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Review of Scientific Instruments

The Hong–Ou–Mandel interferometer: A new procedure for alignment

Peter James Thomas, Jessica Y. Cheung, Christopher J. Chunnilall, Malcolm H. Dunn

Published Mar 1, 2009• 6 citations
Publisher

Abstract

Two major requirements in the construction of the Hong-Ou-Mandel interferometer are the alignment and length balancing of two optical paths. A new method is presented for meeting these requirements that requires no custom optics or expensive equipment. Using this method, a two photon interferometer sourced by degenerate noncollinear parametric photon pairs was aligned and the optical paths were balanced to within an average of 11.6 microm, yielding two-photon interference features with visibilities of approximately 0.9. The method is applicable to arbitrary noncollinear emission angles, including nondegenerate downconversion situations where the signal and idler emission angles differ.

DOI
10.1063/1.3080559

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The European Physical Journal Special Topics•Jun 5, 2026•1 citation
Two major requirements in the construction of the Hong-Ou-Mandel interferometer are the alignment and length balancing of two optical paths. A new method is presented for meeting these requirements that requires no custom optics or expensive equipment. Using this method, a two photon interferometer sourced by degenerate noncollinear parametric photon pairs was aligned and the optical paths were balanced to within an average of 11.6 microm, yielding two-photon interference features with visibilities of approximately 0.9. The method is applicable to arbitrary noncollinear emission angles, including nondegenerate downconversion situations where the signal and idler emission angles differ.
The critical design aspects of a free space Hong-Ou-Mandel interferometer illuminated with pair photons from a non-collinearly phasematched spontaneous parametric downconversion process are discussed. In particular the efficient generation and collection of pair photons at a specified wavelength and a procedure for constructing the interferometer are presented. Two major requirements in the construction are the alignment and length-balancing of two optical paths, which were achieved using a new method incorporating long and short coherence length light sources, and requiring no custom optics or expensive equipment. The method balanced the optical paths to within an average of 11.6 mm, considerably shortening set-up time and two-photon interference features with visibilities ~ 0.9 were produced. The importance of interferometer characterization is discussed in the context of the application of Hong-Ou-Mandel interferometry to single photon source evaluation.
A method is presented for the separation of polarization-entangled, frequency-degenerate photon pairs based on inverse Hong-Ou-Mandel interference. Adjusting the phase between photon states is achieved by exploiting the wavelength dispersion of optical glass, for which process a complete mathematical description is given. To experimentally demonstrate the phenomenon, a custom photon pair source has been built using spontaneous parametric downconversion of type-0 phase matching and a Sagnac interferometer. The effect of phase tuning was characterized by the modulation of coincidence probability. The visibility of quantum interference was found to be 0.820 and 0.720 for photons being in purely horizontal or vertical polarization state. In case of polarization entanglement, the measured state fidelity with respect to the $|{\mathrm{\ensuremath{\Phi}}}^{\ensuremath{-}}\ensuremath{\rangle}$ Bell state was 0.914.
Sources of entanglement are an enabling resource in quantum technology, and pushing the limits of generation rate and quality of entanglement is a necessary prerequisite towards practical applications. Here, we present an ultrabright source of polarization-entangled photon pairs based on time-reversed Hong-Ou-Mandel interference. By superimposing four pair-creation possibilities on a polarization beam splitter, pairs of identical photons are separated into two spatial modes without the usual requirement for wavelength distinguishability or noncollinear emission angles. Our source yields high-fidelity polarization entanglement and high pair-generation rates without any requirement for active interferometric stabilization, which makes it an ideal candidate for a variety of applications, in particular those requiring indistinguishable photons.
We review some recent experiments based upon multimode two-photon interference of photon pairs created by spontaneous parametric down-conversion. The new element provided by these experiments is the inclusion of the transverse spatial profiles of the pump, signal and idler fields. We discuss multimode Hong–Ou–Mandel interference, and show that the transverse profile of the pump beam can be manipulated in order to control two-photon interference. We present the basic theory and experimental results as well as several applications to the field of quantum information.
Abstract Hong–Ou–Mandel (HOM) dip from a biphoton source in a two-photon interferometer provides a myriad of quantum tools for quantum communication and sensing. But the stringent requirements for spatial coherence between the photon pair makes it prohibitively difficult to observe high-fidelity HOM dip in long-distance free-space implementations, e.g., for the photon pairs involved in quantum communication need to match the two path lengths within a few 10 s of micron because of the short coherence width of the two-photon wave-packet. While many techniques for the pathlength balancing of two interferometric arms have been studied and applied extensively, such balancing is further complicated in presence of turbulence in long-distance free-space propagation. On a table-top testbed with each arm of a Mach–Zehnder interferometer (MZI) ∼10\sim 10∼10 ∼ 10 -meter long in our turbulent atmospheric simulator for qubit (TASQ), we have employed below-threshold lasers to achieve classical low-coherence interferometry and show that we could narrow down the interrogation window for the pathlength balancing to obtain interference peak at the center. This method allowed us to reduce the interrogation distance for searching the HOM dip by four orders of magnitude, i.e., from tens of centimeters to a few tens of micron. Such pathlength balancing allowed realizing the HOM dip within $$160\,\upmu \textrm{m}$$ 160 μ m distance from the peak of the classical interference fringe. This technique could be helpful in a wide variety of long-pathlength interference-based quantum sensing.