OSA/ CLEO 2011
JThB24.pdf
Polarization Entanglement Generation Based on Birefringence in Polarization Maintained Dispersion Shifted Fiber at 1.5 µm Qiang Zhou, 1 Wei Zhang, 1 Pengxiang Wang, 1 Yidong Huang, 1 and Jiangde Peng 1 1 Tsinghua National Laboratory for Information Science and Technology, Department of Electronic Engineering, Tsinghua University, Beijing, 100084, P. R. China E-mail:
[email protected];
[email protected].
Abstract: 1.5 µm polarization entanglement generation is experimentally demonstrated based on birefringence in polarization maintained dispersion shifted fiber. Two-photon interference with visibility of >89% without subtracting background counts is achieved, indicating its polarization entanglement property. ©2010 Optical Society of America
OCIS codes: (270.0270) Quantum optics; (190.4380) Nonlinear optics, four-wave mixing 1. Introduction 1.5 µm polarization entangled photon pair sources are essential devices for quantum information application [1]. Recently, generation of 1.5 µm polarization entangled photon pairs by spontaneous four-wave mixing (SFWM) processes in optical fibers focus much attention due to its potential for realizing efficient, compact and all fiber 1.5 µm polarization entangled photon pair source. Three schemes have been experimentally demonstrated, such as the time-multiplexing, the polarization diversity loop and the birefringence based scheme [2-4]. Among them, the realization of birefringence based scheme is direct and simple. In this paper, we experimentally demonstrate the polarization entanglement generation based on the birefringence in polarization maintained dispersion shifted fiber (PM-DSF), showing its great potential on developing practical quantum light sources. 2. Experiment of polarization entanglement generation in PM-DSF
7.5
Fitting Curve Experimental Results
7.0
-3
Idler side photon count rates (10 /Pulse)
The experiment setup is shown in Fig. 1. The pulsed pump is generated by a passive mode-locked fiber laser. Its central wavelength is 1552.75 nm, with a pulse width of about twenty pico-seconds (estimated by its spectral width) and 1 MHz repetition rate. A side-band rejection of >115 dB is achieved at wavelengths where the signal and idler photon detection is performed. The polarization state of the pump light are controlled by a polarizer (P), a rotatable half wavelength plate (HWP1) and a polarization controller (PC1). A variable optical attenuator (VOA) and a 50/50 fiber coupler with power meter (PM) are used to adjust and monitor the power of the pump light. The nonlinear medium is a piece of 150 m long PM-DSF (λ0=1550 nm, fabricated by Fujikura Ltd.), which is submerged in liquid nitrogen (77 K) to suppress noise photon generation by spontaneous Raman scattering. The output of PM-DSF is directed into a filtering and splitting system based on a 100 GHz/40-channels arrayed waveguide grating (AWG, Scion Photonics Inc.), two fiber Bragg gratings (FBGs), and two tunable optical band-pass filters (TOBFs). Total pump isolation is >110 dB at either signal (1555.15 nm) or idler wavelength (1550.35 nm). The two single photon detectors (SPDs, Id Quantique, id201) are operated under Geiger mode with a 2.5 ns detection window and a detection efficiency of 21.83% and 22.56% for SPD1 and SPD2, respectively. The two SPDs are triggered with residual pump light detected by a photo-detector (PD).
6.5 6.0 5.5 5.0 4.5 4.0 3.5 0
20
40
60
80
100
120
140
160
180
θ (deg.)
Fig. 1. Experiment setup.
Fig. 2. Idler side photon count rates under different θ.
