## A versatile waveguide source of photon pairs for chip-scale quantum information processing

Optics Express, Vol. 17, Issue 8, pp. 6727-6740 (2009)

http://dx.doi.org/10.1364/OE.17.006727

Acrobat PDF (736 KB)

### Abstract

We demonstrate a bright, bandwidth-tunable, quasi-phasematched single-waveguide source generating photon pairs near 900 nm and 1300 nm. Two-photon coincidence spectra are measured at a range of operating temperatures of a periodically-poled KTiOPO_{4} (PPKTP) waveguide, which supports both type-0 and type-II spontaneous parametric down-conversion. We map out relative contributions of two-photon to one-photon fluorescence for a range of operating parameters. Such a versatile device is highly promising for future chip-scale quantum information processing.

© 2009 Optical Society of America

## 1. Introduction

15. S. Sauge, M. Swillo, S. Albert-Seifried, G. B. Xavier, J. Waldebäck, M. Tengner, D. Ljunggren, and A. Karlsson, “Narrowband polarization-entangled photon pairs distributed over a WDM link for qubit networks,” Opt. Express **15**, 6926–6933 (2007). [CrossRef] [PubMed]

16. W. P. Grice and I. A. Walmsley, “Spectral information and distinguishability in type-II down-conversion with a broadband pump,” Phys. Rev. A **56**, 1627–1634 (1997). [CrossRef]

17. Y. Kim and W. P. Grice, “Measurement of the spectral properties of the two-photon state generated via type II spontaneous parametric downconversion,” Opt. Lett. **30**, 908–910 (2005). [CrossRef] [PubMed]

18. H. S. Poh, C. Y. Lum, I. Marcikic, A. Lamas-Linares, and C. Kurtsiefer, “Joint spectrum mapping of polarization entanglement in spontaneous parametric down-conversion,” Phys. Rev. A **75**, 043816 (2007). [CrossRef]

19. A. Ling, P. Y. Han, A. Lamas-Linares, and C. Kurtsiefer, “Preparation of Bell States with controlled white noise,” Laser Phys. **16**, 1140–1144 (2006). [CrossRef]

6. A. B. U’Ren, C. Silberhorn, K. Banaszek, and I.A. Walmsley, “Efficient Conditional Preparation of High-Fidelity Single Photon States for Fiber-Optic Quantum Networks,” Phys. Rev. Lett. **93**, 093601 (2004). [CrossRef] [PubMed]

12. M. Fiorentino, S. M. Spillane, R. G. Beausoleil, T. D. Roberts, P. Battle, and M. W. Munro, “Spontaneous parametric down-conversion in periodically poled KTP waveguides and bulk crystals,” Opt. Express **15**, 7479–7488 (2007). [CrossRef] [PubMed]

## 2. Experimental setup

_{s}= 0.17nm to match the fixed bandwidth of the idler filter (Δλ

_{i}=0.33nm), so that they contain the same frequency bandwidths. After filtering, idler photons are detected with an InGaAs avalanche photodiode in gated Geiger mode, with a 1 MHz gate frequency and a gate width of 1.28 ns. The gating signal is obtained by using a beam splitter (BS in Fig. 1) to pick off a part of the laser output, which is detected with an analogue photodiode. The 80 MHz detection output is then sent through a down-counter/delay generator and converted to a 1 MHz pulse train with suitable delay. A fiber polarization controller (FPC in Fig. 1) is placed in front of the idler filter to maximize its transmission which is polarization dependent. Signal photons are detected with a silicon avalanche single-photon detector. Coincidences are recorded through start and stop inputs, with the detection pulses from the idler (signal) acting as the start (stop).

