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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 16, Iss. 1 — Jan. 7, 2008
  • pp: 32–44
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Spectral study of photon pairs generated in dispersion shifted fiber with a pulsed pump

Xiaoying Li, Xiaoxin Ma, Zhe Yu Ou, Lei Yang, Liang Cui, and Daoyin Yu  »View Author Affiliations


Optics Express, Vol. 16, Issue 1, pp. 32-44 (2008)
http://dx.doi.org/10.1364/OE.16.000032


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Abstract

Spectral correlation of photon pairs generated in dispersion shifted fiber by a pulsed pump is theoretically analyzed and experimentally investigated. We first calculate the spectral function of photon pairs according to the deduced two-photon state generated by spontaneous four wave mixing under the assumptions close to the real experimental conditions. We then experimentally study the spectral property of the signal and idler photon pairs generated in optical fiber by photon correlation measurements, and the experimental results agree with the calculation. The investigation is useful for developing fiber-based sources of entangled photon pairs and for studying multi-photon quantum interference with multiple photon pairs.

© 2008 Optical Society of America

1. Introduction

Quantum correlated photon pairs have been a critical resources for quantum information processing (QIP). For the QIP protocols, such as entanglement swapping, quantum teleportation, generation of GHZ states, and quantum computing etc, the quantum state manipulation depends on quantum interference among different photon pair sources [1

1. D. Bouwmeester, J. W. Pan, K. Mattle, M. Eibl, H. Weinfurter, and A. Zeilinger, “Experimental quantum teleportation,” Nature 390, 575–579 (1997). [CrossRef]

, 2

2. J. W. Pan, D. Bouwmeester, H. Weinfurter, and A. Zeilinger, “Experimental entanglement swapping: entangling photons that never interacted,” Phys. Rev. Lett. 80, 3891–3894 (1998). [CrossRef]

, 3

3. E. Knill, R. Laflamme, and G. J. Milburn, “A scheme for efficient quantum computation with linear optics,” Nature 409, 46–52 (2001). [CrossRef] [PubMed]

, 4

4. Z. Zhao, Y. A. Chen, A. N. Zhang, T. Yang, H. J. Briegel, and J. W. Pan, “Experimental demonstration of five-photon entanglement and open-destination quantum teleportation,” Nature 430, 54–58 (2004). [CrossRef] [PubMed]

]. To obtain high visibility in interference, photons originated from different sources must be identical in their spectral, spatial, polarization and temporal modes, so that it is impossible to determine their origins [5

5. B. Yurke and D. Stoler, “Einstein-Podolsky-Rosen effects from independent particle sources,” Phys. Rev. Lett. 68, 1251–1254 (1992). [CrossRef] [PubMed]

, 6

6 . M. Zukowski, A. Zeilinger, and H. Weinfurter, “Entangling Photons Radiated by Independent Pulsed Sources,” Ann. (N. Y.) Acad. Sci. 755, 91–102 (1995). [CrossRef]

]. So far the workhouse for implementing the various QIP tasks has been entangled photon pairs produced by parametric down conversion (PDC) in χ(2) nonlinear crystals [7

7 . P. G. Kwiat, K. Mattle, H. Weinfurter, A. Zeilinger, A. V. Sergienko, and Y. H. Shih, “New high-intensity source of polarization-entangled photon pairs,” Phys. Rev. Lett. 75, 4337–4341 (1995). [CrossRef] [PubMed]

, 1

1. D. Bouwmeester, J. W. Pan, K. Mattle, M. Eibl, H. Weinfurter, and A. Zeilinger, “Experimental quantum teleportation,” Nature 390, 575–579 (1997). [CrossRef]

, 2

2. J. W. Pan, D. Bouwmeester, H. Weinfurter, and A. Zeilinger, “Experimental entanglement swapping: entangling photons that never interacted,” Phys. Rev. Lett. 80, 3891–3894 (1998). [CrossRef]

]. Due to the independent nature of the down-converted fields, stationary fields cannot be used unless the coherence length of the fields is longer than the temporal resolution of the detectors [8

8. Z. Y. Ou and Y. J. Lu, “Cavity Enhanced Spontaneous Parametric Down-Conversion for the Prolongation of Correlation Time between Conjugate Photons,” Phys. Rev. Lett. 83, 2556–2559 (1999). [CrossRef]

, 9

9. M. Halder, A. Beveratos, N. Gisin, V. Scarani, C. Simon, and H. Zbinden, “Entangling independent photons by time measurement,” Nature Physics 3, 692–695 (2007). [CrossRef]

], which is usually of the order of 10-10 s. Therefore, an ultrashort pulse is often used for pumping the PDC, because the timing provided by the ultrashort pump pulse can overcome the problem of slow detector [6

6 . M. Zukowski, A. Zeilinger, and H. Weinfurter, “Entangling Photons Radiated by Independent Pulsed Sources,” Ann. (N. Y.) Acad. Sci. 755, 91–102 (1995). [CrossRef]

]. However, a problem arises when the widely used χ(2) nonlinear crystal is pumped by a broadband laser. Because of the complicated dispersion relationship in χ(2) materials, the down-converted fields are not transform-limited even though the pump field is [10

10. Z. Y. Ou, “Parametric Down-Conversion with Coherent Pulse Pumping and Quantum Interference between Independent Fields,” Quantum Semiclass Opt. 9, 599–614 (1997). [CrossRef]

]. The incoherence in the down-converted fields makes the high visibility in interference between independent fields impossible without using optical filters to clean the modes, which will reduce the count rate significantly. This problem can be solved to some extent by properly tailoring the dispersive properties of the χ(2) crystals [11

11. W. P. Grice, A. B. U’ren, and I. A. Walmsley, “Eliminating frequency and space-time correlations in multi-Photon States,” Phys. Rev. A 64, 063,815 (2001). [CrossRef]

]. However, it is difficult to implement in the general sense because of the limitation of the χ(2) materials.

