November 1, 2013 / Vol. 38, No. 21 / OPTICS LETTERS
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Poled-fiber source of broadband polarization-entangled photon pairs E. Y. Zhu,1,* Z. Tang,1,2 L. Qian,1 L. G. Helt,2,3 M. Liscidini,2,4 J. E. Sipe,2 C. Corbari,5 A. Canagasabey,5 M. Ibsen,5 and P. G. Kazansky5 1
Department of Electrical and Computer Engineering, University of Toronto, 10 King’s College Rd., Toronto, Ontario M5S 3G4, Canada 2
3
Department of Physics, University of Toronto, 60 St. George St., Toronto, Ontario M5S 1A7, Canada Currently at Centre for Ultrahigh bandwidth Devices for Optical Sytems (CUDOS), MQ Photonics Research Centre, Department of Physics and Astronomy, Faculty of Science, Macquarie University, NSW 2109, Australia 4 Currently at Dipartimento di Fisica, Universita degli Studi di Pavia, Via Bassi 6 I-27100, Italy 5
Optoelectronics Research Centre, University of Southampton, SO17 1BJ, UK *Corresponding author: [email protected]
Received July 29, 2013; revised September 17, 2013; accepted September 17, 2013; posted September 19, 2013 (Doc. ID 194700); published October 25, 2013 We demonstrate broadband polarization-entangled photon pair generation in a poled fiber phase matched for Type II downconversion in the 1.5 μm telecom band. Even with signal–idler separation greater than 100 nm, we observe fringe visibilities greater than 97% and tangle greater than 0.8. A Hong–Ou–Mandel interference experiment is also used to experimentally confirm the broadband nature of the entanglement. © 2013 Optical Society of America OCIS codes: (270.0270) Quantum optics; (190.4370) Nonlinear optics, fibers. http://dx.doi.org/10.1364/OL.38.004397
Entangled photons are a fundamental resource in quantum optics and quantum information [1]. Broadband entangled photon pairs have also been found to be useful in quantum metrology and high-resolution interferometry [2] where the effects of group velocity dispersion can be mitigated by entanglement [3]. Additionally, the combination of a broadband-entangled source and wavelength division multiplexing gives rise to the ability to distribute entangled biphotons to multiple users [4] for such applications as quantum key distribution [5]. Creating such sources remains a challenge, however. Spontaneous parametric downconversion (SPDC) in bulk nonlinear crystals has traditionally been used to generate polarization-entangled photon pairs. Broader band SPDC phase matching can be achieved by using shorter crystals, but in some cases, such as Type II downconversion in periodically poled lithium niobate, the generation of inline broadband entangled photon pairs requires impractically short crystals due to constraints in phase matching and dispersion [6]. Aperiodically chirped poled nonlinear crystals [7,8] have been shown to achieve downconversion bandwidths greater than 100 nm, but poor spectral overlap between signal and idler [8] means that the amount of polarization entanglement is poor. Fiber-based sources of entangled photon pairs are especially desirable for their ease of integration with current telecom infrastructure. However, the inversion symmetry of fused silica prevents SPDC (and other second-order parametric processes) from occurring in conventional fiber. The third-order nonlinear process of spontaneous four-wave mixing, then, is often used in optical fibers to generate correlated photon pairs [9,10], but spontaneous Raman scattering presents a significant source of noise in these schemes. Optical fibers that have been thermally poled, though, have been found to allow for second-harmonic generation (SHG) [11] and Type II SPDC, provided that quasiphase-matching via periodic UV erasure is performed 0146-9592/13/214397-04$15.00/0
[12]. Exploiting this Type II SPDC allows for the direct generation of polarization-entangled photon pairs in a poled fiber at 1.5 μm [13,14]. In this work, we demonstrate an inline poled fiberbased source of broadband polarization-entangled photon pairs. The broadband nature of the entanglement results from the dispersion characteristics of the fiber. The SPDC bandwidth ΔωSPDC [full width at halfmaximum (FWHM)] is a function of the group velocity dispersion β2 at the signal (idler) frequency (ωF ) and nonlinear interaction length L of the fiber: 1 ΔωSPDC ∝ q : F βω L 2
(1)
