandel effect also shows a high degree of purity.
关键词： ppKTP crystal,quantum-information applications,high clock rates,Sagnac-interferometer,InGaAs detectors,entangled-photon sources,ps-pump laser,spectral purity,spontaneous parametric downconversion
更新于2025-09-11 14:15:04
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Compact Femtosecond Laser Direct Written Integrated Retarders Based on Embedded Nanogratings
摘要： Femtosecond Laser Direct Writing (FLDW) is a well-known rapid prototyping method to fabricate integrated optical circuits in glass chips. These circuits have been used to show various quantum information applications, using the states of photons as qubits. Generally, when transmitting information via single photons, it is desirable to make use of all possible degrees of freedom that this photon has to offer, in order to increase the amount of information transferred per photon. One of these degrees of freedom is the photon’s polarization. To make use of this degree of freedom, devices capable of manipulating the polarization are required. Various approaches for manipulating the polarization of photons in a FLDW circuit have been demonstrated before, of which some were used for quantum information applications. In our work, we present a novel method of polarization control using embedded nanogratings as waveplates. These nanogratings are highly birefringent self-assembled structures. Due to their relatively high form birefringence on the order of Δn = 10^-3, they can be used as a compact waveplate enabling further miniaturization of integrated optical circuits. The properties of these gratings can be altered by using different laser parameters during the inscription process. We have added various waveplates made of nanogratings into FLDW waveguides. We demonstrate their functionality as waveplates of different retardation and optical axis orientations using crossed polarizer birefringence measurements. Due to the chosen writing geometry, a full control over the direction of the optical axis can be achieved. The thickness of these structures is on the order of a few hundred micrometers. Former approaches for polarization control in FLDW circuits required structures in the range of millimetres to centimeters. Some of these approaches were limited in the achievable optical axis orientations. Our waveplates can be both used for classical applications and as single qubit quantum gates, which will be demonstrated. Waveplate structures usable as Hadamard, Pauli-x, Pauli-z and Pi/8th gates have been fabricated. The transferability to fibers will be discussed.
关键词： integrated optical circuits,polarization control,quantum information applications,waveplates,nanogratings,Femtosecond Laser Direct Writing
更新于2025-09-11 14:15:04