Recently there are growing interests in developing the various fiber-based entangled photon-pair sources [12

12. M. Fiorentino, P. L. Voss, J. E. Sharping, and P. Kumar, “All-fiber photon-pair source for quantum communication,” Photon. Technol. Lett. 14, 983–985 (2002). [CrossRef]

, 13

13. X. Li, P. L. Voss, J. E. Sharping, and P. Kumar, “Optical-fiber source of polarization-entangled photons in the 1550 nm telecom band,” Phys. Rev. Lett. 94, 053,601 (2005). [CrossRef]

, 14

14. H. Takesue and K. Inoue, “Generation of 1.5-um band time-bin entanglement using spontaneous fiber four-wave mixing and planar lightwave circuit interferometers,” Phys. Rev. A 72, 041,804 (2005). [CrossRef]

, 15

15. 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]

, 16

16. J. Fulconis, O. Alibart, W. J. Wadsworth, P. S. Russell, and J. G. Rarity, “High brightness single mode source of correlated photon pairs using a photonic crystal fiber,” Opt. Express 13, 7572–7582 (2005). [CrossRef] [PubMed]

, 17

17. 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]

], because of the inherent compatibility with the transmission medium, the excellent single spatial-mode purity, and a better nonlinear vs. loss figure of merit over the χ(2) nonlinear crystals. Although Raman scattering (RS) may degrade the fidelity of such sources [18

18. X. Li, J. Chen, P. L. Voss, J. Sharping, and P. Kumar, “All-fiber photon-pair source for quantum communications: Improved generation of correlated photons,” Opt. Express 12, 3737–3744 (2004). [CrossRef] [PubMed]

, 19

19. K. Inoue and K. Shimizu, “Generation of Quantum-Correlated Photon Pairs in Optical Fiber: Influence of Spontaneous Raman Scattering,” Jpn. J. Appl. Phys 43, 8048–8052 (2004). [CrossRef]

], it can be significantly suppressed by cooling the dispersion-shifted fibers (DSF) [20

20. H. Takesue and K. Inoue, “1.5 um 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]

, 17

17. 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]

], by pumping a microstructure fiber (MF) in the normal dispersion region [21

21. J. G. Rarity, J. Fulconis, J. Duligall, W. J. Wadsworth, and P. S. J. Russell, “Photonic crystal fiber source of correlated photon pairs,” Opt. Express 13, 534–544 (2005). [CrossRef] [PubMed]

], and by pumping the optical fibers with a pulsed pump [16

16. J. Fulconis, O. Alibart, W. J. Wadsworth, P. S. Russell, and J. G. Rarity, “High brightness single mode source of correlated photon pairs using a photonic crystal fiber,” Opt. Express 13, 7572–7582 (2005). [CrossRef] [PubMed]

]. Apart from the experimental progresses [12

12. M. Fiorentino, P. L. Voss, J. E. Sharping, and P. Kumar, “All-fiber photon-pair source for quantum communication,” Photon. Technol. Lett. 14, 983–985 (2002). [CrossRef]

, 13

13. X. Li, P. L. Voss, J. E. Sharping, and P. Kumar, “Optical-fiber source of polarization-entangled photons in the 1550 nm telecom band,” Phys. Rev. Lett. 94, 053,601 (2005). [CrossRef]

, 21

21. J. G. Rarity, J. Fulconis, J. Duligall, W. J. Wadsworth, and P. S. J. Russell, “Photonic crystal fiber source of correlated photon pairs,” Opt. Express 13, 534–544 (2005). [CrossRef] [PubMed]

, 14

14. H. Takesue and K. Inoue, “Generation of 1.5-um band time-bin entanglement using spontaneous fiber four-wave mixing and planar lightwave circuit interferometers,” Phys. Rev. A 72, 041,804 (2005). [CrossRef]

, 15

15. 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]

, 16

16. J. Fulconis, O. Alibart, W. J. Wadsworth, P. S. Russell, and J. G. Rarity, “High brightness single mode source of correlated photon pairs using a photonic crystal fiber,” Opt. Express 13, 7572–7582 (2005). [CrossRef] [PubMed]

, 17

17. 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]

], theoretical models analyzing the fiber based two-photon state for the pulsed pump in the normal and anomalous dispersion regions, respectively, had been successively investigated [22

22. J. Chen, X. Li, and P. Kumar, “Two-photon-state generation via four-wave mixing in optical fibers,” Phys. Rev. A 72, 033,801 (2005). [CrossRef]

, 23

23 . O. Alibart, J. Fulconis, G. K. L. Wong, S. G. Murdoch, W. J. Wadsworth, and J. G. Rarity, “Photon pair generation using four-wave mixing in a microstructured fibre: theory versus experiment,” New J. of Phys. 8, 67 (2006). [CrossRef]

]. Moreover, Garay-Palmett [24

24. K. Garay-Palmett, H. J. McGuinness, O. Cohen, J. S. Lundeen, R. Rangel-Rojo, A. B. U’Ren, M. G. Raymer, C. J. McKinstrie, S. Radic, and I. A. Walmsley, “Photon pair-state preparation with tailored spectral properties by spontaneous four-wave mixing in photonic-crystal fiber,” Opt. Express 15, 14,870–14,886 (2007). [CrossRef]

] et al. have theoretically studied photon pair-state preparation with tailored spectral properties by spontaneous four-wave mixing (FWM) in MF, showing the flexibility of engineering dispersion in MF can provide a possible solution to generate two-photon state with specific spectral correlation suitable for QIP. However, comparing with its χ(2) crystal counterparts, the characteristics of the χ(3) based fiber source, such as the spectral and temporal mode properties of various fiber sources, have not been fully explored.

In this paper, the spectral property of fiber based photon pairs is investigated. We focus ourselves on the case in which the DSF is pumped by a pulse with a duration of a few picoseconds. We first theoretically analyze the spectral property of the signal and idler fields by studying the non-degenerate χ(3)-based two-photon state in DSF [22

22. J. Chen, X. Li, and P. Kumar, “Two-photon-state generation via four-wave mixing in optical fibers,” Phys. Rev. A 72, 033,801 (2005). [CrossRef]

]. In the derivation of the two-photon state, the magnitudes of the pump, signal, and idler wave-vectors are expanded around the pump central frequency up to the third-order dispersion, rather than only to the second-order dispersion as presented in Ref.[22

22. J. Chen, X. Li, and P. Kumar, “Two-photon-state generation via four-wave mixing in optical fibers,” Phys. Rev. A 72, 033,801 (2005). [CrossRef]

]. The calculated results show that it is possible to make the phase matching function of FWM ineffective in its corresponding joint spectral function by properly managing the parameters, such as the central wavelength of pump, the detuning between the signal (idler) and pump photons, and the pump power etc. We then experimentally investigate the spectral property of the two-photon state in DSF by measuring the true coincidence of signal and idler photons as a function of the central wavelength of signal photons when the pump and idler photons with fixed wavelengths are set in several different ways. The experimental results agree with the theoretical prediction. The investigation is useful for developing fiber-based sources of entangled photon pairs and for studying multi-photon quantum interference with multiple photon pairs.

