Temporally versatile polarization entanglement from Bragg reflection waveguides

Abstract

Bragg-reflection waveguides emitting broadband parametric down-conversion (PDC) have been proven to be well suited for the on-chip generation of polarization entanglement in a straightforward fashion [R. T. Horn et al., Sci. Rep. 3, 2314]. Here, we investigate how the properties of the created states can be modified by controlling the relative temporal delay between the pair of photons created via PDC. Our results offer an easily accessible approach for changing the coherence of the polarization entanglement, in other words, to tune the phase of the off-diagonal elements of the density matrix. Furthermore, we provide valuable insight in the engineering of these states directly at the source.The verification of the non-classical nature of quantum objects, such as photons, plays an important role to ensure a successful implementation of forthcoming quantum technologies. Among theses tasks is the preparation and characterization of polarization entanglement that provides some of the strongest evidence of the quantum features of light [1][2][3]. Since the first realizations with parametric down-conversion (PDC) in bulk crystals more than two decades ago [4][5][6], a lot of effort has been put in replacing them both with ferroelectric [7][8][9][10][11] and semiconductor waveguides [12][13][14], which provide higher brightnesses and easier alignment in collinear configurations.In one of such configurations, the cross-polarized PDC photon pairs-called signal and idler-that have at least some nanometers of spectral extent, are divided on a dichroic beam splitter into two paths [15][16][17][18][19]. Due to a strict frequency or energy correlation between signal and idler, a two-partite polarization superposition is created between the output paths of the dichroic mirror. Often, a continuous-wave pump laser is utilized to guarantee tight correlations in the spectral domain [14,20].Bragg-reflection waveguides (BRWs) made of AlGaAs generate indistinguishable photon pairs over several tens of nanometers [14,21] and they have been utilized for narrowband multiplexing of polarization entanglement [22,23]. Due to the very small birefringence between the orthogonally-polarized signal and idler [24], BRWs may work reasonably well in this configuration even without compensating their temporal walk-off. However, their group index difference is large enough to result into an uncompensated phase. Therefore, the engineering of the spectro-temporal degree of freedom of PDC photon pairs from BRWs is essential for the controlled creation and manipulation of the polarization entanglement.Changes in the coherence of the polarization entangled states, that is their phase, provide an interesting degree of freedom for the state manipulation [2]. When regarding PDC sources, this is usually accomplished by * kaisa.laiho@uibk.ac.at modifying the characteristics of one of the entangled parties with birefringent retarders [14,25,26]. However, the phase control over the joint state can possibly be realized more easily [27][28][29]. Here, we generat...

Compensation-free broadband entangled photon pair sources

“…In addition, the spectral brightness of PPSF with the M<10fs/m presented in this paper is ∼10× higher than the previous PPSF [24,25] because of the enhancement in poling technique. The spectral brightness and total brightness of PPSF source is comparable to other nonlinear crystal or waveguide based SPDC broadband polarization entanglement sources [13,23,40], but provide better entanglement quality. Even though the χ (2) in PPSF ( χ (2) eff ∼ 0.09pm/V) is one or two orders of magnitude lower than nonlinear crystals [13,41] (∼1-30pm/V), PPSF can offer one or two orders of magnitudes longer nonlinear interaction length, while the SPDC emission brightness is propotional to ( χ (2) L) 2 .…”

Optimizing the spectro-temporal properties of photon pairs from Bragg-reflection waveguides

“…Some applications such as the generation of polarization entanglement, which we investigate in more detail in Section 5, are very sensitive to this average differential group delay (DGD). Only if it is negligible, such a scheme can be implemented without external optical delay compensation [19,33]. The average DGD between the signal and idler wavepackets is given by…”

“…Finally, we use the scheme in Figure 9 for generating polarization entangled states with multiple spectral bands that are free from temporal delay compensation [17,18,33]. In the absence of background light, the density matrix of the polarization entangled state takes the form [19], ρ = α |HV HV| + D |VH HV| The comparison of the HOM dips at the degeneracy for (a) the graded and (b) M-core BRW structures without any filters reveals the clear difference in the spectral indistinguishability of signal and idler. In (c) and (d) we illustrate the coincidence click probabilities without any filters for the graded and M-core BRW structures when the pump wavelength is moved to 776.4 nm and 775.0 nm, respectively.…”

Understanding photoluminescence in semiconductor Bragg-reflection waveguides: Towards an integrated, GHz-rate telecom photon pair source

“…Recently, BRWs have become increasingly popular for PDC [17,[22][23][24], and considerable effort was dedicated towards optimizing their performance for different tasks. For example, not only integrated electrically injected pump lasers on the same chip as the PDC sources could be demonstrated [25,26], but also various aspects of the preparation of polarization entangled states [27][28][29], like the compensation of the birefringent group delay [30].…”