F The values are measured to be L 17 cm and βω 2 2 15 ps ∕km [15] for our fiber, and the bandwidth is calculated to be 2π10.6 THz. We demonstrate that the polarization entanglement is indeed broadband by using coarse wavelength division multiplexers (WDMs) to divide the downconverted light into three frequency-conjugate sets and performing fringe-visibility measurements. A Hong–Ou–Mandel (HOM) interference experiment is also carried out (without filters) to show that the downconverted photons are broadband; the resulting bandwidth is observed to be more than 120 nm [or greater than ▵ωSPDC 2π 15 THz], centered at 1550 nm. Earlier results for this work were published in a conference proceeding [16]. The fiber used is a step-index fiber with a core radius a 2.0 μm and numerical aperture 0.20. Thermal poling is applied to induce a uniform second-order nonlinearity along the length of the fiber. Periodic UV erasure of the nonlinearity allows for the quasi-phase-matched SHG of 775 nm light in the LP 01 transverse mode (the fiber is multimode at this wavelength) in this periodically poled silica fiber (PPSF). Three spectrally separated (Types 0, I, and II) SHG phase matchings [17] are present due to the
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OPTICS LETTERS / Vol. 38, No. 21 / November 1, 2013
birefringence in the fiber when the fiber is untwisted [18]. The Type II phase matching is exploited to generate polarization-entangled photon pairs when the fiber is pumped from 774.4 (degeneracy) to 775 nm. The secondorder nonlinear coefficient responsible for the Type II process (χ 2 yxy ) is measured from SHG to have a value of 0.022 pm∕V. The extremely low walkoff (60 fs∕m) between the two principal (H and V ) polarizations of the poled fiber results in a compensation-free source of polarization-entangled photon pairs. The poled fiber is pumped with an actively modelocked Ti:sapphire laser, set to a central wavelength of 774.6 nm, with 100 ps pulsewidth and 81.6 MHz repetition rate (12.25 ns period). The computed joint-spectral amplitude shown in Fig. 1(a) is characteristic of a spectrally entangled state with a large (≫1) Schmidt number. Exploiting this spectral entanglement, we use coarse WDM filters [Fig. 1(b)] to separate the biphotons into three frequency-conjugate parts, each of which can be p treated as a polarization-entangled state jHV i jVHi∕ 2 for the experiments presented here; central wavelengths for the signal (idler) filters are 1530 nm (1570 nm), 1510 nm (1590 nm), and 1490 nm (1610 nm), with each filter having a top-hat transmission profile and a 3 dB bandwidth of 16 nm. The end-to-end loss of the system from the output of the PPSF to the output of the WDM filters is 6.7 dB (6.2 dB) for the signal (idler) photons. For each set of frequency-conjugate filters, the photon pairs are detected by free-running idQuantique id220 single photon detectors (SPDs), with quantum efficiencies
calibrated to 20% at 1550 nm, but varying from 24% at 1490 nm to 10% at 1610 nm; detector dead times (that follow each detection event) are set to 20 μs and dark count rates (DCRs) of 1.7 kHz are observed at these settings. A field-programmable gate array-based (FPGA) time-interval analyzer (TIA) checks the arrival times of the photons to determine accidental and coincidence counts; a 125-ns-wide histogram of counts is recorded and a 1.5 ns coincidence window is used (Fig. 2 inset). Coincidence-to-accidental ratio (CAR) measurements are performed at various pair generation rates (Fig. 2) at a constant integration time of 50 s and the CAR is found to agree well with the expression CAR
μαs αi 1; μαs ds μαi di
(2)
where μ is the average number of photon pairs generated per pulse, αs (αi ) is the measured transmission efficiency (including detector efficiency) of the signal (idler) arm, and ds (di ) is the DCR of the signal (idler) detector in a 1.5 ns window; the agreement with experiment indicates that there is very little noise due to fluorescence. Polarization analyzers (PAs) are then placed in between the WDMs and SPDs so that two-photon interference and quantum state tomography can be performed [Fig. 3(a)]. Each PA consists of achromatic half-wave and quarter-wave plates (HWPs, QWPs). However, while a polarizer (POL) is used on the idler (λi ) arm, a polarizing-beam-splitter (PBS)-based Mach–Zehnder
Fig. 1. (a) The computed joint-spectral amplitude of the downconverted Type II photon pairs generated in the poled fiber. (b) The spectrally entangled nature of the biphoton allows us to carve up the spectrum into separate frequency-conjugate pairs (λs , λi ) using WDM filters.