2. Theory

When the central wavelength of the pump pulses is in the anomalous-dispersion regime of DSF, the FWM parametric process is phase-matched and the probability of spontaneous FWM is significantly enhanced. In this process, two pump photons at frequency ωp scatter through the Kerr (χ(3)) nonlinearity of the fiber to create energy-time entangled signal and idler photons at frequencies ωs and ωi, respectively, such that 2ωp=ωs+ωi. Because of the isotropic nature of the Kerr nonlinearity in fused-silica-glass fiber, the generated correlated-photons are predominantly co-polarized with the pump photons. We assume that the pump, and the signal and idler modes are linearly polarized and propagating in the z direction, which is parallel with the fiber axis. The state vector of signal and idler photon pair calculated by means of the first-order perturbation theory can be written as [22

22. J. Chen, X. Li, and P. Kumar, “Two-photon-state generation via four-wave mixing in optical fibers,” Phys. Rev. A 72, 033,801 (2005). [CrossRef]

]

Ψ(ωs,ωi)=AdtL0dzEs()(t,z)Ei()(t,z)Ep(+)(t,z)Ep(+)(t,z)0,
(1)

where

Es()(t,z)=dωsas+ei(kszωst)
(2)

and

Ei()(t,z)=dωiai+ei(kizωit)
(3)

The positive-frequency electric field operator of pump E (+) p (t, z) in Eq. (1) is taken to be a classical narrow pulse, having a Gaussian spectral envelope. For the fiber with a length of hundreds meters, its propagation loss can be neglected. Therefore the self phase modulation (SPM) of pump can be included in a straightforward manner [22

22. J. Chen, X. Li, and P. Kumar, “Two-photon-state generation via four-wave mixing in optical fibers,” Phys. Rev. A 72, 033,801 (2005). [CrossRef]

]:
Ep(+)(t,z)=eiωp0teiγPpzdΩpeΩp22σp2ei(kpzΩpt),
(4)
where ω p0 is the central frequency of pump pulse, σp is the optical bandwidth of the pump, Pp is the peak pump power, kp is the wave-vector of pump, γ is the nonlinear coefficient of the optical fiber, and Ωp=ωp-ω p0.

Substituting Eqs. (2)–(4) into Eq.(1) leads to the following form of state vector

Ψ(ωs,ωi)=gdωsdωiF(ωs,ωi)ωsωi,
(5)
gπ2χ(3)Ppσp,
(6)
F(ωs,ωi)=L0dzexp{iΔkz2iγPpz}1ikσp2zi2k(Ωs+Ωi)zσp2exp{(Ωs+Ωi)24σp2},
(7)
Δk=k4(ΩsΩi+Δ)2+k8Δ2(Ωs+Ωi),
(8)

It is well known that for a pump pulse with a central wavelength close to λ 0 and a duration T 0 of a few picoseconds, if the fiber length L is of the order of hundreds meters, the pulse broadening due to group velocity dispersion (GVD) and third order dispersion can be neglected [25

25. G. P. Agrawal, Nonlinear fiber optics (Elsevier Pte Ltd., Singapore, 2005).

]. This is because the fiber length satisfies the relations: LT02k , LT03k . Therefore, the pulse broadening factor 11ikσp2zi2k(Ωs+Ωi)zσp2 in Eq. (7) can be approximated as 1. Then the spectral function F(ωs, ωi) is simplified as

F(ωs,ωi)=exp[iL(Δk2+γPp)]α(ωs,ωi)ϕ(ωs,ωi),
(9)

with

α(ωs,ωi)=exp[(Ωs+Ωi)24σp2]
(10)
ϕ(ωs,ωi)=sinc[kL8(ΩsΩi+Δ)2+kL16Δ2(Ωs+Ωi)+γPpL]
(11)

The mathematical structure of the actual parameter of the sinc function in Eq. (11) is complicated, however, the second order dispersion term kL8(ΩsΩi+Δ)2 and the SPM term γPpL can cancel out each other when the central wavelength of the pump pulse λ p0 in the anomalous dispersion region of DSF is properly optimized and |ω p0-ω 0|≪Δ. Under this condition, we have ϕ(ωs,ωi)sinc[kL16Δ2(Ωs+Ωi)] , here the actual parameter of the sinc function ϕ(ωs, ωi) is the sum of the frequencies Ωs and Ωi, which indicates the sinc function has the property of exchange symmetry: ϕ(ωs, ωi)=ϕ(ωi, ωs).

Equation (9) shows that the shape of |F(ωs, ωi)|2, proportional to the two-photon joint spectral correlation of photon pairs, depends on the functions |α(ωs, ωi)|2 and |ϕ(ωs, ωi)|2, which are determined by the spectrum of pump field and the phase matching condition of FWM, respectively. To understand the relationship between |F(ωs, ωi)|2, |α(ωs, ωi)|2 and |ϕ(ωs, ωi)|2, it is instructive to examine plots of these functions individually at various conditions. Figure 1 shows the envelop of |α(ωs, ωi)|2 for the pump with λ p0=1541 nm and full-widthhalf-maximum (FWHM) of 1 nm (σp/2π=75 GHz). Since the argument of |α(ωs, ωi)|2 is the sum of the frequencies Ωs and Ωi, the function has constant value along lines defined by Ωsi=const, namely, ωs+ωi=const, which reaches its peak value at every point on the line ωs+ωi= 2ω p0. The graphical representation in Fig. 2, (a), (b) and (c) are the sinc function |ϕ(ωs, ωi)|2 in DSF with λ 0=1540 nm and γPpL=1 for the frequency difference Δ/2π=2.17 THz, 2.67 THz, and 3.18 THz, respectively. For each frequency difference Δ, the frequency of the signal and idler photon pairs satisfying the relation Ωis=0 is labeled by a crosshairs. For Δ/2π=2.67 THz, the sum of the second order dispersion term and SPM is negligibly small, so that the sinc function can be approximated as ϕ(ωs,ωi)sinc[kL16Δ2(Ωs+Ωi)] . One sees that in the vicinity of the cross-hairs in Fig. 2(b), the strip of the sinc function is about the same as that of the exponential function in Fig. 1; whereas the strips in the vicinity of the cross-hairs are shifted away from the line ωs+ωi=2ω p0 when Δ/2π is not equal to 2.67 THz, as shown in Fig. 2(a) and (c), respectively. Figure 2(d), (e) and (f) are the normalized joint spectral intensity |F(ωs, ωi)|2 corresponding to Fig. 2(a), (b), and (c), respectively. It is clear that the spectral property of the function |F(ωs, ωi)|2 highly depends on that of the sinc function ϕ (ωs, ωi).

