Professor Urbasi Sinha along with her team work with Photons and she feels that this year has been excellent for photons with the Noble Prize in Physics 2022 being given to entangle photons and experiments in Quantum Information which has inspired a lot of the work they all are working on globally. She also feels that it would be unfair to say that photons don't matter just because they are massless, Along with antimatter and dark matter, the recent discovery of doesn't matter which appears to have no effect on the universe whatsoever. So, she wants her readers to be ensured that despite being massless, chargeless, photons matter a lot and in fact, secure Quantum Communications wouldn't quite be where it is today if photons ceased to matter. So, with those thoughts, their journey began from Raman Research Institute (RRI), Bengaluru in 2012!

## Problems with Classical Communication and Why to use Quantum Communication?

Quantum Communication is a very vast area while one of the key Quantum elements being the key distribution problem and is the most practical Quantum Technology. This technology will prove to be most useful for strategic communications such as used for Credit Card purchase, Online Banking, Electoral polling system and Defense. All of this is at risk of being compromised (mostly by Quantum Computers) as currently information is communicated by classical cryptography. The problem with classical cryptography is that unconditional security is not possible. The RSA (Oldest Public Key Cryptosystem that is widely used for secure data transmission) protocol that forms the basis for a lot of our public key crypto systems is based on the mathematical complexity of factoring large numbers. These are one-way hard problems and so a lot of our security is based on this one-way hardness of these problems which could be solved using enough Quantum gates and Qubits that can perform factorization in polynomial time (Shor's algorithm).

Moreover, computational resources grow very fast, possibly solving today's hard problem tomorrow by developing new algorithm for classical and realizing quantum computers. The solution to this problem is that our security should be independent of future advancements in computational power, new algorithms or new technology. This brings forth the need for Quantum Cryptography, where security is based on laws of nature and not on the mathematical complexity of a problem.

The way Quantum Mechanics can be used to protect Information could be summarized as follows:

1. Measurement of a signal disturbs it on an average.

2. If the signals are not perpendicular to each other, it is impossible to determine which one it is (by the Uncertainty principle)

3. Unknown Quantum States can't be cloned (No-cloning theorem)

4. Quantum correlations can be used to protect information (e.g., Quantum Entanglement (It is a bizarre, counterintuitive phenomenon that explains how two subatomic particles can be intimately linked to each other even if separated by billions of light-years of space. Despite their vast separation, a change induced in one will affect the other.)

These are the principles in Quantum Mechanics which primarily go into the key distribution problem to make it secure from a different secure than mathematical hardness.

## Novel Approach of RRI to solve this problem

The approach used by RRI is using a Satellite, and Quantum Repeaters where we have entanglement-based quantum key distribution (qkd) and is practically very viable and useful method for long-distance based communications. [In public key cryptography, the key distribution of public keys is done through public key servers. When a person creates a key-pair, they keep one key private and the other, known as the public-key, is uploaded to a server where it can be accessed by anyone to send the user a private, encrypted, message.] Here, we can have a satellite which has an entangled photon source, then it can distribute one of the two photonsto one of the ground stations second one to the second ground station, and by doing that we can establish downlink based qkd. But this approach has a limitation in terms of simultaneous visibility of two ground stations, so depending on the height of the satellite, you can only simultaneously access a certain distance and not more than that because of a solid angle issue.

On the other hand, if we now do an Uplink based security where the satellite is spending some time on one ground station, then we know that it is in its orbit going over to a second one and spending some time. These two need not be simultaneously useable, this is a way in which we can have much longer distances possible. Research IIT Tech Ambit

The big picture is that we have a satellitebased Quantum Communication between two distant ground stations and then in turn the key is distributed to alll these different kinds of service providers, maybe through fiber networks and so essentially going towards the global Quantum Communication Network.

## Scope in Future

It is no exaggeration to state that everyone will benefit from secure quantum communications. With chip-based solutions to quantum security, these will be found in all our future mobile phone handsets, making our everyday transactions more secure. Hospitals and banks will use these solutions to protect sensitive patient data and transaction security, respectively. India has been developing quantum security solutions for the past few years with significant progress, both in free space as well as fiber-based solutions.

Through the mission-mode execution of National Quantum Mission (NQM), large-scale quantum security solutions will be developed. With NQM, India has taken a major step towards assuming a global leadership role in this domain. Its focus on secure quantum communications could very well revolutionize life as we know it.