Two-photon joint temporal density measurements via
ultrafast single-photon upconversion
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Kuzucu, O. et al. “Two-photon joint temporal density
measurements via ultrafast single-photon upconversion.” Lasers
and Electro-Optics, 2009 and 2009 Conference on Quantum
electronics and Laser Science Conference. CLEO/QELS 2009.
Conference on. 2009. 1-2. ©2009 Institute of Electrical and
Electronics Engineers.
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© 2009 OSA/CLEO/IQEC 2009
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IThJ6.pdf
IThJ6.pdf
Two-photon joint temporal density measurements via
ultrafast single-photon upconversion
Onur Kuzucu and Franco N.C. Wong
Research Laboratory of Electronics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139
email: onur@alum.mit.edu
Sunao Kurimura and Sergey Tovstonog
National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba-shi, Ibaraki 305-0044, Japan
Abstract: We have developed the technique of two-photon joint temporal density measurements
for temporal state characterization, thus facilitating two-photon generation with high temporal
entanglement or nearly factorizable outputs by controlling the ultrafast pump bandwidth.
”2008 Optical Society of America
OCIS codes: (270.5585) Quantum information and processing; (190.7220) Upconversion; (270.5290) Photon Statistics.
Ultrafast-pumped spontaneous parametric downconversion (SPDC) is a reliable technique to generate two-photon
states with precise timing for quantum information processing (QIP) applications. The ultrafast SPDC-generated
signal and idler photon pairs are typically highly entangled in frequency and time, and one can employ spectral (or
temporal) engineering to tailor the ultrafast SPDC source for specific quantum information processing applications.
For instance, positive frequency-entangled photon pairs can be utilized to overcome the standard quantum limit for
time-of-flight measurements [1]. On the other hand, completely unentangled photon pairs are ideally suited for
generating ancilla photons for linear optics quantum computation [2]. A common method for characterizing
frequency entanglement involves a measurement of the joint spectral density (JSD) of the two-photon state by
coincident detection of the narrowband-filtered signal and idler [2]. However, JSD measurements do not yield the
full picture of the two-photon state unless it is transform limited. A time-domain method is useful to capture the
temporal correlations between signal and idler photons. In this paper we present the first joint temporal density
(JTD) measurements of a two-photon state via ultrafast single-photon frequency upconversion. The method enables
us to manipulate the two-photon temporal correlations by varying the SPDC pump bandwidth and modifying the
JTD distribution to yield outputs ranging from highly-entangled to almost unentangled photons [3].
(a)
(b)
Fig. 1. (a) Sketch of downconversion and upconversion setups driven by the same ultrafast pump. Fiber coupled signal
and idler are launched into a 1-mm PPMgSLT crystal at an angle as shown in the close-up diagram. IF, interference filter;
PBS, polarizing beam splitter; FPBS, fiber PBS; DM, dichroic mirror; HWP, half-wave plate; BPF, band-pass filter. (b)
Normalized singles and coincidence histograms by time-resolved upconversion. The pump pulse was scanned through
collocated signal and idler arrival windows. The sharp coincidence peak as the center (165 fs FWHM) was a consequence
of the temporally anti-correlated two-photon state.
For time-resolved detection of signal and idler arrivals we applied our recently developed ultrafast single-photon
upconversion technique with a ~150 fs temporal resolution [4]. The pump was an ultrafast Ti:sapphire laser (150-fs
pulses, 80-MHz repetition rate, 6-nm bandwidth at 790 nm) for synchronous downconversion and time-resolved
upconversion as shown in Fig. 1(a). Type-II frequency-degenerate SPDC at 1580 nm in a 1-cm long periodically
poled KTiOPO4 (PPKTP) crystal ( = 46.1 μm) generated positive frequency-entangled photon pairs under
extended phase-matching conditions [5]. For a 6-nm pump bandwidth, the single-photon and two-photon coherence
times for signal and idler were previously measured to be 350 fs and 1.4 ps, respectively, indicating a high temporal
and spectral entanglement [5]. The orthogonally polarized signal and idler were filtered with a 25-nm interference
filter and coupled into a polarization maintaining (PM) fiber and then separated at a fiber polarizing beam splitter.
