Spooky but secure: A big leap for ISRO in quantum communications

Scientists from Ahmedabad’s Space Applications Centre and Physical Research Laboratory have used quantum entanglement to create a hack-proof communications system. This is a terrific achievement putting India in the first rank when it comes to developing knowhow about quantum-encryption.

The press release says real-time quantum key distribution (QKD) was used to conduct hack-proof communications between two places separated by 300 metres. Scientists created a channel to share secure text, image transmissions and two-way video calling. The experiment was conducted between two buildings at the Space Applications Centre.

The experiment was repeated multiple times to ensure the repeatability and robustness of an indigenously developed QKD system capable of seamlessly generating and utilising secure keys for various applications.

To understand what this means, consider the basic challenges of encrypted communication. If two parties are to exchange encrypted information, they must share the same key (or keys) to encrypt and decrypt the data. They must transfer the key, and they must transfer the encrypted data. If somebody intercepts the transmission of either key, or message, or steals the key somehow from either party, they may be able to decrypt the information.

Quantum cryptography uses quantum physics — the properties of elementary particles — to create a secure communications system. Quantum physics is a weird, counterintuitive realm, with laws markedly different from the “big world”.

According to Heisenberg’s Uncertainty Principle, it is impossible to know the exact velocity of a particle, and its exact position, at the same time. We can calculate velocity, or position, to precision, but not both at the same time.

The act of observing a particle also changes its state and disturbs it. This is the Observer Effect. The Observer Effect leads into the “superposition” concept. Elementary particles are not only particles — they are also waves!

A wave has the mathematical probabilities of being in one place or another, or doing one thing or another, until it is actually observed. Under observation, the wave “collapses”, and becomes a particle doing something specific.

Einstein described another key quantum effect, entanglement, as “spooky”. Particles can be paired. If the state of one paired particle changes, the state of the other paired particle instantly changes. This is true, even if they are separated by great distances. The paired particles somehow “know” instantly about changes in each other’s state.

These peculiar properties can be used to create quantum cryptographic systems. Entanglement can be used to create secure keys. A quantum-entangled transmission has a built-in “alarm”. If two entangled particles are separated, a change in the state of one causes a change in the state of the other. If somebody observes a particle during transmission — that is, if the transmission is intercepted — the observer effect leads to changes in the state of the paired particle. So, the users of such a quantum communication system can immediately tell if it has been snooped on.

Another exotic property is that it is impossible to make an exact copy of a quantum state. This “no-cloning” theorem means that, unlike with conventional data, it is impossible to just copy a quantum communication to try and crack it as leisure. By real-time exchange of “quantum-ized” keys, the “alarm” can be continuously updated.

If this can be done over 300 metres, it can be done over greater distances. China claims to have created a system over 40 km, and it is trying to develop a quantum-encrypted satellite communication system using a specialised satellite focussed on studying quantum effects.

For ISRO too, this is a big step towards satellite-based quantum communications. Obviously if ISRO is going to develop a satellite-based system it will have to scale up considerably. But every nation with space-going ambitions must develop such systems.

This is especially true because, as and when quantum computing really gets going, the decryption of conventional encryption would happen very fast. Currently, encryption depends on the fact that massive number-crunching is required to crack it. Computer encryption can be broken, given millions of hours of calculations on super-computers.

Using quantum computers for calculations involves using the superpower of superposition. A normal electrical circuit is either on, or off. Conventional computers use such circuits with each bit coded as 0 (off) or 1 (on). A quantum bit or Qbit is both on and off due to superposition, and can therefore carry much more information than a normal bit. They crunch numbers way faster, and this could make them capable of breaking current encryption.

There are tricky issues. When observed, waves collapse and the Qbit is either on or off. Handling this transition and correcting for errors is very hard. Another massive engineering problem: quantum systems need to be kept very cold, and stored in stable places. A passing train can create problems.

Quantum encryption is relatively easier to develop than other computing efforts using quantum physics but it is still very difficult. Developing this capacity is not just impressive; it will be foundational to keeping ISRO’s satellite assets secure in future.