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Quantum secure direct communication (QSDC)

Quantum communication refers to a quantum information exchange that uses photons as quantum information carriers over optical fibre or free-space channels.


Today, quantum data transfer rates remain quite low, and so communicating entire messages is not yet practical. Instead, Quantum Cryptography or Quantum key distribution (QKD) is being used that employs single or entangled photons to generate shared secret key between the parties that is perfectly secure. The security is guaranteed by Heisenberg’s uncertainty principle. This ensures that any attempts to intercept and measure quantum transmissions, will introduce an anomalously high error rate in the transmissions and therefore will be detectable.


QKD technology requires single-photon sources (SPSs), single-photon detectors, modulating schemes, and protocols. Sensitive superconducting detectors also require cryogenic refrigerated devices. Currently point to point fiber optic links are commercially available with limited distance due to photon losses. QKD is also being tested on free space channels from ground to satellites and drones. They are now being expanded to quantum network that contains elements such as a quantum repeater and quantum switch.


QKD systems have been shown to be unconditionally secure, however, this is true only for an ideal system. Practical implementations have been shown vulnerable to many attacks such as photon number splitting (PNS) attacks that uses extra photons generated by photon sources, Trojan-horse attack that sends bright light from the quantum channel and analyzes the back-reflections, and high-power laser attack that may cause detector blinding.


Researchers are developing new QKD systems. One is Measurement Device Independent (MDI) QKD that can remove all of the detector-based side-channel attacks but still remains vulnerable to source-based attacks.


Quantum secure direct communication (QSDC) is another important quantum communication protocol that transmits private information directly between communication parties without producing secret keys in advance . In general, QSDC can function as a deterministic QKD when the parties exchange random keys instead of private information. No basis reconciliation is involved in QSDC.

The framework of general Quantum Secure Direct Communication (QSDC)... |  Download Scientific Diagram

QSDC takes advantage of entanglement as a means of securing network transmission over unsecured data lines. Because such particles are linked in a way that cannot be changed, protocols using them cannot be hacked without being detected by systems on the intended receiving end of such messages. As research has progressed to allow for the use of QSDC in real-world applications, the goal has been to reduce errors, increase transmission rates and, above all, extend the distance that messages using the protocol can be sent. Earlier the record was just 18 km.


China Claims ‘World Record’ In Quantum Communications (QSDC)

Long Guilu, the developer of quantum-based secure direct communication technology, and his team announced in April 2022 that they have achieved a new distance record by safely transmitting data over 100 Km (62 miles), reported SCMP. The observations were published in the journal Light: Science & Applications in early April in an article titled “Realization of quantum secure direct communication over 100 Km fiber with time-bin and phase quantum states.”


To extend that distance, the researchers devised a new QSDC protocol, one that involves the use of photonic time-bin states for monitoring signals and phase states for the actual communication messages. The researchers suggest adding such features to the QSDC protocol protects against phase errors and polarization. Further, it does not rely on feedback nor accurate matching of pairs of interferometers. They also suggest it makes such systems more reliable as well, which in turns leads to a lower error rate. And lowering the error-rate allows for extending the distance messages using the protocol can be sent.


Despite transmission speeds being slow (0.54 bits per second), the paper noted that it was a major improvement over Long’s previous record of 18.5 Km set in 2020, two decades after he devised the device that can identify and prevent eavesdropping threats. Long, a Tsinghua University physics professor and vice-president of the Beijing Academy of Quantum Information Sciences, noted the transmission speeds, saying they were good enough for phone calls and text messaging at roughly 30 Km.


He claimed that the technology was ready to be integrated with standard encryption techniques to create a secure network with classical relay points. “If we replace parts of the internet today, where more eavesdropping attacks happen, with quantum channels, those parts will have the added ability to sense and prevent eavesdropping, making communication even safer,” Long added.


A bank account password, for example, could be securely communicated between two devices 90 Km away using three 30 Km quantum channels connected by two relay points and protected by encryption, according to Long. The most notable aspect is that any eavesdropping attempt during quantum transmission would be spotted, whereas information at the relay points would be safeguarded by classical encryption.


“The experiment shows that intercity quantum secure direct communication through the fiber is feasible with present-day technology,” the team noted, adding that the technique also has “great potential” to secure the 6G technology. The longest QSDC distance published before this breakthrough was 18.5 Km. “The rapid progress of quantum computing causes anxiety over the security of those traditional communications,” the Chinese quantum team wrote.


China’s Quantum Communication Advances
China has made strides in quantum technology’s industrial utilization. In recent years, it has made several quantum technological advances, such as the world’s first quantum satellite, a 2,000 Km quantum communication line between Beijing and Shanghai, and the world’s first optical quantum computing machine prototype.

Additionally, one of Beijing’s aims for its 14th five-year plan, which ends in 2025, is to establish an intercity quantum demonstration network based on secure relays. In November of last year, the goal was also incorporated into the city’s international science and technology innovation center team the construction plans.


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