Quantum Communication Using Entangled Photons

In the middle of a night a few weeks ago, scientists in the dark corridors of the Tata Institute of Fundamental Research (TIFR) achieved a milestone in quantum communication when they successfully measured the preservation of entanglement between pairs of photons separated by a distance of 100 metres.

With the advent of quantum computers, current cryptographic methods that secure the internet and private communications are at risk. Algorithms like RSA and ECC (Elliptic Curve Cryptography) are based on mathematical problems that quantum computers could solve exponentially faster than classical computers. Once this happens, current encryption methods will become obsolete, putting sensitive data—from financial transactions to personal communications—at risk.

This is where quantum mechanics comes to the rescue. Quantum states are fragile, and their measurement has a degree of uncertainty. These properties prohibit their duplication, the essence of the so-called no-cloning theorem. This diktat protects quantum states against eavesdroppers. When you put two such indistinguishable quantum states together you get the strange phenomenon of entanglement. Entanglement leads to correlated results of arbitrary measurements between the pair of particles (states) regardless of the distance between them. This feature of nature can be exploited to send unhackable messages.

The scientists generated entangled pairs of photons using a nonlinear crystal and used a homemade telescope to send one of the photons to another telescope kept 100 metres away. They also deployed an in-house developed method to synchronize the two ends of the communication channel to within a billionth of a second. This fact turned out to be crucial in maintaining entanglement upto a degree where secure quantum communication can be discharged.

The work marks an important milestone for future efforts where the group plans on measuring the preservation of entanglement over distances of tens and hundreds of kilometres in free space.