The total PM fiber length was ~55 cm to ensure low dispersion and synchronous arrival with the upconverting pump
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© 2009 OSA/CLEO/IQEC 2009
a685_1.pdf
IThJ6.pdf
IThJ6.pdf
pulse at the 1-mm long periodically poled MgO-doped stoichiometric LiTaO4 (PPMgSLT) crystal ( = 8.5 μm),
designed for noncollinear type-0 sum-frequency generation at 526.7 nm. We controlled the arrival times of signal
and idler photons with respect to the pump pulse with separate translation stages for the signal and idler beam
collimation setups and for the upconversion pump delay line. The upconverted single-photon outputs were filtered
with 10-nm interference filters at 530 nm and subsequently coupled into single-mode fibers for detection with two
PerkinElmer Si avalanche photodetectors. We aligned the three interacting beams at the PPMgSLT crystal in a nonplanar geometry that provided background-free coincidence measurements of the upconverted signal and idler
within a 1.8-ns coincidence window. Figure 1(b) shows the observed singles and coincidence profiles as the
upconverting pump pulse was swept through the collocated signal and idler arrival windows without background
subtraction. The peak singles (coincidence) rate was 5300/s (17/s) including the constant fluorescence background
contribution of 1900/s. Using the full pump bandwidth, the singles (coincidence) width of 1.3 ps (165 fs) was
consistent with previous frequency domain characterization of the two-photon state, confirming the state was highly
time anticorrelated.
For JTD measurements, we varied the signal and idler delay stages independently while keeping the
upconversion pump delay constant. In addition, through a set of interference filters, we varied the downconversion
pump bandwidth (but not the upconversion pump) to modify the JTD distributions. From the coincidence
measurements we plot a normalized surface plot over a two-dimensional measurement grid, 2 × 2 ps with 133 fs
delay steps for each channel in Figs. 2(a)-(c), and 4 × 4 ps with 266 fs increments in Fig. 2(d). Each data point was
averaged over a 60-s measurement interval. With the full pump bandwidth, the two-dimensional coincidence profile
exhibited clear time anti-correlation, thus verifying the coincident-frequency entanglement in the time domain. As
the pump bandwidth was reduced, the resulting JTD profiles became more symmetric with reduced entanglement.
Fig.2. Experimental joint temporal densities as normalized surface and contour plots over the measurement grid for various
3-dB downconversion pump bandwidths: (a) 6 nm, (b) 3.6 nm, (c) 2.1 nm, (d) 1.1 nm. Reduced pump bandwidth produces
a more symmetric distribution yielding a less entangled two-photon state.
We quantified the temporal entanglement for various pump bandwidths via Schmidt decomposition [6]. Using
the experimentally obtained JTD profiles for the near transform-limited output state, we calculated the purity of the
heralded single-photon state for 6-nm downconversion pump bandwidth to be 0.38, whereas for 1.1 nm SPDC pump
bandwidth the purity increased to 0.88, indicating a nearly unentangled joint state. Further increase in purity should
be possible with enhanced control over the pump spectrum and filter bandwidth.
In summary, the time-resolved single-photon upconversion and subsequent JTD measurement technique enabled
us to verify temporal entanglement and facilitated in manipulating the entanglement to obtain a nearly factorizable
two-photon state. This technique complements existing frequency domain methods for enhanced characterization of
ultrafast-pumped SPDC sources. This work was partially supported by the National Institute of Information and
Communications Technology, Japan.
References
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Single Photons in Pure Quantum States”, Phys. Rev. Lett. 100, 133601 (2008).
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Phys. Rev. Lett. 101, 153602 (2008).
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