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Cryptography

Cryptography

Quantum secrets can be teleported and shared between multiple senders and receivers

27 Feb 2020 Isabelle Dumé
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Decentralised quantum teleportation of a shared quantum secret. Courtesy: Seung-Woo Lee, Korea Institute for Advanced Study

A novel “decentralized” protocol makes it possible to share secret information among multiple senders and receivers using quantum teleportation. According to the South Korea-based team of researchers who developed it, the new method is the first of its kind, and might be used to make the first networked quantum computers.

Quantum teleportation – a way of instantly transferring a quantum state between distant parties without actually sending a particle in that state through space – is a fundamental building block of quantum computation and communication and works thanks to quantum entanglement. This “spooky action at a distance”, as Albert Einstein called it, allows two or more interacting particles to remain linked in a manner not possible in classical physics – no matter how far apart they are.

Conventional quantum teleportation begins when the sender and the receiver share a pair of entangled particles (for example, photons). The sender then interacts her half of the entangled pair with a third particle in an unknown state. Next, she measures the outcome of this interaction and then communicates the result to the receiver via a classical channel. Armed with this information and a measurement on his half of the entangled pair, the receiver is thus able to recover the state of the unknown state that has been teleported.

From one party to many

The first experimental demonstration of quantum teleportation came in 1997, when researchers succeeded in teleporting the spin (or polarization) of a photon. Since then, various groups have teleported the states of atomic spins, nuclear spins and trapped ions – to cite but three examples.

According to the South Korea team, however, there is no scheme that allows quantum information to be teleported to (and securely shared by) multiple parties at the same time. The researchers – Sang Min Lee and Hee Su Park of the Korea Research Institute of Standards and Science in Daejeon; Seung-Woo Lee of the Quantum Universe Center at the Korea Institute for Advanced Study in Seoul; and Hyunseok Jeong of the Department of Physics and Astronomy at Seoul National University – now propose such a protocol. Importantly, their scheme involves teleporting these “quantum secrets” in a decentralized way, so that the information does not have to be concentrated at a single location (a so-called “trusted node”).

“Unlike all previous teleportation protocols, our scheme allows quantum information shared by an arbitrary number of senders to be transferred to another arbitrary number of receivers,” they tell Physics World. “If any unauthorized group or individual tries to access the hidden secret, this break-in attempt is detected by the other parties.”

Proof-of-principle experiment

The researchers say they have already performed a proof-of-principle experiment between two senders and two receivers using a four-photon entanglement network. Unlike previous techniques, no single- or sub-party of senders and receivers can fully access the secret information, they explain. The results clearly indicate that the full information cannot be owned by individual parties and remains hidden until everyone involved agrees to reveal it.

The scheme facilitates quantum information relay over a network without requiring fully trusted central – or even intermediate – nodes, and the researchers say it could be further extended to include error corrections against photon losses or even quantum bit- or phase-flip errors. Such “decoherence” phenomena must be accounted for if quantum computation is to succeed. The work could thus eventually become a building block for a distributed network of quantum computers.

The researchers report their scheme in Physical Review Letters.

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