## 3. Experimental results

### 3.1. Temperature dependence of single-photon spectra

*H*and

*V*pump polarizations. Note that these counts are taken without any measurement on the signal channel (i.e., they are not heralded counts). For both types of SPDC, we observe a dramatic dependence of single-photon production rate on temperature. While we cannot ascertain that all of the collected photons are produced by SPDC (in fact, a portion of them are produced by single-photon fluorescence due to defects in the waveguide [6

6. A. B. U’Ren, C. Silberhorn, K. Banaszek, and I.A. Walmsley, “Efficient Conditional Preparation of High-Fidelity Single Photon States for Fiber-Optic Quantum Networks,” Phys. Rev. Lett. **93**, 093601 (2004). [CrossRef] [PubMed]

12. M. Fiorentino, S. M. Spillane, R. G. Beausoleil, T. D. Roberts, P. Battle, and M. W. Munro, “Spontaneous parametric down-conversion in periodically poled KTP waveguides and bulk crystals,” Opt. Express **15**, 7479–7488 (2007). [CrossRef] [PubMed]

*T*= 55.0°C), almost all of the collected photons are produced by single-photon fluorescence, since no peak structure in the single-count spectrum is observable.

*T*

_{opt}= 34.6°C); (ii) type-0 SPDC has a much wider phase-matching bandwidth [full width at half maximum (FWHM) ≈ 12nm for idler] than type-II SPDC (FWHM ≈ 1.4nm for idler); (iii) type-II SPDC is spectrally brighter (5X) than type-0 SPDC at their peak values, and (iv) in terms of the overall brightness (i.e., unfiltered output), we find that type-II is slightly brighter (~ 7%) than type-0 at the optimal temperature

*T*

_{opt}. The same characteristics are also seen in the signal single-count spectra (not shown). We must acknowledge the unexpected coincidence that both type-0 and type-II SPDC are phase matched at the same optimal temperature. While it is possible that this is simply a coincidence, it is more likely that there is some physical reason, although at this point we have not found it. As to the differences in terms of bandwidths and brightnesses between Fig. 2(a) and (b), a combination of two factors is responsible. First, because the PPKTP waveguide is birefringent, a horizontally-polarized pump travels at a different group velocity than a vertically-polarized pump, and therefore must satisfy a different phase-matching condition for efficient down-conversion, which gives rise to the different phase matching bandwidths for the two types of SPDC. Second, the two types of SPDC processes rely on different second-order nonlinear susceptibility tensor components: χ

^{(2)}

_{zzz}(or

*d*

_{eff}=

*d*

_{33}) for type-0 and χ

^{(2)}

_{zyy}(or

*d*

_{24}) for type-II. One might think that this helps to explain why type-II is brighter both spectrally and in overall output than type-0. However, comparing the magnitudes of the nonlinear-optic coefficients alone would lead one to draw the opposite conclusion, since

*d*

_{eff}>

*d*

_{24}. To solve this dilemma and to offer deeper insight into the phase matching mechanisms for the two types of SPDC in a waveguide, we will compare the two processes in more detail in section 3.5, and provide a possible reason for why this is the case.

### 3.2. Coincidence-to-accidental ratio

*T*

_{opt}, we set the tunable filters in both channels. For type-0, we use the wavelength pair {λ

_{s}= 899.18nm, λ

_{i}= 1304.00nm}; for type-II: {λ

_{s}= 904.00nm, λ

_{i}= 1294.00nm}. For each wavelength pair, we varied the pump power (by rotating HWP1 in Fig. 1) and recorded coincidences and accidental coincidences at each pump power level. The coincidence-to-accidental ratio (CAR), a commonly used two-photon source purity measure [25

25. J. Chen, K. F. Lee, C. Liang, and P. Kumar, “Fiber-based telecom-band degenerate-frequency source of entangled photon pairs,” Opt. Lett. **31**, 2798–2800 (2006). [CrossRef] [PubMed]

26. J. Fan, A. Dogariu, and L. J. Wang, “Generation of correlated photon pairs in a microstructure fiber,” Opt. Lett. **30**, 1530–1532 (2005). [CrossRef] [PubMed]

