100 kilometers of quantum-encrypted transfer

Phys.org  April 2, 2024 Continuous-variable (CV) quantum key distribution (QKD) with coherent states and coherent detection integrates well with existing telecommunication networks. Thus far, long-distance CV-QKD has only been demonstrated using a highly complex scheme where the local oscillator is transmitted, opening security loopholes for eavesdroppers, and limiting potential applications. Researchers in Denmark have demonstrated a long-distance CV-QKD experiment with a locally generated oscillator over a 100-kilometer fiber channel with a total loss of 15.4 decibels. This was done by controlling the phase noise–induced excess noise through a machine learning framework for carrier recovery and optimizing the modulation variance. They […]

Toward metropolitan free-space quantum networks

Phys.org   October 10, 2023 An international team of researchers (Germany, Austria) developed a deployable free-space QKD system and demonstrated its use in realistic scenarios. They developed and launched a low-Earth-orbit satellite for implementing decoy-state QKD—a form of QKD that uses weak coherent pulses at high channel loss and was secure because photon-number-splitting eavesdropping could be detected. They achieved a kilohertz key rate from the satellite to the ground over up to 1,200 kilometres. The key rate was around 20 orders of magnitudes greater than that expected using an optical fibre of the same length. According to the researchers their work […]

Telecom-wavelength quantum repeater node transmits quantum information over tens of kilometers

Phys.org  May 23, 2023 An international team of researchers (Austria, France) has presented a quantum repeater node based on trapped ions that act as single-photon emitters, quantum memories, and an elementary quantum processor. They demonstrated the node’s ability to establish entanglement across two 25-km-long optical fibers independently, then to swap that entanglement efficiently to extend it over both fibers. The resultant entanglement was established between telecom-wavelength photons at either end of the 50 km channel. Finally, they calculated the system improvements to allow for repeater-node chains to establish stored entanglement over 800 km at hertz rates revealing a near-term path […]

High-visibility quantum interference between two independent semiconductor quantum dots achieved

Phys.org  December 28, 2022 There has been exciting recent progress in satellite-based quantum communications and quantum repeaters, but a lack of suitable single-photon sources has hampered further advances. The visibility of quantum interference between independent QDs rarely went beyond the classical limit of 50%, and the distances were limited from a few meters to kilometers. An international team of researchers (China, Germany) has shown quantum interference between two single photons from independent QDs separated by a 302 km optical fiber. The single photons were generated from resonantly driven single QDs deterministically coupled to microcavities. Quantum frequency conversions were used to […]

Quantum leap for research into unhackable communications networks

Phys.org  November 30, 2022 Quantum steering relaxes the strict technological constraints of Bell nonlocality by reframing it in an asymmetric manner, with a trusted party only on one side. However, tests of quantum steering still require either extremely high-quality entanglement or very low loss. An international team of researchers (UK, Switzerland) constructed a test of quantum steering for qudits harnessing the advantages of high-dimensional entanglement to be simultaneously noise robust and loss tolerant. It was designed for single-detector measurements and was able to close the fair-sampling loophole in a time-efficient manner. They demonstrated quantum steering in up to 53 dimensions, […]

A scalable quantum memory with a lifetime over 2 seconds and integrated error detection

Phys.org  November 28, 2022 Because of their long coherence times and efficient optical interface, color centers in diamond are promising candidates for quantum memory nodes. Researchers at Harvard University integrated two-qubit network node based on silicon-vacancy centers (SiVs) in diamond nanophotonic cavities. The qubit register consisted of the SiV electron spin acting as a communication qubit and the strongly coupled silicon-29 nuclear spin acting as a memory qubit with a quantum memory time exceeding 2 seconds. They demonstrated electron-photon entangling gates at temperatures up to 1.5 kelvin and nucleus-photon entangling gates up to 4.3 kelvin and efficient error detection in […]

Silicon nanopillars for quantum communication

Science Daily  September 20, 2022 The integration of recently discovered single-photon emitters in silicon into photonic structures is advantageous to exploit their full potential for integrated photonic quantum technologies. Researchers in South Korea have developed a top-down nanofabrication method, enabling the production of thousands of nanopillars per square millimeter with state-of-the-art photonic-circuit pitch, all the while being free of fabrication-related radiation damage defects. They found a waveguiding effect of the 1278 nm-G center emission along individual pillars accompanied by improved brightness compared to that of bulk silicon. Their results unlock clear pathways to monolithically integrating single-photon emitters into a photonic platform […]

The entanglement of two quantum memory systems 12.5 km apart from each other

Phys.org  August 16, 2022 Researchers in China have reported the establishment of post selected entanglement between two atomic quantum memories physically separated by 12.5 km. They created atom-photon entanglement in one node and sent the photon to a second node for storage via electromagnetically induced transparency. They harnessed low-loss transmission through a field-deployed fiber of 20.5 km by making use of frequency down-conversion and up-conversion. The final memory-memory entanglement was verified to have a fidelity of 90% via retrieving to photons. According to the researchers their experiment makes a significant step forward toward the realization of a practical metropolitan-scale quantum […]

Chinese team breaks distance record for quantum secure direct communication

Phys.org  April 20, 2022 Researchers in China have designed an elaborate physical system and protocol with much enhanced performance for quantum secure communication. The design increased the secrecy capacity greatly by achieving an ultra-low quantum bit error rate of <0.1%, one order of magnitude smaller than that of existing systems. Compared to previous systems, the proposed scheme used photonic time-bin and phase states, operating at 50 MHz of repetition rate, which can be easily upgraded to over 1 GHz using current on-the-shelf technology. Their experiments demonstrated that the proposed system could tolerate more channel loss, from 5.1 dB, which is about 28.3 km in […]

Quantum teleportation: The express lane for quantum data traffic

Phys.org  April 11, 2022 Short-distance quantum encryption is already used commercially. However, to implement a global quantum network, photon loss becomes in issue. An international team of researchers (Australia, USA – NIST) demonstrated an error reduction method that improved the performance of a channel. They sent a photon which is not carrying any useful information, through the loss. Using a noiseless linear amplifier, they corrected for the effects of loss and recovered the lost quantum state to teleport the information they wanted to transmit into the now corrected carrier, avoiding all the loss on the channel. The arbitrary quantum information […]