Fig. 1. The envelope of the function |α(ωsi)|2 for the Gaussian shaped pump pulse, whose central wavelength and FWHM are 1541 nm and 1 nm, respectively. In this and subsequent figures, the two axes represent the signal and idler frequencies, although as an aid to the reader, the frequencies have been converted to wavelengths. The symmetry line of the function |α(ωs, ωi)|2 can be defined by ωs+ωi=2ω p0, i.e., λi=λsλ p0/(2λs-λ p0).
Fig. 2. The three top plots are the envelopes of phase-matching function |ϕ(ωs, ωi)|2, the frequency (wavelength) difference Δ/2π in plots (a), (b), and (c) is 2.17 THz (17.16 nm), 2.67 THz (21.16 nm), and 3.18 THz (25.16 nm), respectively; the three bottom plots are the normalized joint spectral intensity |F(ωs, ωi)|2 for spontaneous FWM in DSF, the frequency (wavelength) difference Δ/2π in plots (d), (e), and (f) is 2.17 THz (17.16 nm), 2.67 THz (21.16 nm), and 3.18 THz (25.16 nm), respectively. The solid line in each plot is the symmetry axis of the exponential function |α(ωs, ωi)|2; the cross-hairs in each plots corresponding to Ωis=0. The parameters in Eq. (11) are λ 0=1540 nm, γ=2/W·km, L=0.3 km, Dslope=0.075 ps/(nm2·km), λ p0=1541 nm and γPpL=1.

To obtain an unabridged understanding of the spectral function F(ωs, ωi), it is useful to study the conditional spectrum S s(i)(ω) of the individual signal (idler) photon wave packets for a given Gaussian shaped filter with width σ 0 at the idler (signal) field

Ss(i)(Ω)=dΩi(s)eΩi(s)2σ02F(ωs,ωi)2=dΩi(s)eΩi(s)2σ02α(ωs,ωi)ϕ(ωs,ωi)2
(12)

It is worth noting when the parameters Ωs and Ωi, associated with the bandwidths of signal and idler fields, are much less than Δ, i.e., |Ωs|, |Ωi|≪Δ, and the sum of the second order dispersion term and SPM term in the function ϕ(ωs, ωi) (see Eq. (11)) is negligibly small, the sinc function ϕ(ωs, ωi) in Eq. (12) can be approximated to the Gaussian function: sinc(ϕ2)eΓϕ2 , where ϕ=(k″′L/8)Δ2si), and Γ=0.04822. In this case, the conditional spectrum S s(i)(ω) can be expressed as

Ss(i)(Ω)=gexp(Ω2[1+Γσp2(kLΔ24)2]2σp2+σ02+Γσp2σ02(kLΔ24)2)
(13)

where g=2πσ0σp2σp2+σ02+Γσp2σ02L2(kΔ24)2. Equation (13) shows that the conditional spectra of the individual signal and idler photon wave packet, Ss(ω) and Si(ω), are identical. Moreover, if the condition σpk′″LΔ2/4<1 is satisfied, we have

Ss(Ω)=Si(Ω)2πσ0σp2σp2+σ02exp[Ω22σp2+σ02]
(14)

Equation (14) shows S s(i)(Ω) only depends on the bandwidth of pump and that of the filter placed in idler (signal) field, which implies the sinc function ϕ(ωs, ωi) in Eq. (12) does not play a role. We note that the required conditions |Ωs|, |Ωi|≪Δ and σpk′″LΔ2/4<1 are practically realizable. For example, the signal and idler photon pairs at telecom band, with the frequency difference Δ/2π=1.8 THz (15 nm) and bandwidth σ 0/2π=25 GHz, can be obtained by pumping 300m DSF with a pulsed pump and by using the off-the-shelf filters, such as fiber Bragg-grating filters and fiber-pigtailed wavelength-division-multiplexing filters. For the pump pulse with a bandwidth σp/2π=100 GHz, we have σpk′″LΔ2/4=0.13<1.

Knowing the special conditions under which the sinc function ϕ(ωs, ωi) has the value of about 1 and can be viewed as nonexistence, we investigate how to deduce the spectral property of the spectral function F(ωs, ωi) in the whole range of the frequency difference Δ from the function S s(i)(Ω). Assuming the bandwidth in idler field σ 0 is much less than that of the pump σp, so that Ωi0 is valid, then the trait of Ss(Ω) can be found by projecting the joint spectral intensity |F(ωs, ωi)|2 onto the ωs axis for a given frequency difference Δ. Under this condition, we have

Ss(Ωs)F(ωs,ωi)2=exp(Ωs22σp2)sinc[kL8(Ωs+Δ)2+kL16Δ2Ωs+γPpL]2
(15)

Fig. 3. Plots of phase-matching function sinc[kL8(Ωs+Δ)2+kL16Δ2Ωs+γPpL]2=sinc[kL8(ωsωi0)2+kL4(ωi0ωp0)2(ωs+ωi02ωp0)+γPpL]2 (a, c, e) and joint spectral intensity |F(ωs, ωi)|2 (b, d, f) under the assumption Ωi=0. The zero dispersion wavelength and dispersion slope of the DSF are λ 0=1540 nm and Dslope=0.075 ps/(nm2·km), respectively. For (a) and (b), the central wavelength of pump and SPM term are λ p0=1541 nm and γPpL=1, respectively; for (c) and (d), λ p0=1541 nm and γPpL=0.5; for (e) and (f), λ p0=1542 nm and γPpL=1. In the plots (a), (c) and (e), the solid line is the symmetry axis of the corresponding exponential function |α(ωs, ωi)|2, and in the vicinity of the cross-hairs, whose corresponding frequency difference is Δ0, the sum of second order dispersion term and SPM term in Eq. (15) (and Eq. (11)) is negligible.

3. Experiment

We experimentally test the above theoretical analysis by using the experimental setup shown in Fig. 4. Signal and idler photon-pairs at frequencies ωs and ωi, respectively, are produced in 300m DSF by a pulsed pump. The zero-dispersion wavelength λ 0 of the DSF is about 1539±2nm. The efficiency of spontaneous FWM in DSF is low because of the relatively low magnitude of Kerr (χ(3)) nonlinearity. To reliably detect the scattered photon-pairs, a pump to photon-pair rejection ratio in excess of 100 dB is required. This is achieved with a filter F2 by cascading a free-space double-grating spectral filter (DGSF) that provides a pump-rejection ratio in excess of 90 dB [13

13. X. Li, P. L. Voss, J. E. Sharping, and P. Kumar, “Optical-fiber source of polarization-entangled photons in the 1550 nm telecom band,” Phys. Rev. Lett. 94, 053,601 (2005). [CrossRef]

] with a tunable filter (TF) (Santec, OTM-30M-03D) and a 40-channel array-waveguide-grating (AWG) in signal and idler band (see Fig. 4), respectively. Both the TF and AWG can provide a pump rejection ratio greater than 40 dB. The central wavelength of signal channel can be tuned by properly adjusting the DGSF and TF, and the central wavelength of idler channel can be changed by adjusting the DGSF and by choosing the different channel of AWG. The FWHM in signal and idler band is 0.25nm and 0.35nm, respectively.