27. K. F. Lee, J. Chen, C. Liang, X. Li, P. L. Voss, and P. Kumar, “Generation of high-purity telecom-band entangled photon pairs in dispersion-shifted fiber,” Opt. Lett. **31**, 1905–1907 (2006). [CrossRef] [PubMed]

28. H. Takesue and K. Inoue, “1.5-*μ*m band quantum-correlated photon pair generation in dispersion-shifted fiber: suppression of noise photons by cooling fiber,” Opt. Express **13**, 7832–7839 (2005). [CrossRef] [PubMed]

6. A. B. U’Ren, C. Silberhorn, K. Banaszek, and I.A. Walmsley, “Efficient Conditional Preparation of High-Fidelity Single Photon States for Fiber-Optic Quantum Networks,” Phys. Rev. Lett. **93**, 093601 (2004). [CrossRef] [PubMed]

13. Q. Zhang, X. Xie, H. Takesue, S.W. Nam, C. Langrock, M. M. Fejer, and Y. Yamamoto, “Correlated photon-pair generation in reverse-proton-exchange PPLN waveguides with integrated mode demultiplexer at 10 GHz clock,” Opt. Express **15**, 10288–10293 (2007). [CrossRef] [PubMed]

29. S. D. Dyer, M. J. Stevens, B. Baek, and S.W. Nam, “High-efficiency, ultra low-noise all-fiber photon-pair source,” Opt. Express **16**, 9966–9977 (2008). [CrossRef] [PubMed]

13. Q. Zhang, X. Xie, H. Takesue, S.W. Nam, C. Langrock, M. M. Fejer, and Y. Yamamoto, “Correlated photon-pair generation in reverse-proton-exchange PPLN waveguides with integrated mode demultiplexer at 10 GHz clock,” Opt. Express **15**, 10288–10293 (2007). [CrossRef] [PubMed]

**93**, 093601 (2004). [CrossRef] [PubMed]

13. Q. Zhang, X. Xie, H. Takesue, S.W. Nam, C. Langrock, M. M. Fejer, and Y. Yamamoto, “Correlated photon-pair generation in reverse-proton-exchange PPLN waveguides with integrated mode demultiplexer at 10 GHz clock,” Opt. Express **15**, 10288–10293 (2007). [CrossRef] [PubMed]

29. S. D. Dyer, M. J. Stevens, B. Baek, and S.W. Nam, “High-efficiency, ultra low-noise all-fiber photon-pair source,” Opt. Express **16**, 9966–9977 (2008). [CrossRef] [PubMed]

**93**, 093601 (2004). [CrossRef] [PubMed]

29. S. D. Dyer, M. J. Stevens, B. Baek, and S.W. Nam, “High-efficiency, ultra low-noise all-fiber photon-pair source,” Opt. Express **16**, 9966–9977 (2008). [CrossRef] [PubMed]

**15**, 10288–10293 (2007). [CrossRef] [PubMed]

### 3.3. Coincidence spectra

32. D. C. Burnham and D. L. Weinberg, “Observation of Simultaneity in Parametric Production of Optical Photon Pairs,” Phys. Rev. Lett. **25**, 84–87 (1970). [CrossRef]

33.
We note that another method for measuring joint spectrum is given by W. Wasilewski *et*. *al*., “Joint spectrum of photon pairs measured by coincidence Fourier spectroscopy,” Opt. Lett. **31**, 1130–1132 (2006). [PubMed]

_{s},λ

_{i}} pair with {Δλ

_{s},Δλ

_{i}} resolution at a discrete step size of {Δλ

_{s},Δλ

_{i}}. The results are color-coded as 2-dimensional coincidence spectra shown in Fig. 4. The wavelength resolutions are kept the same for all measurements (Δλ

_{s}= 0.17nm, Δλ

_{i}= 0.33nm), and the step sizes used in obtaining each coincidence spectrum are indicated on the figures. Note that the resolutions in this experiment are limited not by our homemade signal grating filter, but by the fixed passband of the commercial idler filter.

