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Frontiers in Quantum Science and Technology

Single and entangled photon pair generation using atomic vapors for quantum communication applications

Sumit Achar, Abhijit Kundu, Chilukoti Ashok, Arijit Sharma

Published Oct 22, 2024• 7 citations
Publisher

Abstract

Significant progress has been achieved in leveraging atomic systems for the effective operation of quantum networks, which are essential for secure and long-distance quantum communication protocols. The key elements of such networks are quantum nodes that can store or generate both single and entangled photon pairs. The primary mechanisms leading to the production of single and entangled photon pairs revolve around established techniques such as parametric down-conversion, four-wave mixing, and stimulated Raman scattering. In contrast to solid-state platforms, atomic platforms offer a more controlled approach to the generation of single and entangled photon pairs, owing to the progress made in atom manipulation techniques such as trapping, cooling, and precise excitation schemes facilitated by the use of lasers. This review article delves into the techniques implemented for generating single and entangled photon pairs in atomic platforms, starting with a detailed discussion of the fundamental concepts associated with single and entangled photons and their characterization techniques. The aim is to evaluate the strengths and limitations of these methodologies and offer insights into potential applications. Additionally, the article will review the extent to which these atomic-based systems have been integrated into operational quantum communication networks.

DOI
10.3389/frqst.2024.1438340

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Significant progress has been achieved in leveraging atomic systems for the effective operation of quantum networks, which are essential for secure and long-distance quantum communication protocols. The key elements of such networks are quantum nodes that can store or generate both single and entangled photon pairs. The primary mechanisms leading to the production of single and entangled photon pairs revolve around established techniques such as parametric down-conversion, four-wave mixing, and stimulated Raman scattering. In contrast to solid-state platforms, atomic platforms offer a more controlled approach to the generation of single and entangled photon pairs, owing to the progress made in atom manipulation techniques such as trapping, cooling, and precise excitation schemes facilitated by the use of lasers. This review article delves into the techniques implemented for generating single and entangled photon pairs in atomic platforms, starting with a detailed discussion of the fundamental concepts associated with single and entangled photons and their characterization techniques. The aim is to evaluate the strengths and limitations of these methodologies and offer insights into potential applications. Additionally, the article will review the extent to which these atomic-based systems have been integrated into operational quantum communication networks.
In the emerging field of quantum information technology the two basic subfields are quantum communication and quantum computation. Photonic qubits are considered as most promising information carriers for this new technology due to the immense advantage of suffering negligible decoherence. Additionally, the very small photon-photon interactions can be replaced by inducing effective nonlinearities via measurements which allow for the implementation of crucial two-qubit gate operations. Although the spontaneous parametric down-conversion gives access to the generation of highly entangled few-photon states, such as four-qubit cluster states which allow to demonstrate the new concept of the one-way quantum computer, its applicability is highly limited due to the poor scaling of the simultaneous emission of more than one-entangled photon pair. Therefore of particular interest is the reversible mapping of qubits from photon states to atomic states. This might allow the implementation of photonic quantum repeaters for long-distance quantum communication or the generation of arbitrary multi-photon states as required for linear-optics quantum computing. Thus for the realization of such a quantum network several approaches for achieving the required quantum control between matter and photons have been studied during the past few years. Recent experiments demonstrating the generation of narrow-bandwidth single photons using a room-temperature ensemble of 87Rb atoms and electromagnetically induced transparency should emphasize the progress towards such a quantum network.
Recent progress in quantum technologies has brought quantum communication and networking from a largely theoretical idea to an emerging practical reality. By exploiting fundamental quantum phenomena such as superposition and entanglement, quantum communication promises capabilities that are fundamentally different and out of reach for classical communication systems, particularly in terms of security and information sharing. This review aims to present the essential theoretical concepts required to understand quantum communication, alongside an experimental and engineering perspective that highlights how these ideas are realized using real hardware. We explain core principles and protocols in a manner that connects the physics to practical system design, allowing readers to see how quantum theory and communication technology jointly enable quantum networking. The paper further surveys recent experimental and technological advances that have made practical quantum communication possible, emphasizing the role of photonic hardware, sources, detectors, and integrated platforms in enabling key operations, now leading to deployment of multi-node long-distance quantum networks. Finally, we also present the current developments in quantum-secure communication and quantum networking efforts undergoing in India, placing them in the context of the global progress.