In the experiment, the PM-DSF is divided into two 75 m long pieces and spliced together with 90 degree principle axes offset, shown in the inset of Fig. 1. The linearly polarized pulsed pump light with a peak power of Pp
© Optical Society of America
OSA/ CLEO 2011
JThB24.pdf
is injected into the PM-DSF with a polarization direction of θ with respect to one of the fiber principle axes, and then split into two components along the two fiber principle axes with peak power of Ppsin2θ and Ppcos2θ, respectively. Thanks to the birefringence of the fiber, the correlated photon pairs (CPPs) might generate through the scalar and vector four-photon scattering processes in fiber [5]. However, because of the walk-off effect, the vector scattering processes can only take place at the input and output ends of the PM-DSF, while the scalar scattering processes take place along the entire fiber. Theoretical calculation shows that the CPPs from scalar scattering processes are about 200 times more than the CPPs from vector scattering processes. Under this condition, the total CPPs generation rate ξ should satisfy ξ 1-0.5sin2(2θ). First, the CPP generation in the PM-DSF under different θ is investigated. Figure 2 shows the measured idler side photon count rates under different θ. The experiment results agree well with ξ 1-0.5sin2(2θ), indicating that the CPPs are generated from the scalar scattering processes along the two polarization axes. It can be expected that if θ ≠ 0 polarization entangled photon pairs can be generated. To realize the maximum polarization entanglement, θ is set to 45 degree. The output photon should be in state of 1 2 ( H s H i + eiφ Vs Vi ) , where H and V denote the two fiber principle axes; s and i denote the signal and idler photons; φ is the relative phase difference between H s H i and Vs Vi . In order to measure the polarization entanglement property of generated photon pairs, two polarization analyzers are used (the dashed square in Fig. 1), which consist two PCs, two rotatable HWPs and two polarization beam splitters (PBSs). Figure 3 shows the measured entanglement properties of the generated photon pairs. The coincidence counts per 10 seconds (without subtracting accidental coincidence count) under different idler side detecting direction θi are given in Fig. 3 (a). The squares and circles are the experiment data when signal side detecting polarization direction θs is set to 0 and 135 degree, respectively. The solid and dashed lines are fitting curves, showing that the fringe visibilities of two photon interference are 92% and 89% for θs=0 and 135 degree, respectively. The idler side photon count rates under different θi are shown in Fig. 3 (b). They are almost unchanged under different θi, except a small ripple caused by the polarization dependent loss of HWP1, demonstrating the polarization indistinguishable property of generated polarization entangled photon pairs. Coincidence counts (/10 s)
1.8
(a)
θ s=0 deg.
200
-3
θ s=135 deg.
250
Idler side photon count rates (10 /Pulse)
∝
∝
Idler side photon count rates
(b)
1.5 1.2
150
0.9
100
0.6
50
0.3
0 0
25 50 75 100 125 150 175 200 225 250 275 300 325 350
Idler side detecting direction θi (deg.)
0.0 0
25 50 75 100 125 150 175 200 225 250 275 300 325 350
Idler side detecting direction θi (deg.)
Fig. 3. (a) Coincidence counts under different θi, (b) Idler side photon count under different θi.
3. Conclusion We have demonstrated polarization entanglement generation based on the birefringence in polarization maintained dispersion shifted fiber at 1.5 µm. A two-photon interference fringe visibility of >89% is achieved without subtracting the counts caused by the background photons, showing the great potential of this simple fiber based entanglement generation scheme on developing practical quantum light sources. This work is supported in part by National Natural Science Foundation of China under Grant No. 60777032, 973 Programs of China under Contract No. 2010CB327600, Science Foundation of Beijing under Grant No. 4102028, and Basic Research Foundation of Tsinghua National Laboratory for Information Science and Technology (TNList). 4. References [1] N.Gisin, G. Ribordy., W. Tittel, and H. Zbinden, “Quantum Cryptography,” Rev. Mod. Phys. 74, 145-195 (2002). [2] X. Li, Paul L.Voss, Jay E. Sharping, and P. Kumar, “Optical fiber-source of polarization-entangled photons in the 1550 nm telecom band,” Phys. Rev. Lett. 94, 053601 (2005). [3] H. Takesue and K. Inoue, “Generation of polarization entangled photon pairs and violation of Bell's inequality using spontaneous four-wave mixing in fiber loop,” Phys. Rev. A 70, 031802 (2004). [4] Q. Zhou, W. Zhang, J. Cheng, Y. Huang, and J. Peng, “Polarization-entangled Bell states generation based on birefringence in high nonlinear microstructure fiber at 1.5 µm,” Opt. Lett. 34, 2706 (2009). [5] E. Brainis, “Four-photon scattering in birefringent fibers,” Phys. Rev. A, 79, 023840 (2009).
