Broadband polarization-entanglement over 100 nm is demonstrated in a poled fiber phase-matched for type-II downconversion in the 1.5µm telecom-band. Two-photon interference and Hong-Ou-Mandel interference are used experimentally to confirm the broadband nature of the entanglement

Canagasabey, A.

Corbari, C.

Helt, L.G.

Ibsen, M.

Kazansky, P.G.

Liscidini, M.

Qian, L.

Sipe, J.E.

Tang, Z.

Zhu, E.Y.

E. Y. Zhu

Z. Tang

L. Qian

L. G. Helt

M. Liscidini

J. E. Sipe

C. Corbari

A. Canagasabey

M. Ibsen

P. G. Kazansky

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Broadband Polarization-Entanglement in a Poled Fiber

Broadband polarization-entanglement over 100 nm is demonstrated in a poled fiber phase-matched for type-II downconversion in the 1.5-micron telecom-band. Two-photon interference and Hong-Ou-Mandel interference are used experimentally to confirm the broadband nature of the entanglement.2 page(s

Zhu, Eric Y

Tang, Zhiyuan

Qian, Li

Helt, Lucas G

Liscidini, Marco

Sipe, John E

Corbari, Costantino

Canagasabey, Albert

Ibsen, Morten

Kazansky, Peter G

Macquarie University ResearchOnline

Broadband polarization-entanglement in a poled fiber

Southampton (e-Prints Soton)