The pump is a train of picoseconds pump pulses with a duration of about 4 ps, which is spectrally carved out from a mode-locked femtosecond fiber laser with a repetition rate of 40 MHz. Its central wavelength can be tuned from 1530 to 1560 nm. To achieve the required power, the pump pulses are then amplified by an erbium-doped fiber amplifier (EDFA). Before launching the pump pulses into DSF, photons at the signal and idler wavelength from the fiber laser that leak through the spectral-dispersion optics and from the amplified spontaneous emission from the EDFA are suppressed by passing the pump through a 1nm-bandwidth TF (Newport, model TBF-1550-1.0) F1, whose FWHM is about 1 nm. Single photon detectors (SPD, id200) operated in the gated Geiger mode are used to detect the signal and idler photons. The 2.5-ns-wide gate pulses arrive at a rate of about 600 KHz, which is 1/64 of the repetition rate of the pump pulses, and the deadtime of the gate is set to be 10 µs to suppress the after-pulse effect. The electrical signals produced by the SPDs in response to the incoming photons (and dark counts) are acquired by a photon-counting system. Thus, single counts in both the signal and idler bands and coincidences acquired from different time bins can be determined. The quantum efficiency of SPD1 (SPD2) is about 10% (7%), with a corresponding dark-count probability of 1.5×10-4 (2×10-5) /pulse. The total detection efficiencies for both the signal and idler photons are about 1%, when the transmission efficiencies of DSF (~90%), and F2 composed of DGSF (~40%), TF(~40%), and AWG (~30%) are included.

Fig. 4. A schematic of the experimental setup; scattered signal and idler photons emerging from the port labeled “Out” are detected; F, filter; G, grating; TF, tunable filter; AWG, array waveguide grating.

We study the spectral property of our fiber source by measuring the true coincidence rate as a function of the wavelength of signal photons when the pump and idler photons with fixed wavelengths are combined in several different manners, which is equivalent to measuring the conditional joint spectrum function Ss(Ω) in Eq. (12). In the first experiment, the average power of pump is about 0.4mW (corresponding to γPpL≈1), the central wavelength of the pump pulses is fixed at 1540.65±0.05nm, the central wavelength of the detected idler photons is fixed at 1530.18±0.02nm, 1531.74±0.02nm, and 1533.30±0.02nm, respectively. For each setting of the pump and idler photons, we scan the central wavelength of signal channel and conduct the photon counting measurement. At each wavelength, we measure the single counts in both the signal and idler channels, and record the coincidence rate between the detected signal and idler photons. The true coincidence rate is obtained by subtracting the accidental coincidence rate produced by the adjacent pump pulses from the coincidence rate produced by the same pulse. For each set of data, after subtracting the Raman contribution [18

18. X. Li, J. Chen, P. L. Voss, J. Sharping, and P. Kumar, “All-fiber photon-pair source for quantum communications: Improved generation of correlated photons,” Opt. Express 12, 3737–3744 (2004). [CrossRef] [PubMed]

], the single counts in both the signal and idler channel are about the same due to the steady-going detection efficiency in both channels and the flat gain of FWM within the scanning wavelength range. The three sets of data in Fig. 5(a) shows true coincidence rate vs. signal wavelength in three cases. Each set of data fits well with a Gaussian function, and their FWHM are about the same. The peaks of each set of data and its fitting curve are different, the main reason is the detection efficiencies of idler photons are different, depending on the transmission efficiency of the corresponding channel of AWG. Figure 5(b) plots the central wavelength of idler photons vs. signal wavelength, which is the central wavelength of the corresponding fitting function. To show the characteristic of the spectra clearly, a solid line satisfied with the relation λi=λsλ p0/(2λs-λ p0) is also plotted in Fig. 5(b). One sees that all the three data points are sat on the symmetry line, indicating the source has the property of spectral symmetry when the wavelength of the idler (signal) photon is in the range from 1530nm to 1533.30nm (1551nm to 1548.30nm).

Fig. 5. (a) True coincidences versus the central wavelength of filter F2 in signal band when the central wavelength of idler is fixed at 1533.3 nm, 1531.74 nm, and 1530.18 nm, respectively. In the experiment, λ p0=1540.65 nm, and the average pump power is about 0.4mW. The blue, pink and purple solid curves are fits of the Gaussian function f(λ)=2.12exp[(λ1547.961.16)2] , f(λ)=1.48exp[(λ1549.691.26)2] , and f(λ)=1.56exp[(λ1551.181.21)2], respectively. (b) Idler central wavelengths versus the central wavelengths of the corresponding fitting curves. The solid line defined by λi=λsλ p0/(2λs-λ p0) is the symmetry axis of the pump pulses.

In the second experiment, we tune the central wavelength of pump to 1541.7±0.05nm, and make the same measurement at different pump power levels. In the experiment, we measure the photon correlation by fixing the central wavelength of the idler photons at 1531.74±0.02nm, 1533.30±0.02nm, and 1534.88±0.02nm, respectively. At the pump power level of 0.4mW and 0.2mW (corresponding to γPpL≈0.5), we obtain three sets of data for the three kinds of experimental settings, as shown in Fig. 6(a) and (b), and Fig. 6(c) and (d), respectively. We find each set of data in Fig. 6(a) and Fig. 6(c) fits well with a Gaussian function, and their FWHM are about the same at a certain pump level. The FWHM in Fig. 6(a) is wider than that in Fig. 6(c), because the SPM induced pump pulse broadening increases with the increasing pump power. At the lower pump level of 0.2mW, taking the slight broadening of pump pulse due to SPM into account, we find the FWHMs of the fitting curves, pump pulse and the filter in the idler band in Fig. 6(c), which are about 1.4 nm, 0.9 nm, and 0.35 nm, respectively, satisfy the relation shown in Eq. (14) [26

26. X. Li, J. Chen, K. F. Lee, P. L. Voss, and P. Kumar, “All-fiber photon-pair source for quantum communication: Influence of spectra,” Proceeding of Quantum Communication and Measurement QCMC’06, 31–34 (2006).

]. In Fig. 6(b), the cross points of central wavelengths of idler and signal are not all sat on the symmetry line, some of them are below the line, and the offset increases with the increase of the frequency difference Δ; while in Fig. 6(d), all the three data points sit on the symmetry line. The results agree with the calculated results: when the pump power decreases, although the frequency difference Δ0 decreases, the spectral range within which |F(ωs, ωi)|2 exhibits spectral symmetry broadens.