*K*(also known as the cooperativity parameter) and the entropy of entanglement

*S*[34

34. C. K. Law, I. A. Walmsley, and J. H. Eberly, “Continuous frequency entanglement: effective finite Hilbert space and entropy control,” Phys. Rev. Lett. **84**, 5304–5307 (2000). [CrossRef] [PubMed]

_{s},λ

_{i}), and can be conveniently calculated from its coincidence spectrum through Schmidt decomposition [36

36. A. Ekert and P. L. Knight, “Entangled quantum systems and the Schmidt decomposition,” Am. J. Phys. **63**, 415–423 (1995). [CrossRef]

_{n}(i.e., ∑nλ

_{n}= 1), the Schmidt number is defined as

*K*= 1/(∑

_{n}λ

_{n}

^{2}) and the entropy of entanglement is given by

*S*= -∑

_{n}λ

_{n}log

_{2}(λ

_{n}). Both of these increase monotonically with the amount of spectral entanglement present in Ψ(λ

_{s},λ

_{i}). They achieve their minimum values (

*K*

_{min}= 1 and

*S*

_{min}= 0) for a factorable two-photon state [i.e., Ψ(λ

_{s},λ

_{i}) =

*ψ*(λ

_{s})

*ϕ*(λ

_{i}), possessing zero spectral entanglement], which is an important resource for quantum information applications such as heralded pure single-photon states [35] and multi-element Hong-Ou-Mandel interference [37]. The Schmidt number is estimated to be 5.12 for the type-0 two-photon state shown in Fig. 4(a), indicating a high degree of spectral entanglement. In comparison, the Schmidt numbers are much lower (

*K*≈ 1.8) for the type-II two-photon states shown in Fig. 4(b), (c) and (d). This means that the type-II two-photon state is less spectrally entangled than its type-0 counterpart in this waveguide, and thus more closely approaches a factorable state. This also suggests that a factorable two-photon state output is possible through waveguided SPDC.

17. Y. Kim and W. P. Grice, “Measurement of the spectral properties of the two-photon state generated via type II spontaneous parametric downconversion,” Opt. Lett. **30**, 908–910 (2005). [CrossRef] [PubMed]

19. A. Ling, P. Y. Han, A. Lamas-Linares, and C. Kurtsiefer, “Preparation of Bell States with controlled white noise,” Laser Phys. **16**, 1140–1144 (2006). [CrossRef]

18. H. S. Poh, C. Y. Lum, I. Marcikic, A. Lamas-Linares, and C. Kurtsiefer, “Joint spectrum mapping of polarization entanglement in spontaneous parametric down-conversion,” Phys. Rev. A **75**, 043816 (2007). [CrossRef]

*S*in both cases), tight spectral filtering can be applied to reduce the amount of spectral entanglement to allow efficient quantum information processing applications such as polarization entanglement swapping, at the cost of reduced coincidence count rates.

### 3.4. Photon pair and single-photon fluorescence

*η*

_{s}(

*η*

_{i}) as the total efficiency (including all collection and detection losses) for the signal (idler) photon channels. The photon-pair contribution to the total photon flux produced by the waveguide can be written as:

*N*

_{2}= (

*N*· F)/(

_{c}*η*s ·

*η*

_{i}), where

*N*≡

_{c}*C*-

*A*is the detected true coincidence rate and

*F*= 80 is the down-count factor. The single-photon fluorescence production rate can be calculated by subtracting the photon-pair rate from the total production rate in each channel:

*N*

_{1(s)}=

*D*/

_{s}*η*

_{s}-

*N*

_{2}and

*N*

_{1(i)}=

*D*·

_{i}*F*/

*η*

_{i}-

*N*

_{2}, where

*D*and

_{s}*D*

_{i}are the dark-count-subtracted detected photon rate in the signal and idler channel, respectively. This technique is similar to the one used in separating four-wave-mixing photon pairs from spontaneous Raman scattering in the context of fiber-based χ

