The pandemic has significantly affected the dynamics of communication for people around the world, and video conferencing platforms such as Zoom, Skype, and Google Hangout have now become familiar substitutes for in-person meetings. The rise in popularity of this virtual communication method inevitably brings security concerns, especially when it is crucial for the contents of the meetings to be kept from unwanted individuals.
Secure communication can be defined as follows: Alice and Bob are able to send messages back and forth while keeping the contents of their communication secure from Eve, a constant eavesdropper. To establish secure communication across an unknown environment, such as the Internet, Alice and Bob have to devise a way to share a secret key used to encrypt and decrypt their messages . Cryptographic tools that make this possible often rely on the difficulty of solving problems like integer factorization and discrete logarithm problems . However, with the emergence of quantum computing, there have been questions about the ability of quantum computers to solve these problems in a reasonable time and many are looking to quantum cryptography as an alternative to classic cryptographic schemes.
Quantum key distribution, also known as QKD, is a method that allows secret keys to be exchanged between two parties in a cryptographic system by taking advantage of the laws of quantum mechanics . The key in a QKD is encoded in a photon, and the “observer effect,” which states that a quantum system is changed when one disturbs or observes it, is used to keep the key secret from the public . This characteristic ensures that whenever a third party attempts to intercept the key, they would disturb the system and reveal themselves, making it easy to identify any attempted attacks .
For a long time, QKD has only been applicable to communication between two parties. However, recent success in demonstrating a quantum-secured conference call presents a breakthrough in which multiple parties exchanged keys using QKD. Fedrizzi and his colleagues recently succeeded in demonstrating a secure conference call between four parties using “quantum conference key agreement”, a method that allows quantum key distribution to happen at a network level rather than exclusively between two parties .
Alessandro Fedrizzi, a quantum technologist at Heriot-Watt University, and his colleagues presented a quantum-secured conference call in their paper Experimental quantum conference key agreement, published on June 4th, 2021. In their experiment, they distributed four-photon Greenberger-Horne-Zeilinger (GHZ) states, one for each participant of the conference call . The distributed quantum states contain four entangled photons and the participants performed a specific sequence of measurements on the GHZ-state they received to obtain the key . The photons in the system are entangled, which means that observing a part of the system without following the agreed-upon sequence of measurements would disturb the whole system, catching any eavesdropping attempts . Via optical fiber up to 50 km long, the participants of the call exchanged a cryptographic key of about one million bits. The members of the experimental conference call securely shared an image of a Cheshire cat, making it the first quantum-secured conference call .
There have been many efforts to make QKD more efficient and applicable, and Fedrizzi and his colleagues’ demonstration of a secure video conferencing system using QKD is an important step towards real-world goals. The quantum conference key agreement that they proposed steps over the barrier that QKD only allows for key distribution between two parties. The success of their demonstration shows promise that QKD can be used to host meeting calls that require a high level of security, such as government conference calls. Furthermore, their demonstration of secret keys securely shared between more than two parties is a step towards a fully connected quantum network, which is a prerequisite for the ‘quantum internet’ that many have envisioned.
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