Fig. 6. The same as in Fig. 5, except λ p0=1541.7 nm, and the central wavelength of idler is fixed at 1534.88 nm, 1533.3 nm, and 1531.74 nm, respectively. In the experiment, the average pump power is about 0.4 mW (in (a) and (b)) and 0.2 mW (in (c) and (d)), respectively. The blue, purple and pink solid curves in (a) are fits to the Gaussian function f(λ)=2.3exp[(λ1548.471.41)2] , f(λ)=1.64exp[(λ1549.971.3)2] , and f(λ)=2.15exp[(λ1551.421.4)2] , respectively; the blue, purple and pink solid curves in (c) are fits to the Gaussian function f(λ)=3.92exp[(λ1548.640.84)2] , f(λ)=3.16exp[(λ1550.060.85)2] , and f(λ)=3.51exp[(λ1551.780.79)2] , respectively.

Comparing Fig. 6(a) with Fig. 5(a), obtained by using the pump with an average power of 0.4mW, one sees the FWHM in Fig. 6(a) is slighter wider than that in Fig. 5(a), this is because the broadening of the pump bandwidth due to GVD generally increases with the increasing deviation of λ p0 from λ 0. Comparing Fig. 6(b) with Fig. 5(b), one sees the spectrum of the photon pairs shown in Fig. 6(b) starts to become asymmetry when the idler (signal) photon is within the range from 1531.7nm to 1534.9nm (1551.5nm to 1548.5nm). The results agree with the theoretical prediction: when the deviation of λp from λ0 increases, Δ0 decreases; and when Δ is greater than Δ0, the symmetry direction of the function |F(ωs, ωi)|2 will shift away from that of the pump, and the offset increases with the increase of Δ, as shown in Fig. 2 and Fig. 3.

We would like to point out that above experiments are conducted at room temperature and the polarization beam splitters are not used to suppress the photons produced by cross-polarized FWM and Raman scattering (RS) [18

18. X. Li, J. Chen, P. L. Voss, J. Sharping, and P. Kumar, “All-fiber photon-pair source for quantum communications: Improved generation of correlated photons,” Opt. Express 12, 3737–3744 (2004). [CrossRef] [PubMed]

, 19

19. K. Inoue and K. Shimizu, “Generation of Quantum-Correlated Photon Pairs in Optical Fiber: Influence of Spontaneous Raman Scattering,” Jpn. J. Appl. Phys 43, 8048–8052 (2004). [CrossRef]

, 17

17. 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]

]. In our experiments, the detected photons via cross-polarizedFWMare negligibly small. We verify this by making a classical parametric-gain measurement. When the polarizations of the injected weak signal and the pump are orthogonal, at a certain pump level, the measured FWM gain is 20 dB less than that when the polarizations of the signal and pump are the same [18

18. X. Li, J. Chen, P. L. Voss, J. Sharping, and P. Kumar, “All-fiber photon-pair source for quantum communications: Improved generation of correlated photons,” Opt. Express 12, 3737–3744 (2004). [CrossRef] [PubMed]

]. Although RS degrades the quality of fiber-based photon pairs source [18

18. X. Li, J. Chen, P. L. Voss, J. Sharping, and P. Kumar, “All-fiber photon-pair source for quantum communications: Improved generation of correlated photons,” Opt. Express 12, 3737–3744 (2004). [CrossRef] [PubMed]

, 13

13. X. Li, P. L. Voss, J. E. Sharping, and P. Kumar, “Optical-fiber source of polarization-entangled photons in the 1550 nm telecom band,” Phys. Rev. Lett. 94, 053,601 (2005). [CrossRef]

], what we are interested in here is the true coincidence originated from spontaneous FWM, the imperfection contributed by RS is eliminated after the accidental coincidence is subtracted from the total coincidence.

4. Summary and discussion

In summary, we have characterized spectral properties of non-degenerate photon pairs in optical fiber generated by a picoseconds pump pulse from both the theoretical and experimental aspects. The theoretical calculations show that it is possible to properly manage the experimental parameters so that the phase matching function of FWM does not play a role in its corresponding joint spectral function. Using the photon correlation measurement, we have experimentally verified the spectral property of the fiber based two-photon state, and the experimental results agree with the calculation.

The spectral correlation between the signal and idler photons revealed in this study demonstrates the spectral entanglement between the two photons, which is useful in quantum information protocols such as quantum key distribution. However, this type of spectral correlation cannot be used for the generation of heralded single-mode single-photon state for the application in linear optical quantum computing [3

3. E. Knill, R. Laflamme, and G. J. Milburn, “A scheme for efficient quantum computation with linear optics,” Nature 409, 46–52 (2001). [CrossRef] [PubMed]

], which requires the factorization of the joint spectral function F(ωs, ωi) [10

10. Z. Y. Ou, “Parametric Down-Conversion with Coherent Pulse Pumping and Quantum Interference between Independent Fields,” Quantum Semiclass Opt. 9, 599–614 (1997). [CrossRef]

, 24

24. K. Garay-Palmett, H. J. McGuinness, O. Cohen, J. S. Lundeen, R. Rangel-Rojo, A. B. U’Ren, M. G. Raymer, C. J. McKinstrie, S. Radic, and I. A. Walmsley, “Photon pair-state preparation with tailored spectral properties by spontaneous four-wave mixing in photonic-crystal fiber,” Opt. Express 15, 14,870–14,886 (2007). [CrossRef]

, 27

27. Z. Y. Ou, J. K. Rhee, and L. J. Wang, “Photon Bunching and Multiphoton Interference in Parametric Down-Conversion,” Phys. Rev. A 60, 593–604 (1999). [CrossRef]

]. On the other hand, a close examination of the sincfunction in Eq.(11) leads to

ϕ(ωs,ωi)=sinc[kL8Δ2+γPpL+kL4Δ(ΩsΩi)+kL16Δ2(Ωs+Ωi)],
(16)

up to the first order in Ωsi(≪Δ). When we properly adjust the power and the second order dispersion k″ so that the first two terms cancel, the function becomes

ϕ(ωs,ωi)=sinc[A(ΩsΩi)+B(Ωs+Ωi)],
(17)
Fig. 7. Plot of joint spectral function |F(ωs, ωi)|2 with A/B=10 for (a) σp=3.3/A, (b) σp=0.37/A, and (c) σp=1.1/A.
Fig. 8. Plot of joint spectral function |F(ωs, ωi)|2 with σp=3.3/A for (a) A=B and (b) A=-B.