^{(3)}photon-pair sources [38

38. X. Li, P. L. Voss, J. Chen, K. F. Lee, and P. Kumar, “Measurement of co- and cross-polarized Raman spectra in silica fiber for small detunings,” Opt. Express **13**, 2236–2244 (2005). [CrossRef] [PubMed]

39. J. Fan and A. Migdall, “A broadband high spectral brightness fiber-based two-photon source,” Opt. Express **15**, 2915–2920 (2007). [CrossRef] [PubMed]

^{(2)}photon-pair source, and for the first time give a complete and separate description of the SPDC photon spectrum and fluorescence spectrum for the entire down-conversion bandwidth, for both types of SPDC.

*η*

_{s}=3.5% ± 0.3% and

*η*

_{i}=4.0% ± 0.2%. The details of the measured efficiencies are listed in Table 1. These efficiencies were measured for a pair of wavelengths λ

_{s}= 900nm and λ

_{i}= 1300nm using classical light at those wavelengths. It is possible that these efficiencies are dependent on the wavelength of light that is collected. However, since the majority of the down-converted 1300 nm (900 nm) light is emitted in a single spatial mode, and falls within the 12 nm and 1.4 nm (6 nm and 0.7 nm) bands for type-0 and type-II SPDC, respectively, we assume the collection efficiencies are constant within those passbands. It can be clearly seen that when operating at the optimal temperature, the photon-pair component is much higher than the single-photon fluorescence component. On the other hand, the single-photon fluorescence can become comparable to or even higher than the former when the waveguide is away from the optimal temperature or the photon wavelength falls outside the SPDC phase-matching band.

40. D. N. Klyshko, “Use of two-photon light for absolute calibration of photoelectric detectors,” Sov. J. Quantum Electron. **10**, 1112–1117 (1980). [CrossRef]

*C*is the coincidence counts per pulse, and

*D*(

_{j}*j*=s,i) is the detected single counts per pulse in the

*j*th channel. The Klyshko efficiencies are calculated to be

*η*

^{K}

_{s}= 2.3% ± 0.1% and

*η*

^{K}

_{i}= 2.5% ± 0.1% for both types of SPDC. They are less than their measured counterparts, i.e.,

*η*

^{K}

_{s(i)}<

*η*

_{s(i)}. This is because the photon source that we have is not a pure photon-pair source, i.e., one that outputs photon pairs and only photon pairs. For a pure photon-pair source, one would have

*D*

_{s}=

*η*s

*μ*,

*D*

_{i}=

*η*i

*μ*, and

*C*=

*η*

_{s}

*η*i

*μ*, where

*μ*is the produced photon pair per pulse. Therefore, from the definition of the Klyshko efficiencies, we have

*μ*

_{s(i)}in the single photon production rate, i.e.,

*D*′

_{s}=

*η*

_{s}(

*μ*+

*μ*

_{s}),

*D*′

_{i}=

*η*

_{i}(

*μ*+

*μ*

_{i}), and

*C*′ =

*η*

_{s}

*η*

_{i}

*μ*, where

*μ*

_{s(i)}is the produced single-photon noise per pulse. Note that here

*C*′ stands for true coincidence, where accidental coincidence counts from noise photons and multiple photon pairs should be subtracted (

*C*′ =

*C*-

*A*). As a result, we have for our waveguide source,

^{7}/s/mW/THz, and for type-II SPDC is ≈ 2.5×10

^{8}/s/mW/THz. These numbers are the highest spectral brightness efficiencies reported to date (see Table 2 for a detailed comparison).

