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IEEE International Conference on Quantum Computing and Engineering (QCE)

Linear Polarization-Based Entanglement of a Single Photon, 2-Qubit Spatial Mode System

Neha Chandran, Srimaye Peddinti

Published Sep 15, 2024• 0 citations
Publisher

Abstract

Quantum communications offers a revolutionary solution to future data security challenges. Photons, the building blocks of quantum communications, encode quantum information, such as polarization and momentum. We investigate the enhancement of communications protocols by treating the spatial modes of a single polarization-encoded photon as two separate qubits. In effect, we can recreate traditional 2-photon operations in an individual photon, improving the scability of quantum devices. To demonstrate the robustness of our approach, we created path entanglement between the spatial mode qubits in the polarization basis. By introducing a linear approach, we avoid low conversion efficiencies and spectral limitations associated with nonlinear processes like spontaneous parametric down conversion (SPDC). We explore extending the concept of two-photon Hong-Ou-Mandel (HOM) interference to demonstrate instinguishability between two spatial modes. Using the Python-based photonic quantum computing library Perceval, we leveraged a single photon source as the input, combined with beam splitters (BS), waveplates (WP), and polarizing beam splitters (PBS). The HOM effect is observed when both spatial modes bunch together at the port to form a photon coincidence. We used Perceval's Strong Linear Optical (SLOS) simulator to run 10,000 shots and measured equal probabilities at all four detectors, confirming consistent indistinguishability of the modes when the HWP is rotated$\pi/4$radians. Finally by parameterizing the equivalent circuit in Qiskit with angles from$[0,2\pi]$, we showed a clear violation of the Clauser-Horne-Shimony-Holt (CHSH) inequality, more definitively indicating successful entanglement.

DOI
10.1109/qce60285.2024.10426

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Quantum communications offers a revolutionary solution to future data security challenges. Photons, the building blocks of quantum communications, encode quantum information, such as polarization and momentum. We investigate the enhancement of communications protocols by treating the spatial modes of a single polarization-encoded photon as two separate qubits. In effect, we can recreate traditional 2-photon operations in an individual photon, improving the scability of quantum devices. To demonstrate the robustness of our approach, we created path entanglement between the spatial mode qubits in the polarization basis. By introducing a linear approach, we avoid low conversion efficiencies and spectral limitations associated with nonlinear processes like spontaneous parametric down conversion (SPDC). We explore extending the concept of two-photon Hong-Ou-Mandel (HOM) interference to demonstrate instinguishability between two spatial modes. Using the Python-based photonic quantum computing library Perceval, we leveraged a single photon source as the input, combined with beam splitters (BS), waveplates (WP), and polarizing beam splitters (PBS). The HOM effect is observed when both spatial modes bunch together at the port to form a photon coincidence. We used Perceval's Strong Linear Optical (SLOS) simulator to run 10,000 shots and measured equal probabilities at all four detectors, confirming consistent indistinguishability of the modes when the HWP is rotatedπ/4\pi/4π/4radians. Finally by parameterizing the equivalent circuit in Qiskit with angles from[0,2π][0,2\pi][0,2π], we showed a clear violation of the Clauser-Horne-Shimony-Holt (CHSH) inequality, more definitively indicating successful entanglement.
The information-carrying capacity of a single photon can be vastly expanded by exploiting its multiple degrees of freedom: spatial, temporal, and polarization. Although multiple qubits can be encoded per photon, to date only two-qubit single-photon quantum operations have been realized. Here, we report an experimental demonstration of three-qubit single-photon, linear, deterministic quantum gates that exploit photon polarization and the two-dimensional spatial-parity-symmetry of the transverse single-photon field. These gates are implemented using a polarization-sensitive spatial light modulator that provides a robust, non-interferometric, versatile platform for implementing controlled unitary gates. Polarization here represents the control qubit for either separable or entangling unitary operations on the two spatial-parity target qubits. Such gates help generate maximally entangled three-qubit Greenberger-Horne-Zeilinger and W states, which is confirmed by tomographical reconstruction of single-photon density matrices. This strategy provides access to a wide range of three-qubit states and operations for use in few-qubit quantum information processing protocols.Photons are essential for quantum information processing, but to date only two-qubit single-photon operations have been realized. Here the authors demonstrate experimentally a three-qubit single-photon linear deterministic quantum gate by exploiting polarization along with spatial-parity symmetry.
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.
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.
We report an experimental implementation of deterministic preparation and measurement of single-photon two-qubit entangled states, in which the polarization and the spatial modes of a single-photon each represent a quantum bit. It is demonstrated that all four single-photon two-qubit entangled states (Bell states) can be easily prepared and measured deterministically using linear optical elements alone. We also discuss how this scheme can be used for implementing recently proposed single-photon two-qubit quantum cryptography.
Quantum photonic platforms have proven to be essential in realizing fundamentally secure quantum transfer of information, with commercially ready systems already deployed in municipal and terrestrial links. The drive toward higher bit rates and robustness to eavesdropping and noisy channels has focused attention on moving from the present two-dimensional quantum states of polarization, to harnessing all of light's degrees of freedom for multi-dimensional quantum coding with structured photons. In this Perspective, we outline the present state-of-the-art in achieving this control with spatial modes of light, both as single photon and entangled states, highlight the open challenges that remain, and consider the roadmap that might see its full potential realized.
Single-photon sources play a crucial role in quantum information processing. Among the various methods for generating single photons, spontaneous parametric down-conversion (SPDC) is one of the most widely used. Numerous studies have demonstrated the implementation of SPDC sources across different platforms. Among these, crystalline whispering-gallery-mode resonators (WGMRs) represent an intriguing platform for realizing SPDC-based sources. WGMR-based SPDC sources have been shown to be capable of efficiently interacting with alkali atoms due to the narrow and tunable emission bandwidths. Moreover, they operate with relatively low pump power, which is advantageous for large-scale integration and power-constrained applications. In this work, we further demonstrate the versatility of WGMR-based SPDC sources through three key experiments. First, we generate polarization-entangled photon pairs using an interferometric scheme, achieving a Clauser–Horne–Shimony–Holt (CHSH) inequality parameter of 𝑆 =2.54 ±0.06, violating the classical bound by over eight standard deviations. Second, we observe Hong–Ou–Mandel interference with visibilities of 74 ±5% and 76 ±4% for photons from one and two independent WGMRs, respectively, using an in-coupled pump power as low as 50 nW. Finally, we demonstrate photon--ion interaction between a WGMR-generated photon and a ¹⁴⁷Yb⁺ ion with a success probability of 8%. These results highlight the potential of WGMR-based SPDC sources for scalable quantum networks and hybrid quantum systems.