with AkLΔ/4,Bk′″LΔ2/16. Normally, this function shows a stripe with a slope of tanδ=(A+B)/(A-B) and this will not produce a factorized F(ωs, ωi). But an interesting case occurs when AB so that δ≈45°. Combining with the pump spectral function α(ωs, ωi), which lies in 135°, ϕ(ωs, ωi) may provide a factorized joint spectral function F(ωs, ωi) for proper value of the pump band width σp. As shown in Fig. 7, plots (a) and (b) show a non-factorized joint spectral function with too large or too small σp, but Fig. 7(c) shows a factorized F-function with σp=1.1/A. Furthermore when AB and σp≫1/|A|, an asymmetric but factorized F-function can be obtained as shown in Fig. 8. The ideas employed here are the same as those from Ref. [24

24. K. Garay-Palmett, H. J. McGuinness, O. Cohen, J. S. Lundeen, R. Rangel-Rojo, A. B. U’Ren, M. G. Raymer, C. J. McKinstrie, S. Radic, and I. A. Walmsley, “Photon pair-state preparation with tailored spectral properties by spontaneous four-wave mixing in photonic-crystal fiber,” Opt. Express 15, 14,870–14,886 (2007). [CrossRef]

] but the mechanism to achieve these are different: we use a near degenerate frequencies so that all linear dispersion terms are cancelled. We have to rely on higher order dispersion terms to achieve the conditions for factorization. However, for the DSF used in our current experiment, since the pump wavelength λ p0 is close to the zero-dispersion wavelength λ 0, we have |A|≪|B|, and therefore are unable to achieve the conditions mentioned above. But a micro-structured fiber will have all the freedom to adjust k″ and k″′ to achieve these.

Acknowledgement

This work was supported in part by NCET-060238, the National Basic Research Program of China (No. 2003CB314904), 111 Project B07014, the Key Project of Chinese Ministry of Education (No. 107027), and NSFC (No. 60578024).

References and links

1.

D. Bouwmeester, J. W. Pan, K. Mattle, M. Eibl, H. Weinfurter, and A. Zeilinger, “Experimental quantum teleportation,” Nature 390, 575–579 (1997). [CrossRef]

2.

J. W. Pan, D. Bouwmeester, H. Weinfurter, and A. Zeilinger, “Experimental entanglement swapping: entangling photons that never interacted,” Phys. Rev. Lett. 80, 3891–3894 (1998). [CrossRef]

3.

E. Knill, R. Laflamme, and G. J. Milburn, “A scheme for efficient quantum computation with linear optics,” Nature 409, 46–52 (2001). [CrossRef] [PubMed]

4.

Z. Zhao, Y. A. Chen, A. N. Zhang, T. Yang, H. J. Briegel, and J. W. Pan, “Experimental demonstration of five-photon entanglement and open-destination quantum teleportation,” Nature 430, 54–58 (2004). [CrossRef] [PubMed]

5.

B. Yurke and D. Stoler, “Einstein-Podolsky-Rosen effects from independent particle sources,” Phys. Rev. Lett. 68, 1251–1254 (1992). [CrossRef] [PubMed]

6 .

M. Zukowski, A. Zeilinger, and H. Weinfurter, “Entangling Photons Radiated by Independent Pulsed Sources,” Ann. (N. Y.) Acad. Sci. 755, 91–102 (1995). [CrossRef]

7 .

P. G. Kwiat, K. Mattle, H. Weinfurter, A. Zeilinger, A. V. Sergienko, and Y. H. Shih, “New high-intensity source of polarization-entangled photon pairs,” Phys. Rev. Lett. 75, 4337–4341 (1995). [CrossRef] [PubMed]

8.

Z. Y. Ou and Y. J. Lu, “Cavity Enhanced Spontaneous Parametric Down-Conversion for the Prolongation of Correlation Time between Conjugate Photons,” Phys. Rev. Lett. 83, 2556–2559 (1999). [CrossRef]

9.

M. Halder, A. Beveratos, N. Gisin, V. Scarani, C. Simon, and H. Zbinden, “Entangling independent photons by time measurement,” Nature Physics 3, 692–695 (2007). [CrossRef]

10.

Z. Y. Ou, “Parametric Down-Conversion with Coherent Pulse Pumping and Quantum Interference between Independent Fields,” Quantum Semiclass Opt. 9, 599–614 (1997). [CrossRef]

11.

W. P. Grice, A. B. U’ren, and I. A. Walmsley, “Eliminating frequency and space-time correlations in multi-Photon States,” Phys. Rev. A 64, 063,815 (2001). [CrossRef]

12.

M. Fiorentino, P. L. Voss, J. E. Sharping, and P. Kumar, “All-fiber photon-pair source for quantum communication,” Photon. Technol. Lett. 14, 983–985 (2002). [CrossRef]

13.

X. Li, P. L. Voss, J. E. Sharping, and P. Kumar, “Optical-fiber source of polarization-entangled photons in the 1550 nm telecom band,” Phys. Rev. Lett. 94, 053,601 (2005). [CrossRef]

14.

H. Takesue and K. Inoue, “Generation of 1.5-um band time-bin entanglement using spontaneous fiber four-wave mixing and planar lightwave circuit interferometers,” Phys. Rev. A 72, 041,804 (2005). [CrossRef]

15.

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]

16.

J. Fulconis, O. Alibart, W. J. Wadsworth, P. S. Russell, and J. G. Rarity, “High brightness single mode source of correlated photon pairs using a photonic crystal fiber,” Opt. Express 13, 7572–7582 (2005). [CrossRef] [PubMed]

17.

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]

18.

X. Li, J. Chen, P. L. Voss, J. Sharping, and P. Kumar, “All-fiber photon-pair source for quantum communications: Improved generation of correlated photons,” Opt. Express 12, 3737–3744 (2004). [CrossRef] [PubMed]

19.

K. Inoue and K. Shimizu, “Generation of Quantum-Correlated Photon Pairs in Optical Fiber: Influence of Spontaneous Raman Scattering,” Jpn. J. Appl. Phys 43, 8048–8052 (2004). [CrossRef]

20.

H. Takesue and K. Inoue, “1.5 um 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]

21.

J. G. Rarity, J. Fulconis, J. Duligall, W. J. Wadsworth, and P. S. J. Russell, “Photonic crystal fiber source of correlated photon pairs,” Opt. Express 13, 534–544 (2005). [CrossRef] [PubMed]

22.

J. Chen, X. Li, and P. Kumar, “Two-photon-state generation via four-wave mixing in optical fibers,” Phys. Rev. A 72, 033,801 (2005). [CrossRef]

23 .

O. Alibart, J. Fulconis, G. K. L. Wong, S. G. Murdoch, W. J. Wadsworth, and J. G. Rarity, “Photon pair generation using four-wave mixing in a microstructured fibre: theory versus experiment,” New J. of Phys. 8, 67 (2006). [CrossRef]

24.