Broadband Polarization-Entanglement in a Poled FiberE. Y. Zhu,1; Z. Tang,1;2 L. Qian,1 L. G. Helt,2 M. Liscidini,2;4 J. E. Sipe,2 C. Corbari,3 A.Canagasabey,3;5 M. Ibsen,3 P. G. Kazansky31Dept. of Electrical and Computer Engineering, University of Toronto, 10 King’s College Rd., Toronto, ON M5S 3G4, Canada2Dept. of Physics, University of Toronto, 60 St. George St., Toronto, ON M5S 1A7, Canada3Optoelectronics Research Centre, University of Southampton, SO17 1BJ, United Kingdom4Current address: Dipartimento di Fisica, Universita degli Studi di Pavia, Via Bassi 6 I-27100, Italy5Current address: School of Physics, University of Sydney, New South Wales 2006, AustraliaCorresponding author: eric.zhu@utoronto.caAbstract: Broadband polarization-entanglement over 100 nm is demonstrated in a poled fiberphase-matched for type-II downconversion in the 1.5-m telecom-band. Two-photon interferenceand Hong-Ou-Mandel interference are used experimentally to confirm the broadband nature of theentanglement.c 2012 Optical Society of AmericaOCIS codes: (270.0270) Quantum optics; (190.4370) Nonlinear optics, fibers1. IntroductionEntangled photons are a fundamental resource in quantum optics and quantum information [1]. Fiber-based sourcesof entangled photon pairs are especially sought-after for their ease of integration with current telecom infrastructure.While spontaneous parametric downconversion (SPDC) has historically been used to generate such entangled photonsin bulk crystals, the inversion symmetry of fused silica prevents SPDC (and other second-order parametric processes)from occurring in conventional fiber. However, optical fibers subjected to thermal poling and quasi-phase-matchinghave been found to allow for second-harmonic generation [2] and type-II SPDC. Exploiting this type-II SPDC allowsfor the direct generation of polarization-entangled photon pairs in a poled fiber at 1.5-micron, which was recentlydemonstrated [3].Here we demonstrate that the polarization entanglement is in fact broadband, by using coarse wavelength divi-sion multiplexers (WDMs) to divide the downconverted light into three frequency-conjugate sets. A Hong-Ou-Mandel(HOM) interference experiment is also carried out (without filters) to show that the downconverted photons are broad-band.2. ExperimentThe fiber used is a step-index fiber with core radius a = 2:0 m, and numerical aperature 0.20. Thermal poling isapplied to induce a uniform second-order nonlinearity along the length (17 cm) of the fiber. Periodic UV erasureof the nonlinearity allows for the quasi-phase-matched second-harmonic generation (SHG) of 775 nm light in theLP01 transverse mode (the fiber is multimode at this wavelength). Three spectrally-separated phase-matched (type-0, I, and II) SHG signals [4] are present due to the form birefringence in the fiber. The type-II signal is exploitedto generate polarization-entangled photon pairs when the fiber is pumped at 774.6 nm. The extremely low walkoff(60 fs/m) between the two principal polarizations (H;V ) of the poled fiber results in a compensation-free source ofpolarization-entangled photon pairs.1530 ± 8 nm1570 ±8 nmλs (nm)λi(nm)1470 1490 1510 1530 155016101590157015500.80.60.40.2Spectral Brightness (arb. units)1510 ± 8 nm1590 ±8 nm1490 ± 8 nm1610 ±8 nm17-cm Poled Fiber(PPSF)PumpSuppressionFiltersWDM775-nm Pump Pulseλs,1λ i,1λ i,Nλs,N……a) b)Fig. 1. a) The computed joint-spectral amplitude of the downconverted type-II photons generated in the poled fiber. b) The spectrally-entanglednature of the biphoton allows us to carve up the spectrum into separate frequency-conjugate pairs (s; i) using WDM filters.A pulsed source (mode-locked Ti:Sapph) with central wavelength 774.6 nm is used as the pump for the poledfiber. The joint-spectral amplitude shown in Fig. 1a is characteristic of an entangled state with a large ( 1) Schmidtnumber. Exploiting this spectral entanglement, we use coarse WDM filters (Fig. 1b) to separate the biphoton into threefrequency-conjugate parts [5], each of which can be treated as a polarization-entangled state (jHV i + jV Hi) for theexperiments presented here; central wavelengths for the signal (idler) filters are 1530 nm (1570 nm), 1510 nm (1590nm), and 1490 nm (1610 nm), with each filter having a 3 dB bandwidth of 16 nm.At the output of each set of frequency-conjugate filters, polarization analyzers (consisting of a half-wave andquarter-wave plate, and polarizer), followed by gated Geiger-mode single photon detectors (SPDs), are used to detectthe photons. A time-interval analyzer (TIA) checks the arrival times of the photons to determine accidental and coin-cidence counts. Two-photon interference results are shown in Fig. 2. The visibilities for the diagonal basis are foundto exceed 97% for all three frequency-conjugate sets, demonstrating broadband polarization-entanglement.0 50 100 150 200 250 300050100150200250Idler Polarizer Angle (θ )Coincidences in 100 secondsλs= 1530 nm, λi= 1570 nm0 50 100 150 200 250 300050100150200250300350Idler Polarizer Angle (θ )Coincidences in 100 secondsλs= 1510 nm, λi= 1590 nm0 50 100 150 200 250 300020406080Idler Polarizer Angle (θ )Coincidences in 150 secondsλs= 1490 nm, λi= 1610 nm = Diagonal Basis, = Horizontal Basis ○ □Visibility = 97.5 ± 1.2 % Visibility = 98.0 ± 0.3 % Visibility = 97.3 ± 1.7 %i iiFig. 2. Results of the two-photon interference measurements for the three frequency-conjugate pairs. The diagonal-basis visibilities are writtenbelow each plot.Finally, a HOM experiment is used to determine the temporal extent of the biphoton wavepacket, and thus thebandwidth of the downconverted light. Fig. 3a gives the experimental setup; this Mach-Zehnder interferometer (MZI)replaces the WDMs in Fig. 1b. A polarizing beam splitter (PBS) splits the type-II photons deterministically, with theV  and H polarized photons going into separate legs of the interferometer; a variable optical delay (VOD) linecompensates for the path length difference of the MZI. Fiber-based polarization controllers (FPC2, FPC3) changethe polarizations of the photons to be the same when they meet at the beam splitter (BS). SPDs are placed at theoutput of the BS and coincidence measurements are made. Fig. 3b shows the experimental results, demonstrating ahigh-visibility dip of width 43 fs, which is equivalent to an SPDC bandwidth of more than 100 nm at 1.5 microns.93.5 93.6 93.7 93.8 93.9050100150200Optical Delay (ps)Coincidences in 50 secondsCoincidencesAccidentalsBSVODVHPBSFPC3VHFPC2SPDSPDTIAFPC1 τa)                                                                                                                        b)Fig. 3. a) Experimental setup for the Hong-Ou-Mandel (HOM) experiment. b) Results of the HOM experiment, with a full-width at half-maximumdipwidth of 43 fs; this corresponds to a more-than 100 nm bandwidth for the downconverted 1.5-micron biphotons.References1. R. Horodecki, P. Horodecki, M. Horodecki, and K. Horodecki, “Quantum entanglement,” Rev. Mod. Phys. 81, 865–942 (2009).2. A. Canagasabey, C. Corbari, A. V. Gladyshev, F. Liegeois, S. Guillemet, Y. Hernandez, M. V. Yashkov, A. Kosolapov, E. M. Dianov, M. Ibsen,and P. G. Kazansky, “High-average-power second-harmonic generation from periodically poled silica fibers,” Opt. Lett. 34, 2483–2485 (2009).3. E. Y. Zhu, Z. Tang, L. Qian, L. G. Helt, M. Liscidini, J. E. Sipe, C. Corbari, A. Canagasabey, M. Ibsen, and P. G. Kazansky, “Direct generationof polarization-entangled photon pairs in a poled fiber,” Phys. Rev. Lett. (in press) .4. E. Y. Zhu, L. Qian, L. G. Helt, M. Liscidini, J. E. Sipe, C. Corbari, A. Canagasabey, M. Ibsen, and P. G. Kazansky, “Measurement of (2)symmetry in a poled fiber,” Opt. Lett. 35, 1530–1532 (2010).5. W. Grice, R. S. Bennink, P. Evans, T. Humble, R. Pooser, J. Schaake, and B. Williams, “Time-bin entanglement distribution on a wavelength-division-multiplexed network,” in “CLEO:2011 - Laser Applications to Photonic Applications,” (Optical Society of America, 2011), p. JTuA5.

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Broadband polarization-entanglement in a poled fiber

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