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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Photonic hybrid multidimensional systems and their application in quantum communication

Vincenzo D’Ambrosio, Eleonora Nagali, Lorenzo Marrucci, Fabio Sciarrino

Published May 6, 2013• 0 citations
Publisher

Abstract

Qudits, the d-dimensional extension of qubits, open new perspectives in several fields, from fundamental quantum mechanics to quantum cryptography. Although photon polarization is a privileged choice for qubits encoding, it is not suitable for the physical realization of qudits. However, in order to realize multidimensional quantum systems, other degrees of freedom of single photons such as path or orbital angular momentum are available. When two or more degrees of freedom are exploited simultaneously we refer to "hybrid encoding". It is possible for instance to encode information in a four dimensional (ququart) hybrid space spanned by polarization and a bidimensional orbital angular momentum subspace of a single photon. Here we present how high dimensional hybrid systems can be exploited to overcome a major limitation of quantum communication: the need of a shared reference frame. Indeed the joint action of polarization and orbital angular momentum of hybrid ququarts can be exploited to realize quantum communication without a shared reference frame. We experimentally showed that, by using a proper subspace of hybrid ququart space, it is possible to perform any quantum communication protocol and violate CHSH inequalities without any information about the reference frame orientation of the two parties (except the direction of propagation of the photons). Such feature could find application in satellite based communication schemes.

DOI
10.1117/12.1518444

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Qudits, the d-dimensional extension of qubits, open new perspectives in several fields, from fundamental quantum mechanics to quantum cryptography. Although photon polarization is a privileged choice for qubits encoding, it is not suitable for the physical realization of qudits. However, in order to realize multidimensional quantum systems, other degrees of freedom of single photons such as path or orbital angular momentum are available. When two or more degrees of freedom are exploited simultaneously we refer to "hybrid encoding". It is possible for instance to encode information in a four dimensional (ququart) hybrid space spanned by polarization and a bidimensional orbital angular momentum subspace of a single photon. Here we present how high dimensional hybrid systems can be exploited to overcome a major limitation of quantum communication: the need of a shared reference frame. Indeed the joint action of polarization and orbital angular momentum of hybrid ququarts can be exploited to realize quantum communication without a shared reference frame. We experimentally showed that, by using a proper subspace of hybrid ququart space, it is possible to perform any quantum communication protocol and violate CHSH inequalities without any information about the reference frame orientation of the two parties (except the direction of propagation of the photons). Such feature could find application in satellite based communication schemes.
Photon polarization is usually exploited for quantum communication tasks as well as for roll angles measurements. However, it is also possible to encode information in other degrees of freedom of single photons like path or orbital angular momentum. When two or more degrees of freedom are exploited at the same time we refer to it as hybrid encoding. Single photon hybrid states encoded in polarization and orbital angular momentum show properties that make such states a useful resource in quantum communication and in roll angle measurements. In the first case they allow two users to communicate without sharing a reference frame even in presence of turbulences, in the second one they provide a sensitivity enhancement of optical non-contact angular measurements.
Quantum information and quantum optics offer powerful innovative tools for secure communication and cryptography, with Quantum Key Distribution (QKD) becoming a standard technology in the cryptographic world. By exploiting fundamental principles such as quantum superposition, entanglement, and the no-cloning theorem, QKD enables two users to establish a shared secret key with information-theoretic security. Quantum communication is a fundamental element for the realization of the quantum internet, a network of distant nodes connected by quantum channel. The realization of such networks requires methods to distribute states and correlations with high fidelity and low losses. This thesis explores the use of innovative photonic technologies for the realization of experiment aiming at improving quantum communication and quantum channels. Single photons based on QD are generated on-demand and show high purity, near-optimal indistinguishability and low multi-photons emission. By exploiting the features of these photons, a method to realize entangled states is implemented. Using the polarization degree of freedom, these entangled states are ideal candidates for free-space QKD protocols based on the Bell’s inequality. To address the problem of the reference frame alignment using the polarization, hybrid states are studied and implemented. The specific combination of polarization and orbital angular momentum (OAM) can produce photons that are not modified by rotation of the reference frame. We implemented an experiment to realize a QKD that is independent on the relative rotation of the two user. The final goal of my thesis was devoted to the in-field demonstration of quantum teleportation as a fundamental building block of distributed quantum networks, enabling the reliable transmission of quantum information between remote nodes. Exploiting the free-space infrastructure in the "Sapienza" University of Rome, a link of around ∼ 300 m, we implemented the quantum teleportation between two different QDs showing the possibility of using this technology into real-case scenario. The results obtained show not only that QDs are ideal sources for quantum communication, but also the possibility of using new states to improve current technologies, advancing the development of efficient and reliable quantum networks.
High-dimensional quantum states, or qudits, represent a promising resource in the quantum information field. Here we present the experimental generation of four-dimensional quantum states, or ququarts, encoded in the polarization and orbital angular momentum of a single photon. Our technique, based on the qqq-plate device, allows the ququart to be prepared and measured in all five mutually unbiased bases. We report the reconstruction of the four-dimensional density matrix through the tomographic procedure for different ququart states.
The orbital angular momentum carried by single photons represents a promising resource in the quantum information field. In this paper we report a series of optical schemes adopted for the manipulation of quantum states encoded in the orbital angular momentum exploiting the q-plate device. We further demonstrate the capability of generating hybrid entangled states as well as quantum states in dimension higher than two. Indeed we present the manipulation of ququart states encoded in the polarization and orbital angular momentum, which have been adopted for new quantum cryptographic protocols
Photonic entanglement is a fundamental resource in quantum information processing and its distribution between distant parties is a key challenge in quantum communications. Optical satellite links allow transmitting entangled photons over longer distances than currently possible on ground and could provide a path towards global quantum communication networks. The majority of current photonic free-space quantum communication systems, including two recent quantum satellite experiments, use two-dimensional polarization encoding, where each photon can carry at most a single bit of quantum information. Encoding additional qubits in the high-dimensional spatio-temporal degrees of freedom could not only increase channel capacities but also improve robustness with respect to noise and eavesdropping in quantum cryptography. Here, we give an overview of our efforts towards exploiting high-dimensional entanglement in long-distance free-space quantum communications. We report on the results of a first feasibility study, in which we used polarization / energy-time hyperentanglement to increase the dimensionality of the state space and transmit genuine 4-dimensional entanglement via an intra-city free-space link. We discuss how we intend to extend this approach to applications in noise-tolerant large-alphabet quantum key distribution via free-space links, and, ultimately, experiments with satellites.
Higher channel capacity and noise elimination are the key requirements for the implementation of long-distance quantum communication. As the additional degrees of freedom (DoF) of photons can be employed to achieve higher channel capacity and security beyond the polarizations DoF of photons, the photonic qubits are always employed as the flying qubits in quantum communication and quantum information processing. Here, exploiting the multiple DoFs of photons, we present an efficient quantum secure direct communication protocol based on the coding and manipulation of qubits on both the polarization and the orbital angular momentum of photons. Also, the numerical simulation is studied to further clarify the improvement of the channel capacity and the security. It is found that the channel capacity and the error rate (caused by eavesdropping) of the QSDC protocol which encoded on the polarization DoF and the OAM DoF is significantly higher than that of coding on only polarization DoF. We believe this work could provide more evidence for the applications of higher-dimensional qubits in quantum information science.