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International Journal of Quantum Information

EXPERIMENTAL PROPOSAL OF SWITCHED "DELAYED-CHOICE" FOR ENTANGLEMENT SWAPPING

Thomas D. Jennewein, Časlav Brukner, Markus Aspelmeyer, Anton Zeilinger

Published Dec 17, 2004• 8 citations
Publisher

Abstract

Entanglement swapping is a fascinating generalization of quantum teleportation, where the entanglement between pairs is interchanged leading to entanglement between particles that never interacted. Extending the idea of Peres, we propose an experimental setup where the choice which quantum systems are finally entangled is made at a time after they have been registered and do not even exist anymore. Our proposed setup can be used in Third-Man Quantum Cryptography where a third party can control whether Alice and Bob are able to communicate secretly, but he does know their secret key.

DOI
10.1142/s0219749905000505

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International Journal of Apllied Mathematics•Nov 16, 2025•0 citations
Entanglement swapping is a fascinating generalization of quantum teleportation, where the entanglement between pairs is interchanged leading to entanglement between particles that never interacted. Extending the idea of Peres, we propose an experimental setup where the choice which quantum systems are finally entangled is made at a time after they have been registered and do not even exist anymore. Our proposed setup can be used in Third-Man Quantum Cryptography where a third party can control whether Alice and Bob are able to communicate secretly, but he does know their secret key.
Abstract. Two observers (Alice and Bob) independently prepare two sets of singlets. They test one particle of each singlet along an arbitrarily chosen direction and send the other particle to a third observer, Eve. At a later time, Eve performs joint tests on pairs of particles (one from Alice and one from Bob). According to Eve’s choice of test and to her results, Alice and Bob can sort into subsets the samples that they have already tested, and they can verify that each subset behaves as if it consisted of entangled pairs of distant particles, that have never communicated in the past, even indirectly via other particles. To appear in a special issue of Journal of Modern Optics. 1 1. Introduction and
Two observers (Alice and Bob) independently prepare two sets of singlets. They test one particle of each singlet along an arbitrarily chosen direction and send the other particle to a third observer, Eve. At a later time, Eve performs joint tests on pairs of particles (one from Alice and one from Bob). According to Eve's choice of test and to her results, Alice and Bob can sort into subsets the samples that they have already tested, and they can verify that each subset behaves as if it consisted of entangled pairs of distant particles that have never communicated in the past, even indirectly via other particles.
Two observers (Alice and Bob) independently prepare two sets of singlets. They test one particle of each singlet along an arbitrarily chosen direction and send the other particle to a third observer, Eve. At a later time, Eve performs joint tests on pairs of particles (one from Alice and one from Bob). According to Eve's choice of test and to her results, Alice and Bob can sort into subsets the samples that they have already tested, and they can verify that each subset behaves as if it consisted of entangled pairs of distant particles that have never communicated in the past, even indirectly via other particles.
Building on Peres's idea of "Delayed-choice for extanglement swapping" we show that even the degree to which quantum systems were entangled can be defined after they have been registered and may even not exist any more. This does not arise as a paradox if the quantum state is viewed as just a representative of information. Moreover such a view gives a natural quantification of the complementarity between the measure of information about the input state for teleportation and the amount of entanglement of the resulting swapped entangled state.
Quantum teleportation stands as a cornerstone of quantum communication, enabling the transmission of an unknown quantum state—typically a qubit—from a sender to a receiver without the physical transfer of the particle itself. In this study, we explore two novel teleportation protocols within the framework of ternary quantum logic, which extends conventional binary systems by incorporating three distinct quantum states, known as qutrits. This ternary paradigm introduces richer computational possibilities and more nuanced entanglement structures compared to binary logic. Both protocols incorporate a supervisory controller to regulate the transmission. The first proposed scheme utilizes entanglement swapping between two qutrits to achieve the teleportation of a qutrit. This approach demonstrates the feasibility of ternary teleportation using minimal entangled resources. The second scheme builds upon this foundation by employing entanglement swapping among three qutrits, offering enhanced control. In the second scheme, the controller remains actively embedded in the quantum circuit throughout the protocol, with the authority to abort or validate the teleportation either at the beginning of protocol or at the end, thereby introducing dynamic control. Direct entanglement between distant parties like Alice and Bob is often infeasible due to channel noise and losses, but using an intermediary party who shares entangled pairs with each party, enables long-distance entanglement, which is crucial for scalable quantum networks and quantum repeaters. This modular approach not only enhances the security and reliability of quantum communication by managing errors locally but also offers flexibility and practicality in real-world systems by facilitating entanglement through local intermediaries.