A single-crystal source of path-polarization entangled photons at non-degenerate wavelengths

We demonstrate a bright, narrowband, compact, quasi-phase-matched single-crystal source generating path-polarization-entangled photon pairs at 810 nm and 1550 nm at a maximum rate of 3 Mcounts/(s THz mW) after coupling to single-mode fiber, and with two-photon interference visibility above 90%. While the source can already be used to implement quantum communication protocols such as quantum key distribution, this work is also instrumental for narrowband applications such as entanglement transfer from photonic to atomic qubits, or entanglement of photons from independent sources.Comment: 7 pages, 3 figures, published in Optics Express (2008

Quantum-polarization state tomography

We propose and demonstrate a method for quantum-state tomography of qudits encoded in the quantum polarization of N-photon states. This is achieved by distributing N photons nondeterministically into three paths and their subsequent projection, which for N = 1 is equivalent to measuring the Stokes (or Pauli) operators. The statistics of the recorded N-fold coincidences determines the unknown N-photon polarization state uniquely. The proposed, fixed setup manifestly rules out any systematic measurement errors due to moving components and allows for simple switching between tomography of different states, which makes it ideal for adaptive tomography schemes

Bloch mode excitation in two-dimensional photonic crystals imaged by Fourier optics

Coupling into the Bloch modes of a two-dimensional photonic crystal (PhC) field is investigated by Fourier optics. The PhC was designed to operate in the second band above the air-light line, close to the autocollimation regime for TE polarization. The sample was fabricated in an InP-based heterostructure and an access ridge waveguide provides in-plane excitation of the PhC. The spatial Fourier transform of the field maps obtained from finite-difference time-domain simulations and those calculated by plane-wave expansion are compared to the experimentally obtained equifrequency surfaces (EFS). The shape of the imaged EFS and its variation with the excitation wavelength is shown to be consistent with the theoretical simulations. Finally, the results indicate that if combined with different excitation geometries, Fourier optics can be a powerful technique to assess photonic crystal devices and to design efficient structures

Narrowband polarization-entangled photon pairs distributed over a WDM link for qubit networks

We present a bright, narrowband, portable, quasi-phase-matched two-crystal source generating polarization-entangled photon pairs at 809 nm and 1555 nm at a maximum rate of 1.2 10^6 s-1 THz-1 mW-1 after coupling to single-mode fiber. The quantum channel at 1555 nm and the synchronization signal gating the single photon detector are multiplexed in the same optical fiber of length 27 km by means of wavelength division multiplexers (WDM) having 100 GHz (0.8 nm) spacing between channels. This implementation makes quantum communication applications compatible with current high-speed optical networks.Comment: 8 pages, 4 figure

Integrated directional coupler wavelength filter for 1.5 mm

Разработан интегральный узкополосный фильтр для длины волны 1,5 мкм, состоящий из канального волновода на одномерной решетке Брэгга, связанного со стандартным оптическим волноводом. Профиль распределения показателя преломления в волноводе на одномерной решетке Брэгга, а также его параметры выбраны таким образом, чтобы достичь равенства эффективного показателя преломления neff=1,48–1,5 на длине волны ~1,5-1,55 мкм. Значение величины neff соответствует дисперсионным свойствам стандартного оптического волновода. Ширина канала составляет 1,5 мкм, что обеспечивает существование мод только низшего порядка. В качестве второго канала направленного ответвителя выбран стандартный волновод с параметрами: ncw=1,86 — показатель преломления и b=0,42 мкм — толщина волноводного слоя, что позволяет иметь одномодовый режим в области длины волны 1,5 мкм. Проведено моделирование дисперсионных свойств фильтра на основе канального волновода на одномерной решетке Брэгга при использовании векторного формализма метода сшивания. Для расчета его рабочих характеристик применена теория "супермод". Показано, что вследствие существенного различия в ходе дисперсионных характеристик двух каналов направленного ответвителя согласование фазовых скоростей мод обеспечивается только в области λ∼1,5 мкм. Это позволяет иметь высоко селективные свойства для мод ТЕ поляризации в предлагаемом фильтре. Рассчитаны спектры пропускания направленного ответвителя с различными параметрами. Показана возможность достижения значения ширины полосы пропускания порядка субнанометров при общей длине структуры ~1 мм, что подтверждено экспериментально.The narrow band wavelength filter for 1.5 µm based on the coupling between a channel Bragg reflection waveguide (BRW) and a conventional low loss, weakly guiding channel waveguide has been designed. The Bragg reflection waveguide refractive index profile and its parameters were chosen to have the effective refractive index about neff=1,48–1,5 at the wavelength ∼ 1.5–1.55 µm. The effective refractive index value corresponds to conventional optical waveguide dispersion. The channel width is 1.5 µm to provide only low order mode existing. As a second channel of the directional coupler conventional waveguide is taken with the following parameters: ncw=1.86 — refractive index and b=0.42 µm — thickness of core that allowed having a single mode regime in the 1.5 µm wavelength range. The dispersion properties of the narrow band wavelength filter have been vectorially investigated by means the mode matching method. Filter transmission spectra were calculated using the “supermode” theory formalism. The TE and TM mode dispersion of the separated directional coupler channels and the narrow band filter were presented. It was shown that due to significant difference in dispersion of the channels the mode phase velocity agreement could be reached only in the narrow range around 1.5 µm. This fact is allowed having a highly wavelength selective properties for TE mode of the proposed filter. The directional coupler transmission spectra were calculated for different parameters, the channel distance increasing causes decreasing the full width at half maximum (FWHM) and growing coupler length defined by beating length. It is shown that wavelength bandwidth gives the subnanometers range at the filter length about 1 mm that was also proved experimentally