### 3.5. Quasi-phase matching for type-0 and type-II SPDC

43. T. Y. Fan, C. E. Huang, B. Q. Hu, R. C. Eckardt, Y. X. Fan, R. L. Byer, and R. S. Feigelson, “Second harmonic generation and accurate index of refraction measurements in flux-grown KTiOPO_{4},” App. Opt. **26**, 2390–2394 (1987). [CrossRef]

*x*is the light propagation direction in the waveguide,

*y*is the horizontal polarization, and

*z*is the vertical polarization. The Sellmeier equations for

*y*-polarized and

*z*-polarized light fields in the PPKTP waveguide are [43

43. T. Y. Fan, C. E. Huang, B. Q. Hu, R. C. Eckardt, Y. X. Fan, R. L. Byer, and R. S. Feigelson, “Second harmonic generation and accurate index of refraction measurements in flux-grown KTiOPO_{4},” App. Opt. **26**, 2390–2394 (1987). [CrossRef]

*n*is the refractive index for

_{y(z)}*y*(

*z*)-polarized light. The phase-matching equations for type-0 and type-II SPDC in the waveguide are given by:

*m*

_{0}(

*m*

_{2}) is the (integer) order of grating harmonic that contributes to phase matching in type-0 (type-II) SPDC.

*k*

_{wg}to phase matching is the same for the two types of SPDC, which is a valid assumption since the wavelengths involved are very close. Solving Eqs. 3 and 4 simultaneously, we find valid solutions

*only when*

*m*

_{0}= 1 and

*m*

_{2}= 0. This means that type-II phase matching does not need any contribution from periodic poling, and therefore picks up the 0th-order harmonic of the grating (

*m*

_{2}= 0), whereas type- 0 phase matching is made possible by the contribution from the first-order harmonic of the grating (

*m*

_{0}= 1).

_{p}= 0.5322

*μ*m, λ

_{s}= 0.904

*μ*m, and λ

_{i}= 1.294

*μ*m) into Eq. 4 determines the waveguide contribution to be

*k*

_{wg}= −0.1

*μ*m

^{-1}. Similarly, by plugging the peak phase matching wavelengths for type-0 SPDC (λ

_{p}= 0.5322

*μ*

_{m}, λ

_{s}= 0.8992

*μ*

_{m}, and λ

_{i}= 1.304

*μ*m) into Eq. 3, we determine the fit value of the grating period to be Λ ≈ 8.045

*μ*m. This is less than the nominal value of 8.29

*μ*m given by the manufacturer, and we attribute this difference to the temperature dependence of the grating period and possible variations in the periodic poling process (since the poling period is only microns long, there may be variations in uniformity of pole widths and periods).

^{2}(Δ

*k*

*L*

_{eff}/2), where Δ

*k*=

*k*−

_{p}*k*−

_{s}*k*−

_{i}*k*

_{wg}for type-II SPDC and Δ

*k*=

*k*−

_{p}*k*−

_{s}*k*− 2

_{i}*π*/Λ-

*k*

_{wg}for type-0 SPDC. We use these to generate phase matching curves for both signal and idler fields to fit the experimental data shown in Fig. 6(b) and (c). The matching between theory and experiment is remarkably good. An effective length of the entire waveguide of

*L*

_{eff}= 8.5mm matches the experimental FWHM of type-II SPDC. This length is significantly shorter than the specified nominal length of 15 mm, suggesting possible variation in the waveguide fabrication [20

20. M. M. Fejer, G. A. Magel, D. H. Jundt, and R. L. Byer, “Quasi-phase-matched second harmonic generation: tuning and tolerances,” IEEE J. Quantum Electron. **28**, 2631–2654 (1992). [CrossRef]

_{s}≈ 2nm, Δλ

_{i}≈ 4nm). We thus believe that the broadening of type-0 SPDC phase matching bandwidth is due to some small non-uniformity in the grating period over the waveguide [20