The interaction of single photons with single atoms is a fundamental process in quantum optics which manifests the quantum mechanical nature of light and matter. As an experimentally controlled process it enables the implementation of quantum technological applications, such as quantum information processing and quantum metrology. In the last decades, substantial progress has been attained to experimentally control and manipulate single atoms. For example, single or multiple ions can be stored in an ion trap and their quantum states can be controlled with great precision. At the same time, experimental techniques to generate and control single photons and entangled photon pairs have been widely developed. However, only modest progress has been attained in controlling the interaction of single atoms with sin~le photons. The aim of this thesis project was to experimentally study he absorption of single photons and entangled photons pairs by single trapped DCa· ions. To this end, a source of polarization-entangled photon pairs based on the process of spontaneous parametric down-conversion (SPDC) was designed and constructed . Generating photon pairs whose interaction with an atom can be controlled imposed stringent requirements on the bandwidth, tunability, stability and brightness of the source, making its setup much more complex than usual SPDC sources. In a parallel project, a set of two separate ion traps were built, capable of storing a single or a set of a few 4DCa· ions. he first part of the thesis documents the design, construction , operation and characterization of the entangled photon pair source. The characterization proves all the necessary requirements for the photons to interact with either of two transitions, D312 P312 and D5i2 P312 at 850 and 854 nm respectively, in 4DCa· One of the crucial requirements was to make the source bandwidth match the bandwidth of these transitions (22MHz). This was not straightforward since the typical emission bandwidths of the SPDC process is several orders of magnitude higher (1 05MHz). The ion trap setup is also briefly described. One of its crucial features is the inclusion of two high numerical-aperture objectives very close to the ion trap, allowing to enhance the interaction efficiency of the SPDC photons with the ion, and to efficiently collect the fluorescence light they emit. In a second part of the thesis, a concise description of the schemes and techniques used to detect the absorption of single photons by a single trapped ion is given, including a heoretical treatment of the methods used to analyze the data. he core part of the thesis describes the methods used to couple the entangled photon source to one of the ion traps, and explains the results obtained with the combined setup. First it shows how the source is configured to produce heralded single photons. It then presents experiments in which the absorption by a single ion of photons produced by the source is detected, and how these absorption events are time-correlated with the detection of the heralding photon. These experiments are performed under several conditions, with a growing level of control over the interaction process. Finally, two proof-of-principle experiments are presented. In the first one, the ion is pumped to polarization-sensitive Zeeman substates and the polarization dependence of the correlated absorption rate is observed. This is a first step toward more advanced experiments in which the polarization entanglemen of the photon pairs is transferred to the entanglement of one photon with one atom or to two distant atoms. A second experiment performs heralded single photon spectroscopy on a single ion, showing the expected frequency dependence of the absorption process. Finally, methods to extend these experiments to implement entanglement transfer from photons to atoms are outlined. In particular, it is shown how the efficiency of the source can be improved to make these experiments feasible.
The convergence of classical and quantum Internet technologies promises to revolutionize communication networks by leveraging the unique properties of quantum mechanics. Already today, several efforts on quantum key distribution, post-quantum cryptography, and systems-on-chip are paving the way for a seamless and secure quantum-classical hybrid Internet. We explore the role of integrated photonics on-chip systems that generate entangled photon pairs, a critical component for quantum communication. We review how integrated photonics and associated techniques can generate and manipulate photon pairs with the prospect of miniaturization and scalability. In addition, we discuss the challenges and solutions in integrating these quantum systems with existing classical Internet infrastructure, highlighting recent advances and future directions.
Entanglement is one of the essential resources of quantum information and communication technology. Photons are the most popular and promising media to manipulate entanglement. In this review article, concepts and progress in the generation, observation, and characterization of entangled photons are presented. Starting from underlying theoretical concepts, a historical review on the generation of entangled photons is given. Particularly, recent results on the generation of polarization-entangled photons from semiconductor sources are reviewed and discussed.