Fig. 2. CAR measurements for three sets of frequency conjugate photon pairs. Pair generation rates per unit of average pump power are also provided. The solid curves are Eq. (2) plotted as a function of the photon pair generation rate μ. The inset for the first figure shows the histogram collected by the TIA for the 1530–1570 nm pair when the generation rate is μ 3.2 × 10−3 pairs∕pulse.
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Fig. 3. (a) Experimental setup for two-photon interference. A simple PA consisting of an achromatic HWP, QWP, and POL are used in the idler (λi ) arm. A more elaborate time-multiplexing scheme (involving a PBS-based MZI) is used in the signal (λs ) arm so that both H and V (or D∕A, depending on HWP and QWP settings) polarizations can be detected during the same experimental run using only one detector. (b) Representative histograms from the TIA are shown when the idler is A polarized and when the signal is H∕V polarized or D∕A polarized.
Fig. 4. Results of the two-photon interference measurements for the three frequency-conjugate pairs. The diagonal-basis visibilities are written below each plot. The pair generation rate was 3 × 10−3 pairs∕pulse for the 1530∕1570 filter set, while at 1510∕1590 and 1490∕1610 nm the pump power was increased so that the pair generation rate was 5 × 10−3 pairs∕pulse.
interferometer (MZI) allows for the time multiplexing of the basis being interrogated in the signal arm; that is, the horizontal (H) and vertical (V ) (or Diagonal/Antidiagonal) polarizations are time delayed from each other by 2.5 ns so that they can be measured by the same detector. The two time-multiplexed signals are then picked out by the TIA (which has 500 ps resolution); data representative of this is shown in Fig. 3(b). Two-photon interference results are shown in Fig. 4. We emphasize that these are raw results, without subtracting accidentals. The visibilities for the diagonal basis are found to exceed 97% for all three frequencyconjugate sets, demonstrating broadband polarization entanglement [19]. State tomography is also performed, and each state is reconstructed using 36 measurements. The results are shown in Table 1. Even over the large signal–idler separations, the tangle remains high, and fidelity to the nearest maximally entangled state is close to unity. The low (von Neumann) entropy also indicates that the reconstructed states are very nearly pure. Finally, a HOM experiment is used to determine the temporal extent of the biphoton wavepacket and thus the bandwidth of the downconverted light. Figure 5(a)
gives the experimental setup; a HOM interferometer replaces the WDMs and PAs in Figs. 1(b) and 3(a). A fiberbased PBS splits the Type II photons deterministically, with the V - and H-polarized photons going into separate legs of the interferometer; a fiber-coupled free-space variable optical delay (VOD) line compensates for the path length difference between the two legs of the interferometer. Fiber-based polarization controllers (FPC2, FPC3) change the polarizations of the photons to be the same when they meet at the beam splitter (BS). The detectors are placed at the output of the BS and coincidence measurements are made. Figure 5(b) shows the experimental results, demonstrating a high-visibility dip (97.0 0.6% before subtracting accidentals) of width 38.9 0.1 fs Table 1. Summary of Results for State Tomography λs nm λi nm
1530 1570
1510 1590
1490 1610
Fidelity to a maximally entangled state Tangle Entropy
0.987
0.990
0.966
0.945 0.054
0.960 0.075
0.840 0.116
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Fig. 5. (a) Experimental setup for the HOM experiment. (b) Results of the HOM experiment, with a FWHM dipwidth of 38.9 fs; this corresponds to a more than 120 nm bandwidth for the downconverted 1.5 μm biphotons. The raw coincidences are plotted along with the accidentals. The raw visibility is found to be 97.0%, but reaches 98.8% when accidentals are subtracted.