K. Garay-Palmett, H. J. McGuinness, O. Cohen, J. S. Lundeen, R. Rangel-Rojo, A. B. U’Ren, M. G. Raymer, C. J. McKinstrie, S. Radic, and I. A. Walmsley, “Photon pair-state preparation with tailored spectral properties by spontaneous four-wave mixing in photonic-crystal fiber,” Opt. Express 15, 14,870–14,886 (2007). [CrossRef]

25.

G. P. Agrawal, Nonlinear fiber optics (Elsevier Pte Ltd., Singapore, 2005).

26.

X. Li, J. Chen, K. F. Lee, P. L. Voss, and P. Kumar, “All-fiber photon-pair source for quantum communication: Influence of spectra,” Proceeding of Quantum Communication and Measurement QCMC’06, 31–34 (2006).

27.

Z. Y. Ou, J. K. Rhee, and L. J. Wang, “Photon Bunching and Multiphoton Interference in Parametric Down-Conversion,” Phys. Rev. A 60, 593–604 (1999). [CrossRef]

OCIS Codes
(190.4370) Nonlinear optics : Nonlinear optics, fibers
(190.4410) Nonlinear optics : Nonlinear optics, parametric processes
(270.0270) Quantum optics : Quantum optics

ToC Category:
Quantum Optics

History
Original Manuscript: October 22, 2007
Revised Manuscript: December 3, 2007
Manuscript Accepted: December 14, 2007
Published: December 21, 2007

Citation
Xiaoying Li, Xiaoxin Ma, Zhe Yu Ou, Lei Yang, Liang Cui, and Daoyin Yu, "Spectral study of photon pairs generated in dispersion shifted fiber with a pulsed pump," Opt. Express 16, 32-44 (2008)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-16-1-32


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References

  1. D. Bouwmeester, J. W. Pan, K. Mattle, M. Eibl, H. Weinfurter, and A. Zeilinger, "Experimental quantum teleportation," Nature 390, 575-579 (1997). [CrossRef]
  2. J. W. Pan, D. Bouwmeester, H. Weinfurter, and A. Zeilinger, "Experimental entanglement swapping: entangling photons that never interacted," Phys. Rev. Lett. 80, 3891-3894 (1998). [CrossRef]
  3. E. Knill, R. Laflamme, and G. J. Milburn, "A scheme for efficient quantum computation with linear optics," Nature 409, 46-52 (2001). [CrossRef] [PubMed]
  4. Z. Zhao, Y. A. Chen, A. N. Zhang, T. Yang, H. J. Briegel, and J. W. Pan, "Experimental demonstration of fivephoton entanglement and open-destination quantum teleportation," Nature 430, 54-58 (2004). [CrossRef] [PubMed]
  5. B. Yurke and D. Stoler, "Einstein-Podolsky-Rosen effects from independent particle sources," Phys. Rev. Lett. 68, 1251-1254 (1992). [CrossRef] [PubMed]
  6. M. Zukowski, A. Zeilinger, and H. Weinfurter, "Entangling Photons radiated by independent pulsed sources," Ann. Acad. Sci. (NY) 755, 91-102 (1995). [CrossRef]
  7. P. G. Kwiat, K. Mattle, H. Weinfurter, A. Zeilinger, A. V. Sergienko, and Y. H. Shih, "New high-intensity source of polarization-entangled photon pairs," Phys. Rev. Lett. 75, 4337-4341 (1995). [CrossRef] [PubMed]
  8. Z. Y. Ou and Y. J. Lu, "Cavity enhanced spontaneous parametric down-conversion for the prolongation of correlation time between conjugate photons," Phys. Rev. Lett. 83, 2556-2559 (1999). [CrossRef]
  9. M. Halder, A. Beveratos, N. Gisin, V. Scarani, C. Simon, and H. Zbinden, "Entangling independent photons by time measurement," Nat. Phys. 3, 692-695 (2007). [CrossRef]
  10. Q3. Z. Y. Ou, "Parametric down-conversion with coherent pulse pumping and quantum interference between independent fields," Quantum Semiclass Opt. 9, 599-614 (1997). [CrossRef]
  11. W. P. Grice, A. B. U’ren, and I. A.Walmsley, "Eliminating frequency and space-time correlations in multi-photon states," Phys. Rev. A 64, 063,815 (2001). [CrossRef]
  12. M. Fiorentino, P. L. Voss, J. E. Sharping, and P. Kumar, "All-fiber photon-pair source for quantum communication," Photon. Technol. Lett. 14, 983-985 (2002). [CrossRef]
  13. X. Li, P. L. Voss, J. E. Sharping, and P. Kumar, "Optical-fiber source of polarization-entangled photons in the 1550 nm telecom band," Phys. Rev. Lett. 94, 053,601 (2005). [CrossRef]
  14. H. Takesue and K. Inoue, "Generation of 1.5-um band time-bin entanglement using spontaneous fiber four-wave mixing and planar lightwave circuit interferometers," Phys. Rev. A 72, 041,804 (2005). [CrossRef]
  15. 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]
  16. J. Fulconis, O. Alibart, W. J. Wadsworth, P. S. Russell, and J. G. Rarity, "High brightness single mode source of correlated photon pairs using a photonic crystal fiber," Opt. Express 13, 7572-7582 (2005). [CrossRef] [PubMed]
  17. 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]
  18. X. Li, J. Chen, P. L. Voss, J. Sharping, and P. Kumar, "All-fiber photon-pair source for quantum communications: Improved generation of correlated photons," Opt. Express 12, 3737-3744 (2004). [CrossRef] [PubMed]
  19. K. Inoue and K. Shimizu, "Generation of quantum-correlated photon pairs in optical fiber: Influence of spontaneous Raman Scattering," Jpn. J. Appl. Phys 43, 8048-8052 (2004). [CrossRef]
  20. H. Takesue and K. Inoue, "1.5 um 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]
  21. J. G. Rarity, J. Fulconis, J. Duligall, W. J. Wadsworth, and P. S. J. Russell, "Photonic crystal fiber source of correlated photon pairs," Opt. Express 13, 534-544 (2005). [CrossRef] [PubMed]
  22. J. Chen, X. Li, and P. Kumar, "Two-photon-state generation via four-wave mixing in optical fibers," Phys. Rev. A 72, 033,801 (2005). [CrossRef]
  23. O. Alibart, J. Fulconis, G. K. L. Wong, S. G. Murdoch,W. J. Wadsworth, and J. G. Rarity, "Photon pair generation using four-wave mixing in a microstructured fibre: theory versus experiment," New J. of Phys. 8, 67 (2006). [CrossRef]
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