Spectral correlation measurements at the Hong-Ou-Mandel interference dip

T. Gerrits, F. Marsili, V. B. Verma, L. K. Shalm, M. Shaw, R. P. Mirin, and S. W. Nam

Phys. Rev. A 91, 013830 – Published 22 January 2015

Abstract

We present an efficient tool capable of measuring the spectral correlations between photons emerging from a Hong-Ou-Mandel interferometer. We show that for our spectrally factorizable spontaneous down-conversion source, the Hong-Ou-Mandel interference visibility decreases as the photons' frequency spread is increased to a maximum of 165 nm. Unfiltered, we obtained a visibility of $92.0 % \pm 0.2 % .$ The maximum visibility was $97 % \pm 0.2 %$ after applying filtering. We show that the tool can be useful for the study of spectral correlations that impair high-visibility and high-fidelity multisource interference applications. The nature of this tool also allows for arbitrary post-selective spectral filtering and high-rate multiphoton spectral correlation measurements.

Received 4 September 2014

DOI:https://doi.org/10.1103/PhysRevA.91.013830

Authors & Affiliations

T. Gerrits^1,*, F. Marsili², V. B. Verma¹, L. K. Shalm¹, M. Shaw², R. P. Mirin¹, and S. W. Nam¹

¹National Institute of Standards and Technology, 325 Broadway, Boulder, Colorado 80305, USA
²Jet Propulsion Laboratory, 4800 Oak Grove Dr., Pasadena, California 91109, USA

^*thomas.gerrits@nist.gov

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Vol. 91, Iss. 1 — January 2015

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Images

Figure 1
(a) Experimental setup: A femtosecond pulse, pumps a pp-KTP crystal producing type-II SPDC. PBS1 and PBS2 are sandwiched around an optical path delay $Δ t$ for the $H$ (idler) photon. A HWP at angle $ϕ$ and PBS3 serve as a variable beam splitter for both input modes. Two long single-mode fibers are used to encode the photons' frequency into time of arrival. Four SNSPDs probe various spectral correlations of the SPDC after the variable beam splitter. Time-stamping electronics record the arrival time of each photon; fast pd: fast photodiode (trigger). (b) Instrument response functions of all four SNSPDs. (c) Measured fiber dispersion curves for both single-mode fibers.
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Figure 2
Joint probability distribution, integrated for 2 min (a) as measured, log-scale plot of the raw joint time-of-arrival probability distribution, (b) log-scale plot of the joint-spectral intensity (JSI), (c) absolute value of the joint-spectral amplitude (JSA) distribution.
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Figure 3
(a) HOM interference dip; blue crosses: integral of the correlation spectra as a function of HOM delay; black dotted line: first-principles calculation of the HOM interference dip based on Eq. (1); open diamonds/red line: HOM interference dip prediction based on the simplified version of Eq. (1) and the measured |JSA|; green solid and black dashed-dotted lines: bunching of two-photon component at either output port of the beam splitter; (b)–(f) correlation spectra at various HOM delays; each inset consists of four spectra. The scale is $\pm 2 THz$ for both axes; upper left (right): two-photon component, i.e., bunching, in output mode 1 (2), lower left: measured spectral correlation of HOM interference, lower right: spectral correlation of HOM interference based on Eq. (1) and the measured |JSA|. Bracket insets denote the detector channel numbers used for the measurement. Color bar on the right denotes the relative intensity color scale; (g) correlation spectrum at zero time delay showing remaining correlations [same as bottom left of Fig. 3 but with different color scaling].
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Figure 4
(a) Red dashed line: measured HOM dip visibility versus virtual filter bandwidth, based on a variable bandwidth top-hat post-processing filtering method. Black solid line: first-principles calculation assuming asymmetric mode fiber coupling; inset: calculation of maximum HOM visibility as function of off-optical-axis collection angle $θ$ ; (b) schematic of the asymmetric mode coupling.
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