(FWHM), which is equivalent to an SPDC bandwidth of ΔωSPDC 2π15.5 THz, or more than 120 nm at 1.5 μm. In conclusion, our poled fiber entangled source has been found to be broadband (as verified by the HOM interference), free of noise (as seen in the CAR results), and can generate highly polarization-entangled photon pairs even when signal–idler separation is greater than 100 nm (as observed in the fringe visibilities and state tomography). We gratefully acknowledge idQuantique for lending us two free-running SPDs that were used for obtaining the results presented here. References 1. R. Horodecki, P. Horodecki, M. Horodecki, and K. Horodecki, Rev. Mod. Phys. 81, 865 (2009). 2. M. B. Nasr, B. E. A. Saleh, A. V. Sergienko, and M. C. Teich, Phys. Rev. Lett. 91, 083601 (2003). 3. A. M. Steinberg, P. G. Kwiat, and R. Y. Chiao, Phys. Rev. A 45, 6659 (1992). 4. H. C. Lim, A. Yoshizawa, H. Tsuchida, and K. Kikuchi, Opt. Express 16, 16052 (2008). 5. A. K. Ekert, Phys. Rev. Lett. 67, 661 (1991). 6. D. H. Jundt, Opt. Lett. 22, 1553 (1997). 7. M. B. Nasr, S. Carrasco, B. E. A. Saleh, A. V. Sergienko, M. C. Teich, J. P. Torres, L. Torner, D. S. Hum, and M. M. Fejer, Phys. Rev. Lett. 100, 183601 (2008). 8. A. Fraine, O. Minaeva, D. S. Simon, R. Egorov, and A. V. Sergienko, Opt. Lett. 37, 1910 (2012).
9. J. Chen, K. F. Lee, X. Li, P. Voss, and P. Kumar, New J. Phys. 9, 289 (2007). 10. B. J. Smith, P. Mahou, O. Cohen, J. S. Lundeen, and I. A. Walmsley, Opt. Express 17, 23589 (2009). 11. 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, Opt. Lett. 34, 2483 (2009). 12. C. Corbari, A. Canagasabey, M. Ibsen, F. Mezzapesa, C. Codemard, J. Nilsson, and P. Kazansky, in Optical Fiber Communication Conference, Technical Digest (Optical Society of America, 2005), paper OFB3. 13. L. G. Helt, E. Y. Zhu, M. Liscidini, L. Qian, and J. E. Sipe, Opt. Lett. 34, 2138 (2009). 14. E. Y. Zhu, Z. Tang, L. Qian, L. G. Helt, M. Liscidini, J. Sipe, C. Corbari, A. Canagasabey, M. Ibsen, and P. G. Kazansky, Phys. Rev. Lett. 108, 213902 (2012). 15. M. A. Galle, S. S. Saini, W. S. Mohammed, and L. Qian, J. Opt. Soc. Am. B 29, 3201 (2012). 16. 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, in Frontiers in Optics 2012/Laser Science XXVIII (Optical Society of America, 2012), paper FTh1C.4. 17. E. Y. Zhu, L. Qian, L. G. Helt, M. Liscidini, J. E. Sipe, C. Corbari, A. Canagasabey, M. Ibsen, and P. G. Kazansky, Opt. Lett. 35, 1530 (2010). 18. E. Y. Zhu, L. Qian, L. G. Helt, M. Liscidini, J. E. Sipe, C. Corbari, A. Canagasabey, M. Ibsen, and P. G. Kazansky, J. Opt. Soc. Am. B 27, 2410 (2010). 19. J. F. Clauser, M. A. Horne, A. Shimony, and R. A. Holt, Phys. Rev. Lett. 23, 880 (1969).
