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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Spectral Hong-Ou-Mandel Interference Between Independently Generated Single Photons for Scalable Frequency-Domain Quantum Processing

Anahita Khodadad Kashi, Michael Kues

Published Jun 21, 2021• 0 citations
Publisher

Abstract

The frequency degree-of-freedom, being compatible with well-developed omnipresent optical telecommunication infrastructure, allows for stable, controllable and scalable encoding of quantum information [1] . The HOM effect between independent photons, as a central building block to, e.g., quantum key distribution (QKD), quantum teleportation and entanglement swapping, has been mainly employed within the spatial domain [2] . A key element yet missing to enable scalable and thus real-world frequency-based implementation of quantum computation and non-classical communication, is the demonstration of Hong-Ou-Mandel (HOM) interference between independently created photons of different frequencies. To date, spectral bosonic HOM interference between single photons, produced out of the same continuous-wave (cw)-excited spontaneous parametric down conversion (SPDC) process has been demonstrated [3] , which however does not allow scalability due to the spectral impurity inherent to the proposed approach.

DOI
10.1109/cleo/europe-eqec52157.2021.9542329

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The frequency degree-of-freedom, being compatible with well-developed omnipresent optical telecommunication infrastructure, allows for stable, controllable and scalable encoding of quantum information [1] . The HOM effect between independent photons, as a central building block to, e.g., quantum key distribution (QKD), quantum teleportation and entanglement swapping, has been mainly employed within the spatial domain [2] . A key element yet missing to enable scalable and thus real-world frequency-based implementation of quantum computation and non-classical communication, is the demonstration of Hong-Ou-Mandel (HOM) interference between independently created photons of different frequencies. To date, spectral bosonic HOM interference between single photons, produced out of the same continuous-wave (cw)-excited spontaneous parametric down conversion (SPDC) process has been demonstrated [3] , which however does not allow scalability due to the spectral impurity inherent to the proposed approach.
Abstract The photon's frequency degree of freedom, being compatible with mature telecom infrastructure, offers large potential for the stable and controllable realization of photonic quantum processing applications such as the quantum internet. The Hong–Ou–Mandel effect, as a two‐photon interference phenomenon, serves as a central building block for such frameworks. A key element yet missing to enable meaningful frequency‐based implementations as well as scalability in the number of processed photons, is the demonstration of the Hong–Ou–Mandel effect between independently created photons of different frequencies. The experimental implementation of bosonic and fermionic frequency domain Hong–Ou–Mandel interference between independently generated single photons is reported here, with measured visibilities of 74.31% ± 3.56% and 86.44% ± 8.27%, respectively. This is achieved through a scalable photonic frequency circuit that creates two post‐selected pure single photons, which undergo frequency mixing at an electro‐optic phase modulator. The system is on‐the‐fly reconfigurable allowing to probe bosonic and fermionic Hong–Ou–Mandel interference in the same experimental setup. The work demonstrates the versatility of frequency domain processing and its scalability toward higher photon numbers, which enables new quantum gate concepts as well as the establishment of frequency‐based large‐scale quantum networks.
Interference between two completely independent photons lies at the heart of many photonic quantum information applications such as quantum repeaters, teleportation, and quantum key distribution. Here, we report the observation of Hong–Ou–Mandel (HOM) interference with two independent continuous-wave coherent light sources that are neither synchronized nor share any common reference. To prepare indistinguishable photons from two independent laser sources, we employ high-precision frequency-stabilization techniques using the 5 S 1 / 2 ( F = 3 ) − 5 P 1 / 2 ( F ′ = 3 ) transition line of Rb 85 atoms. We successfully observe an HOM interference fringe with two independent continuous-wave coherent photons originating from either the frequency-locked or the frequency-modulated lasers. An interference fringe involving two-photon beating is also observed when the frequency difference between the two interfering photons is beyond the spectral bandwidth of the individual coherent photons. We carry out further experiments to verify the robustness of the source preparation regardless of the separation distance between the two independent photon sources.
The Hong-Ou-Mandel (HOM) interference is one of the most fundamental quantum-mechanical effects that reveal a nonclassical behavior of single photons. Two identical photons that are incident on the input ports of an unbiased beam splitter always exit the beam splitter together from the same output port, an effect referred to as photon bunching. In this Letter, we utilize a single electro-optic phase modulator as a probabilistic frequency beam splitter, which we exploit to observe HOM interference between two photons that are in different spectral modes, yet are identical in other characteristics. Our approach enables linear optical quantum information processing protocols using the frequency degree of freedom in photons such as quantum computing techniques with linear optics.
Entanglement swapping enables the distribution of quantum states over long distances. In this protocol, a Bell state measurement projects photons, which were created in separate entangled photon sources, into an entangled state when their corresponding pairs interfere at a beamsplitter. This process is optimally realized when the photons are indistinguishable in all degrees of freedom, resulting in Hong-Ou-Mandel (HOM) interference. Since its initial proposal by [1], several experimental realizations have used pulsed, synchronized pumps, as in [2] to achieve temporal indistiguishability. In the case of continuous wave (CW) pumping, the sources are not synchronized, requiring post-selection of the coincidence events after detection. This requires detectors with a temporal resolution greater than the coherence time of the photons. Previous experiments have successfully observed HOM interference using periodically poled Lithium Niobate (ppLN) waveguides and superconducting nanowire single-photon detectors (SNSPDs) [3], also with hybrid sources to improve coincidence rates and HOM visibility [4], [5].