The story so far: On December 13, Ajay Chaudhary, Chairman of the Mission Governing Board of the nascent National Quantum Mission, said that India plans to launch a quantum satellite in 2-3 years for quantum communication.
What is the National Quantum Mission?
The National Quantum Mission (NQM) is a program of the Department of Science and Technology to accelerate the use of quantum physics in the development of next-generation communication and sensing systems.
The development of computers changed the course of human history from the middle of the 20th century. Thanks to advances in this field, which continue to this day, mankind has access to telecommunications, weather forecasting, drug-discovery programs, search-and-rescue schemes, artificial intelligence, etc.
But many of these advances are nearing a saturation point because the physics phenomenon on which they are based, called classical physics, is hitting an upper limit of performance. Today, scientists around the world are building new devices to perform similar functions but using quantum physics. New devices are expected to have new capabilities as the laws of quantum physics allow for the results of classical physics as well as ‘bonuses’ not found in the classical paradigm.
The Union Cabinet in April 2023 approved the implementation of the NQM from 2023 to 2031 at a total cost of Rs 6,000 crore. The planned Quantum Satellite is an application of this package.
What is a quantum satellite?
A quantum satellite is a term for a communications satellite that uses quantum physics to protect its signals.
Communication is a broad term that refers to the techniques of sending and receiving signals. An important part of these technologies is security: preventing bad actors from intercepting a message being transmitted over large distances through multiple networks.
The advent of quantum computers threatens the techniques currently used to secure messages. Fortunately, quantum physics has also paved the way for new forms of security, and quantum satellites are expected to facilitate them.
How are messages protected?
Two people named Anil and Selvi are standing on opposite sides of a playground and want to talk to each other. The simplest way is to wave your hand or shout. A third person named Kaushik is also standing in the middle of the ground trying to listen to the conversation. If Anil and Selvi are shouting or using hand signals, Kaushik will have little difficulty intercepting their messages – but if they’re communicating via WhatsApp.
Messages on WhatsApp are encrypted. Encryption means that a message is written in a secret code before it is sent. When the recipient receives it, they will use their knowledge of the code to decrypt and read the message. If a bad actor like Kaushik somehow intercepts the message, he can’t read it without knowing the code.
For example, in the Caesar cipher, the letters of the alphabet are offset by a fixed number. If the number is 5, the words BIRDS FLY AWAY will be GNWIX KQD FBFD.
This paradigm is called cryptographic security. It derives its strength from hiding the key to cracking the code behind an extremely difficult mathematical problem. Anil and Selvi’s devices already have a solution to this problem. If Kaushik wants this, however, he must first solve the problem with a computer—and the harder the problem, the more time (or more computing resources) it needs.
A modern computer can quickly crack a Caesar cipher by repeatedly trying all possible keys (1-26) until the text is readable. But even the most powerful supercomputers today cannot crack the best advanced encryption standard ciphers in a single lifetime. However, quantum computers may be able to do better.
How can quantum physics protect messages?
Quantum cryptography uses the principles of quantum physics to secure messages. The most famous type of this is quantum key distribution (QKD).
In the earlier example, Anil used a secret code to encrypt or ‘lock’ his message and Selvi had the key to decrypt and read the message. QKD is worried about sharing the truth with Anil and Selvi that if Kaushik is listening in on the broadcast, everyone will know and cancel the partnership.
Quantum physics can reveal eavesdropping in a variety of ways. One is quantum measurement—the act of measuring the properties of a quantum system, such as a photon (a subatomic particle of light). According to the laws of quantum physics, a quantum measurement changes the state of the system. If the information about the key is encoded in a stream of photons (in two states, one representing 0 and the other 1) and Kaushik traps and measures to find it, the state of the photon will change and Anil and Selvi will know. The key is compromised.
Another way is to use quantum entanglement: when two photons are entangled, any change in one particle will immediately change the other. (This is necessarily a simplified description.)
QKD is said to provide unconditional security, since the key is lost regardless of the technical capabilities of Kaushik.
Is QKD implemented?
Ravindra Pratap Singh of the Physical Research Laboratory, Ahmedabad, wrote in 2023 that standards for various QKD protocols and the techniques needed to implement them as designed are still a decade away. That said, China currently operates the world’s largest QKD network with three quantum satellites and four ground stations.
Experts are also trying to implement QKD over long distances. In the two decades since its experimental proof in 1992, reliable transmission distances have grown to several hundreds of kilometers via either fiber-optic cable or free space.
In 2013, researchers in China reported that they had implemented QKD between a ground station and a moving hot-air balloon (carrying a payload of devices) 20 km above. This demonstration strengthened the case for quantum satellites.
In an October 2024 study, researchers from the Raman Research Institute, Bengaluru, reported that the Indian Astronomical Observatory at Hanle in Ladakh offered the best atmospheric conditions for transmitting data for a satellite-based QKD system. It was estimated to have a signal loss of 44 dB compared to 50 dB in Chinese use.
“Our main signal will be at 810 nm while the uplink and downlink will use 532 nm and 1550 nm wavelengths, respectively,” Satya Ranjan Behra, lead author of the paper, told the Department of Science and Technology. The planned beam distance is 500 km.
Are there weaknesses in QKD?
Because QKD on paper can be very different than in the real world, the US National Security Agency recommends using post-quantum cryptography instead of quantum cryptography. Criticism of this has focused on five limitations:
(i) “QKD does not provide a means of authenticating a QKD transmission source”;
(ii) “since QKD is hardware-based”, QKD networks cannot be upgraded or easily patched;
(iii) “QKD increases infrastructure costs and insider threat exposure” which “eliminates many use cases from consideration”;
(iv) “the actual security provided by a QKD system is not a theoretical unconditional security from the laws of physics … but a more limited security that can be achieved by hardware and engineering designs”; and
(v) Because eavesdroppers can intercept a transmission, they can deny use of the transmission by its intended users (aka a denial-of-service attack).
Post-quantum cryptography refers to cryptographic techniques that resist attacks using more advanced classical encryption from both quantum and classical devices.
Quantum physics also imposes some restrictions. For example, non-quantum information can be amplified before being transmitted over large distances while the no-cloning theorem prohibits the amplification of quantum information.
published – December 20, 2024 at 06:00 hrs IST