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Poled-fiber source of broadband polarization-entangled photon pairs E. Y. Zhu,1,* Z. Tang,1,2 L. Qian,1 L. G. Helt,2,3 M. Liscidini,2,4 J. E. Sipe,2 C. Corbari,5 A. Canagasabey,5 M. Ibsen,5 and P. G. Kazansky5 1
Department of Electrical and Computer Engineering, University of Toronto, 10 King’s College Rd., Toronto, Ontario M5S 3G4, Canada 2
3
Department of Physics, University of Toronto, 60 St. George St., Toronto, Ontario M5S 1A7, Canada Currently at Centre for Ultrahigh bandwidth Devices for Optical Sytems (CUDOS), MQ Photonics Research Centre, Department of Physics and Astronomy, Faculty of Science, Macquarie University, NSW 2109, Australia 4 Currently at Dipartimento di Fisica, Universita degli Studi di Pavia, Via Bassi 6 I-27100, Italy 5
Optoelectronics Research Centre, University of Southampton, SO17 1BJ, UK *Corresponding author: [email protected]
Received July 29, 2013; revised September 17, 2013; accepted September 17, 2013; posted September 19, 2013 (Doc. ID 194700); published October 25, 2013 We demonstrate broadband polarization-entangled photon pair generation in a poled fiber phase matched for Type II downconversion in the 1.5 μm telecom band. Even with signal–idler separation greater than 100 nm, we observe fringe visibilities greater than 97% and tangle greater than 0.8. A Hong–Ou–Mandel interference experiment is also used to experimentally confirm the broadband nature of the entanglement. © 2013 Optical Society of America OCIS codes: (270.0270) Quantum optics; (190.4370) Nonlinear optics, fibers. http://dx.doi.org/10.1364/OL.38.004397
Entangled photons are a fundamental resource in quantum optics and quantum information [1]. Broadband entangled photon pairs have also been found to be useful in quantum metrology and high-resolution interferometry [2] where the effects of group velocity dispersion can be mitigated by entanglement [3]. Additionally, the combination of a broadband-entangled source and wavelength division multiplexing gives rise to the ability to distribute entangled biphotons to multiple users [4] for such applications as quantum key distribution [5]. Creating such sources remains a challenge, however. Spontaneous parametric downconversion (SPDC) in bulk nonlinear crystals has traditionally been used to generate polarization-entangled photon pairs. Broader band SPDC phase matching can be achieved by using shorter crystals, but in some cases, such as Type II downconversion in periodically poled lithium niobate, the generation of inline broadband entangled photon pairs requires impractically short crystals due to constraints in phase matching and dispersion [6]. Aperiodically chirped poled nonlinear crystals [7,8] have been shown to achieve downconversion bandwidths greater than 100 nm, but poor spectral overlap between signal and idler [8] means that the amount of polarization entanglement is poor. Fiber-based sources of entangled photon pairs are especially desirable for their ease of integration with current telecom infrastructure. However, the inversion symmetry of fused silica prevents SPDC (and other second-order parametric processes) from occurring in conventional fiber. The third-order nonlinear process of spontaneous four-wave mixing, then, is often used in optical fibers to generate correlated photon pairs [9,10], but spontaneous Raman scattering presents a significant source of noise in these schemes. Optical fibers that have been thermally poled, though, have been found to allow for second-harmonic generation (SHG) [11] and Type II SPDC, provided that quasiphase-matching via periodic UV erasure is performed 0146-9592/13/214397-04$15.00/0
[12]. Exploiting this Type II SPDC allows for the direct generation of polarization-entangled photon pairs in a poled fiber at 1.5 μm [13,14]. In this work, we demonstrate an inline poled fiberbased source of broadband polarization-entangled photon pairs. The broadband nature of the entanglement results from the dispersion characteristics of the fiber. The SPDC bandwidth ΔωSPDC [full width at halfmaximum (FWHM)] is a function of the group velocity dispersion β2 at the signal (idler) frequency (ωF ) and nonlinear interaction length L of the fiber: 1 ΔωSPDC ∝ q : F βω L 2
(1)
F The values are measured to be L 17 cm and βω 2 2 15 ps ∕km [15] for our fiber, and the bandwidth is calculated to be 2π10.6 THz. We demonstrate that the polarization entanglement is indeed broadband by using coarse wavelength division multiplexers (WDMs) to divide the downconverted light into three frequency-conjugate sets and performing fringe-visibility measurements. A Hong–Ou–Mandel (HOM) interference experiment is also carried out (without filters) to show that the downconverted photons are broadband; the resulting bandwidth is observed to be more than 120 nm [or greater than ▵ωSPDC 2π 15 THz], centered at 1550 nm. Earlier results for this work were published in a conference proceeding [16]. The fiber used is a step-index fiber with a core radius a 2.0 μm and numerical aperture 0.20. Thermal poling is applied to induce a uniform second-order nonlinearity along the length of the fiber. Periodic UV erasure of the nonlinearity allows for the quasi-phase-matched SHG of 775 nm light in the LP 01 transverse mode (the fiber is multimode at this wavelength) in this periodically poled silica fiber (PPSF). Three spectrally separated (Types 0, I, and II) SHG phase matchings [17] are present due to the
