The team from CAS Key Lab of Quantum Information, LI Chuanfeng and LIU Biheng, used the four-dimensional entangled state to achieve quantum superdense coding for the first time, achieving a channel capacity of 2.09. This achievement demonstrates the advantages of high-dimensional entanglement in quantum communication. The result was published on July 20th in Science Advances.
Quantum superdense coding is one of the most important quantum communication processes. Let’s take the bit system as an example. Initially Alice and Bob share a pair of entangled photons. Alice encodes 2 bits of classical information on her photon, and sends the photon to Bob. Bob then carry out Bell-state measurement on the two photons in his hand, decoding to get the 2 bits of information sent by Alice. In this process, Alice only sents 1 qubit to Bob, but Bob receives 2 bits of classic information. An important parameter for superdense coding is the channel capacity, which is the number of bits that Alice can transmit to Bob by sending a photon. In qubit system, the quantum superdense coding channel capacity limit is 2. The idea of quantum superdense coding was proposed in 1992 and was first realized in optical system in 1996 [PRL 76, 4656 (1996)]. Since full Bell-state measurements could not be achieved, only 1.13 classical bits were transmitted using a pair of entangled photons, i.e., the channel capacity was 1.13. In 2008, with hyper-entanglement, the channel capacity of quantum superdense coding was increased to 1.63 [ Nat. Phys. 4, 282 (2008)]. In 2017, this record was updated to 1.665 [PRL 118, 050501 (2017)] based on full Bell-state measurements.
Compared with the two-dimensional entanglement of bit systems, high-dimensional entanglement has the advantages of high channel capacity and strong ability to resist eavesdropping. Li Chuanfeng and Liu Biheng et al. developed a polarization-path hybrid four-dimensional entangled photon source based on a self-developed high-quality three-dimensional entangled source [PRL 117, 170403 (2016)], with a fidelity of 98%. They used this four-dimensional entanglement source to successfully identify five types of Bell states, and experimentally demonstrated quantum superdense coding, which increased the channel capacity of quantum superdense coding to 2.09, surpassing the theoretical limit of two-dimensional entanglement. The results demonstrate the advantages of high-dimensional entanglement in quantum communication.
School of Physical Sciences, edited by Jane FAN
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