20. M. M. Fejer, G. A. Magel, D. H. Jundt, and R. L. Byer, “Quasi-phase-matched second harmonic generation: tuning and tolerances,” IEEE J. Quantum Electron. **28**, 2631–2654 (1992). [CrossRef]

*not*depend on periodic poling, a variation in grating period will not affect the bandwidth of type-II SPDC. This is supported by the data matching a single sinc

^{2}(Δ

*kL*

_{eff}/2) function in Fig. 6(b). Temperature tuning does affect the type-II SPDC spectra through the temperature dependence inherent in the refractive indices

*n*(λ,

*T*) [20

20. M. M. Fejer, G. A. Magel, D. H. Jundt, and R. L. Byer, “Quasi-phase-matched second harmonic generation: tuning and tolerances,” IEEE J. Quantum Electron. **28**, 2631–2654 (1992). [CrossRef]

*k*

_{wg}(

*T*), while the temperature dependence of the grating period Λ(

*T*) does not affect the type-II spectra because type-II SPDC does not depend on the poling period. In comparison, the type-0 SPDC spectra is affected by the temperature tuning through

*n*(

*λ*,

*T*),

*k*

_{wg}(

*T*), and also Λ(

*T*). We believe the main reason for type-II SPDC to be brighter (both spectrally and overall) than its type-0 counterpart in this waveguide (despite

*d*

_{eff}>

*d*

_{24}) is that the difficulty in maintaining a uniform grating period and the resulting variations in the grating period along the entire waveguide effectively decreases the phase matching efficiency of type-0 SPDC, with different section of the waveguide producing photon pairs at different wavelengths which do not add up coherently. Since type-II SPDC is immune to the grating period change, it is relatively enhanced compared to its type-0 counterpart.

## 4. Conclusion

**93**, 093601 (2004). [CrossRef] [PubMed]

44. B. Boulanger, I. Rousseau, J. P. Feve, M. Maglione, B. Menaert, and G. Marnier, “Optical Studies of Laser-Induced Gray-Tracking in KTP,” IEEE J. Quantum Electron. **35**, 281–286 (1999). [CrossRef]

## Acknowledgments

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18. | H. S. Poh, C. Y. Lum, I. Marcikic, A. Lamas-Linares, and C. Kurtsiefer, “Joint spectrum mapping of polarization entanglement in spontaneous parametric down-conversion,” Phys. Rev. A |

19. | A. Ling, P. Y. Han, A. Lamas-Linares, and C. Kurtsiefer, “Preparation of Bell States with controlled white noise,” Laser Phys. |

20. | M. M. Fejer, G. A. Magel, D. H. Jundt, and R. L. Byer, “Quasi-phase-matched second harmonic generation: tuning and tolerances,” IEEE J. Quantum Electron. |

21. | J. D. Bierlein and H. Vanherzeele, “Potassium titanyl phosphate: properties and new applications,” J. Opt. Soc. Am. B |

22. | M. Ghioni, A. Gulinatti, I. Rech, F. Zappa, and S. Cova, “Progress in silicon single-photon avalanche diodes,” IEEE J. Sel. Top. Quantum Electron. |

23. | K. M. Rosfjord, J. K.W. Yang, E. A. Dauler, A. J. Kerman, V. Anant, B. M. Voronov, G. N. Gol’tsman, and K. K. Berggren, “Nanowire single-photon detector with an integrated optical cavity and anti-reflection coating,” Opt. Express |

24. | A. E. Lita, A. J. Miller, and S. W. Nam, “Counting near-infrared single-photons with 95% efficiency,” Opt. Express |

25. | J. Chen, K. F. Lee, C. Liang, and P. Kumar, “Fiber-based telecom-band degenerate-frequency source of entangled photon pairs,” Opt. Lett. |

26. | J. Fan, A. Dogariu, and L. J. Wang, “Generation of correlated photon pairs in a microstructure fiber,” Opt. Lett. |