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OPTICS LETTERS / Vol. 38, No. 21 / November 1, 2013
birefringence in the fiber when the fiber is untwisted [18]. The Type II phase matching is exploited to generate polarization-entangled photon pairs when the fiber is pumped from 774.4 (degeneracy) to 775 nm. The secondorder nonlinear coefficient responsible for the Type II process (χ 2 yxy ) is measured from SHG to have a value of 0.022 pm∕V. The extremely low walkoff (60 fs∕m) between the two principal (H and V ) polarizations of the poled fiber results in a compensation-free source of polarization-entangled photon pairs. The poled fiber is pumped with an actively modelocked Ti:sapphire laser, set to a central wavelength of 774.6 nm, with 100 ps pulsewidth and 81.6 MHz repetition rate (12.25 ns period). The computed joint-spectral amplitude shown in Fig. 1(a) is characteristic of a spectrally entangled state with a large (≫1) Schmidt number. Exploiting this spectral entanglement, we use coarse WDM filters [Fig. 1(b)] to separate the biphotons into three frequency-conjugate parts, each of which can be p treated as a polarization-entangled state jHV i jVHi∕ 2 for the experiments presented here; central wavelengths for the signal (idler) filters are 1530 nm (1570 nm), 1510 nm (1590 nm), and 1490 nm (1610 nm), with each filter having a top-hat transmission profile and a 3 dB bandwidth of 16 nm. The end-to-end loss of the system from the output of the PPSF to the output of the WDM filters is 6.7 dB (6.2 dB) for the signal (idler) photons. For each set of frequency-conjugate filters, the photon pairs are detected by free-running idQuantique id220 single photon detectors (SPDs), with quantum efficiencies
calibrated to 20% at 1550 nm, but varying from 24% at 1490 nm to 10% at 1610 nm; detector dead times (that follow each detection event) are set to 20 μs and dark count rates (DCRs) of 1.7 kHz are observed at these settings. A field-programmable gate array-based (FPGA) time-interval analyzer (TIA) checks the arrival times of the photons to determine accidental and coincidence counts; a 125-ns-wide histogram of counts is recorded and a 1.5 ns coincidence window is used (Fig. 2 inset). Coincidence-to-accidental ratio (CAR) measurements are performed at various pair generation rates (Fig. 2) at a constant integration time of 50 s and the CAR is found to agree well with the expression CAR
μαs αi 1; μαs ds μαi di
(2)
where μ is the average number of photon pairs generated per pulse, αs (αi ) is the measured transmission efficiency (including detector efficiency) of the signal (idler) arm, and ds (di ) is the DCR of the signal (idler) detector in a 1.5 ns window; the agreement with experiment indicates that there is very little noise due to fluorescence. Polarization analyzers (PAs) are then placed in between the WDMs and SPDs so that two-photon interference and quantum state tomography can be performed [Fig. 3(a)]. Each PA consists of achromatic half-wave and quarter-wave plates (HWPs, QWPs). However, while a polarizer (POL) is used on the idler (λi ) arm, a polarizing-beam-splitter (PBS)-based Mach–Zehnder
Fig. 1. (a) The computed joint-spectral amplitude of the downconverted Type II photon pairs generated in the poled fiber. (b) The spectrally entangled nature of the biphoton allows us to carve up the spectrum into separate frequency-conjugate pairs (λs , λi ) using WDM filters.
Fig. 2. CAR measurements for three sets of frequency conjugate photon pairs. Pair generation rates per unit of average pump power are also provided. The solid curves are Eq. (2) plotted as a function of the photon pair generation rate μ. The inset for the first figure shows the histogram collected by the TIA for the 1530–1570 nm pair when the generation rate is μ 3.2 × 10−3 pairs∕pulse.
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Fig. 3. (a) Experimental setup for two-photon interference. A simple PA consisting of an achromatic HWP, QWP, and POL are used in the idler (λi ) arm. A more elaborate time-multiplexing scheme (involving a PBS-based MZI) is used in the signal (λs ) arm so that both H and V (or D∕A, depending on HWP and QWP settings) polarizations can be detected during the same experimental run using only one detector. (b) Representative histograms from the TIA are shown when the idler is A polarized and when the signal is H∕V polarized or D∕A polarized.
