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

Qudit implementations with broadband energy-time entangled photons

Bänz Bessire, Christof Bernhard, André Stefanov, Thomas Feurer

Published May 1, 2013• 1 citation
Publisher

Abstract

Entanglement is a key feature of quantum mechanics and is best suited to study the peculiar nature of non-classical correlations. Higher dimensional entangled bipartite systems beyond qubits allow for new insights in generalized Bell inequalities and for enhanced bit rates within protocols for quantum key distribution. Entangled qutrits and ququarts in the time domain have been implemented interferometrically. This paper proposes and demonstrates the capability to encode qudits in the spectrum of energy-time entangled photons by means of spectral amplitude and phase modulation.

DOI
10.1109/cleoe-iqec.2013.6801719

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Summary form only given. Entanglement is a fundamental resource for the quantum processing of information and to study the intrinsic nature of quantum correlations. Entangling higher dimensional bipartite systems (qudits) has been shown to give more insight on the nature of entanglement compared to the simplest entanglement system composed of two two level systems (two qubits) [1, 2]. For quantum key distribution, increasing the dimension of the alphabet by using qudits increases the effective bit rate of the protocol, still being secure [3]. Experimentally, higher dimensional entanglement in photonic systems have been demonstrated for various degrees of freedom of light, including time-bins [4,5], transverse momentum [6, 7] or orbital angular momentum modes [8].We demonstrate here a new way to encode qudits in the energy spectrum of the entangled photons by defining frequency bins within their spectra. Broadband entangled photons are generated by continuous wave spontaneous down-conversion and detected in coincidence by sum frequency generation. By means of experimental methods used for shaping fs-laser pulses, frequency bins can be addressed in the photons spectra. Furthermore, controlling each frequency component individually allows to manipulate and characterize the quantum states. The dimension of the generated states is in practice limited by the optical resolution of the setup only.
We experimentally study the violation of the CGLMP inequality for entangled 2-qubit and 2-qutrit states with different degrees of entanglement using numerically optimized measurement settings. The qudits are encoded and manipulated in the frequency spectrum of broadband energy-time entangled photons by taking into account a spatial light modulator (SLM). The latter allows to discretize the spectrum into bins. By controlling each frequency bin individually, the generation of maximally and non-maximally entangled qutrits is verified through quantum state tomography (QST).
Abstract. We have developed a scheme to generate, control, transmit and measure entangled photonic qutrits (two photons each of dimension d = 3). A Bell test of this source has previously been reported elsewhere [1], therefore, here we focus on how the control of the system is realized. Motivated by these results, we outline how the scheme can be used for two specific quantum protocols, namely key distribution and coin tossing and discuss some of their advantages and disadvantages. Performing quantum communication with high-dimensional systems would appear to be an obvious and straightforward extension to many of the qubit protocols that have driven quantum information science in recent years. There have been theoretical proposals for Quantum Key Distribution (QKD) [2] with greater security [3] as well as specific proposals for qutrits such as quantum coin tossing [4]. Fundamentally, high-dimensional Bell inequalities have revealed greater violations of non-locality [5] while increasing the dimensions of the entangled systems has been shown to facilitate closing the detection loop-hole [6] for such tests. Optics has been able to provide a realistic test-bed for these proposals with several different schemes being realized for generating high-dimensional entanglement [7, 8] as well as the scheme presented here [1, 9]. This source of entangled qutrits is analogous to the energy-time qubit arrangement of Franson [10]. The experimental scheme has been developed for telecom wavelengths and with proven long distance quantum communication architecture [11, 12] to optimise the usefulness of this high dimensional entanglement resource. We have already performed
Abstract High-dimensional quantum key distribution (QKD) based on entangled qudits has emerged as a promising route for advancing the performance and practicality of quantum communication. By extending beyond the qubit framework, qudit entanglement enables the encoding of multiple bits per particle and offers greater robustness against noise, loss, and eavesdropping. In this review, we present qudit-based entanglement QKD protocols and their experimental demonstrations. We first introduce the basic concepts of qudits and provide an overview of foundational qubit-based entanglement QKD schemes, such as Ekert '91 (E91) and Bennett–Brassard–Mermin '92 (BBM92), which establish the core principles of entanglement-based secure key generation. Subsequently, we examine the generalization of these protocols to entangled qudits, highlighting how higher dimensionality enhances secure key rates and noise tolerance. Finally, we survey experimental demonstrations of qudit-based entanglement QKD across photonic platforms and outline prospects, showing that high-dimensional entanglement can be reliably generated, distributed, and measured for QKD with high stability and scalability. Remarkably, these achievements not only verify the feasibility of practical high-dimensional QKD, but also demonstrate clear advantages over qubit-based systems. Overall, theoretical frameworks and experimental realizations reviewed here position photonic qudit-based entanglement QKD as a promising resource for scalable, high-speed, and secure quantum communication networks, unveiling its potential as a central component in the future of quantum information technologies.