27. | K. F. Lee, J. Chen, C. Liang, X. Li, P. L. Voss, and P. Kumar, “Generation of high-purity telecom-band entangled photon pairs in dispersion-shifted fiber,” Opt. Lett. |

28. | H. Takesue and K. Inoue, “1.5- |

29. | S. D. Dyer, M. J. Stevens, B. Baek, and S.W. Nam, “High-efficiency, ultra low-noise all-fiber photon-pair source,” Opt. Express |

30. | J. Chen, “Development and Applications of Fiber-based Entanglement Sources”, Ph.D. thesis, Northwestern University (2007), http://terpconnect.umd.edu/̃junchen/files/dissertation.pdf. |

31. | J. Chen, X. Li, and P. Kumar, “Two-photon-state generation via four-wave mixing in optical fibers,” Phys. Rev. A |

32. | D. C. Burnham and D. L. Weinberg, “Observation of Simultaneity in Parametric Production of Optical Photon Pairs,” Phys. Rev. Lett. |

33. |
We note that another method for measuring joint spectrum is given by W. Wasilewski |

34. | C. K. Law, I. A. Walmsley, and J. H. Eberly, “Continuous frequency entanglement: effective finite Hilbert space and entropy control,” Phys. Rev. Lett. |

35. | A. B. U’Ren, C. Silberhorn, K. Banaszek, I. A. Walmsley, R. Erdmann, W. P. Grice, and M. G. Raymer, “Generation of pure-state single-photon wavepackets by conditional preparation based on spontaneous parametric downconversion,” Laser Phys. |

36. | A. Ekert and P. L. Knight, “Entangled quantum systems and the Schmidt decomposition,” Am. J. Phys. |

37. | A. B. U’Ren, K. Banaszek, and I. A. Walmsley, “Photon engineering for quantum information processing,” Quant. Inf. and Comp. |

38. | X. Li, P. L. Voss, J. Chen, K. F. Lee, and P. Kumar, “Measurement of co- and cross-polarized Raman spectra in silica fiber for small detunings,” Opt. Express |

39. | J. Fan and A. Migdall, “A broadband high spectral brightness fiber-based two-photon source,” Opt. Express |

40. | D. N. Klyshko, “Use of two-photon light for absolute calibration of photoelectric detectors,” Sov. J. Quantum Electron. |

41. | A. Fedrizzi, T. Herbst, A. Poppe, T. Jennewein, and A. Zeilinger, “A wavelength-tunable fiber-coupled source of narrowband entangled photons,” Opt. Express |

42. | S. Sauge, M. Swillo, M. Tengner, and A. Karlsson, “A single-crystal source of path-polarization entangled photons at non-degenerate wavelengths,” Opt. Express |

43. | T. Y. Fan, C. E. Huang, B. Q. Hu, R. C. Eckardt, Y. X. Fan, R. L. Byer, and R. S. Feigelson, “Second harmonic generation and accurate index of refraction measurements in flux-grown KTiOPO |

44. | B. Boulanger, I. Rousseau, J. P. Feve, M. Maglione, B. Menaert, and G. Marnier, “Optical Studies of Laser-Induced Gray-Tracking in KTP,” IEEE J. Quantum Electron. |

**OCIS Codes**

(190.4410) Nonlinear optics : Nonlinear optics, parametric processes

(230.7370) Optical devices : Waveguides

(270.5585) Quantum optics : Quantum information and processing

**ToC Category:**

Quantum Optics

**History**

Original Manuscript: February 11, 2009

Revised Manuscript: March 20, 2009

Manuscript Accepted: March 21, 2009

Published: April 8, 2009

**Citation**

Jun Chen, Aaron J. Pearlman, Alexander Ling, Jingyun Fan, and Alan L. Migdall, "A versatile waveguide source of photon pairs for chip-scale quantum information processing," Opt. Express **17**, 6727-6740 (2009)

http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-8-6727

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