Fig. 4. Results of the two-photon interference measurements for the three frequency-conjugate pairs. The diagonal-basis visibilities are written below each plot. The pair generation rate was 3 × 10−3 pairs∕pulse for the 1530∕1570 filter set, while at 1510∕1590 and 1490∕1610 nm the pump power was increased so that the pair generation rate was 5 × 10−3 pairs∕pulse.
interferometer (MZI) allows for the time multiplexing of the basis being interrogated in the signal arm; that is, the horizontal (H) and vertical (V ) (or Diagonal/Antidiagonal) polarizations are time delayed from each other by 2.5 ns so that they can be measured by the same detector. The two time-multiplexed signals are then picked out by the TIA (which has 500 ps resolution); data representative of this is shown in Fig. 3(b). Two-photon interference results are shown in Fig. 4. We emphasize that these are raw results, without subtracting accidentals. The visibilities for the diagonal basis are found to exceed 97% for all three frequencyconjugate sets, demonstrating broadband polarization entanglement [19]. State tomography is also performed, and each state is reconstructed using 36 measurements. The results are shown in Table 1. Even over the large signal–idler separations, the tangle remains high, and fidelity to the nearest maximally entangled state is close to unity. The low (von Neumann) entropy also indicates that the reconstructed states are very nearly pure. Finally, a HOM experiment is used to determine the temporal extent of the biphoton wavepacket and thus the bandwidth of the downconverted light. Figure 5(a)
gives the experimental setup; a HOM interferometer replaces the WDMs and PAs in Figs. 1(b) and 3(a). A fiberbased PBS splits the Type II photons deterministically, with the V - and H-polarized photons going into separate legs of the interferometer; a fiber-coupled free-space variable optical delay (VOD) line compensates for the path length difference between the two legs of the interferometer. Fiber-based polarization controllers (FPC2, FPC3) change the polarizations of the photons to be the same when they meet at the beam splitter (BS). The detectors are placed at the output of the BS and coincidence measurements are made. Figure 5(b) shows the experimental results, demonstrating a high-visibility dip (97.0 0.6% before subtracting accidentals) of width 38.9 0.1 fs Table 1. Summary of Results for State Tomography λs nm λi nm
1530 1570
1510 1590
1490 1610
Fidelity to a maximally entangled state Tangle Entropy
0.987
0.990
0.966
0.945 0.054
0.960 0.075
0.840 0.116
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Fig. 5. (a) Experimental setup for the HOM experiment. (b) Results of the HOM experiment, with a FWHM dipwidth of 38.9 fs; this corresponds to a more than 120 nm bandwidth for the downconverted 1.5 μm biphotons. The raw coincidences are plotted along with the accidentals. The raw visibility is found to be 97.0%, but reaches 98.8% when accidentals are subtracted.
(FWHM), which is equivalent to an SPDC bandwidth of ΔωSPDC 2π15.5 THz, or more than 120 nm at 1.5 μm. In conclusion, our poled fiber entangled source has been found to be broadband (as verified by the HOM interference), free of noise (as seen in the CAR results), and can generate highly polarization-entangled photon pairs even when signal–idler separation is greater than 100 nm (as observed in the fringe visibilities and state tomography). We gratefully acknowledge idQuantique for lending us two free-running SPDs that were used for obtaining the results presented here. References 1. R. Horodecki, P. Horodecki, M. Horodecki, and K. Horodecki, Rev. Mod. Phys. 81, 865 (2009). 2. M. B. Nasr, B. E. A. Saleh, A. V. Sergienko, and M. C. Teich, Phys. Rev. Lett. 91, 083601 (2003). 3. A. M. Steinberg, P. G. Kwiat, and R. Y. Chiao, Phys. Rev. A 45, 6659 (1992). 4. H. C. Lim, A. Yoshizawa, H. Tsuchida, and K. Kikuchi, Opt. Express 16, 16052 (2008). 5. A. K. Ekert, Phys. Rev. Lett. 67, 661 (1991). 6. D. H. Jundt, Opt. Lett. 22, 1553 (1997). 7. M. B. Nasr, S. Carrasco, B. E. A. Saleh, A. V. Sergienko, M. C. Teich, J. P. Torres, L. Torner, D. S. Hum, and M. M. Fejer, Phys. Rev. Lett. 100, 183601 (2008). 8. A. Fraine, O. Minaeva, D. S. Simon, R. Egorov, and A. V. Sergienko, Opt. Lett. 37, 1910 (2012).
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