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Review of Scientific Instruments

A method for balancing the paths of a two-photon interferometer

Scott E. Spence, D.C. Stoudt, Nancy L. Swanson, Allen Parks, Frank E. Peterkin

Published Jan 1, 2000• 9 citations
Publisher

Abstract

We present and demonstrate a method for balancing the two paths of a two-photon interferometer of the Hong-Ou-Mandel (HOM) variety. This method removes the difficulties of balancing the paths in this type of interferometer where a type-I crystal is used for the generation of the entangled photon pairs. Using this method we show that we are consistently able to balance the ∼2000 mm path lengths in our HOM interferometer on average to within ∼60 μm.

DOI
10.1063/1.1150154

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arXiv:1605.07428•Physical Review A•Sep 28, 2016•33 citations
We present and demonstrate a method for balancing the two paths of a two-photon interferometer of the Hong-Ou-Mandel (HOM) variety. This method removes the difficulties of balancing the paths in this type of interferometer where a type-I crystal is used for the generation of the entangled photon pairs. Using this method we show that we are consistently able to balance the ∼2000 mm path lengths in our HOM interferometer on average to within ∼60 μm.
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.
Quantum interference forces two indistinguishable input photons to depart a beamsplitter in the same (of two possible) spatial output modes; this is known as the Hong-Ou-Mandel (HOM) effect. It provides a quantitative way of measuring the distinguishability between two photons and also has been utilised to determine their relative temporal delays [1, 2]. HOM interferometry offers numerous advantages over its classical counterpart, most importantly it lacks any dependence on the phase of the photons and therefore can potentially provide reliable measurements in environments where the phase is unstable. To date there have been very few investigations on the precision that is achievable with HOM interferometry. Experiments with collinear geometries have yielded sensitivities as low as 0.1fs however these approaches are limited to measurements of birefringence and other polarisation dependent effects [3, 4], as the precision derives from the inherently stable shared path of the photon pairs. Typical non-shared path HOM interferometers exhibit resolution of roughly 1–3 fs (0.3–1 μm).
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.
Two-photon interference effects arise because photons are indistinguishable particles. In the wellknown Hong-Ou-Mandel (HOM) effect, the transmission of two photons at a beam splitter interferes destructively with the reflection of both photons, requiring both photons to "bunch up" by leaving the beam splitter on the same side. Here, we show that the interference between locally propagating photons and photons exchanged by a mode swap can be implemented by post-selecting spatially separated photon outputs of a four-path interferometer. Even though the photons detected at spatially separated locations must have travelled along paths that never met up at the same beam splitter, the Hong-Ou-Mandel effect can be observed in correlations between the output ports that originate from the association of detection events with non-local output modes defined by the two single photon inputs. Local phase shifts can be used to map out non-classical correlations between the photons detected at different output locations, clarifying the role of linear optics in generating entanglement between spatially separated photons. Our work thus establishes a fundamental relation between multiphoton interference and entanglement, opening the door to new possibilities in optical quantum technologies.
Hong-Ou-Mandel (HOM) interference is demonstrated experimentally for entangled photon pairs in the Hermite-Gauss (HG) basis. We use two Dove prisms in one of the paths of the photons to manipulate the entangled quantum state that enters the HOM interferometer. It is demonstrated that, when entangled photon pairs are in a symmetric Bell state in the Laguerre-Gauss (LG) basis, they will remain symmetric after decomposing them into the HG basis, thereby resulting in no coincidence events after the HOM interference. On the other hand, if the photon pairs are in an antisymmetric Bell state in the LG basis, then they will also be antisymmetric in the HG basis, thereby producing only coincidence events as a result of the HOM interference.