TSDSI Corner

Secure communication is always a challenge. While numerous tools and techniques exist today to address this issue, the hackers are always a step ahead. Breach of security can lead to compromise in sensitive information, manipulation of the data, introduction of malicious data or code etc. The traditional key based cryptography has become vulnerable for attacks. Today, hackers have adequate computing power to retrieve the key from the encrypted text in real time. The loophole here is the continuation of the communication as the message sending patty is unaware of the compromise. An alert when the intruder meddles with the encrypted data or the key that gets transferred to the receiver would be very helpful to stop further damage. In addition, if it is possible for the sensor of the message to replicate the information at the receiver without the help of the underlying channel, the issue of intruder does not arise at all ! Quantum communication supports both these paradigms.

Working of the mechanism

Quantum communication encompasses a plethora of technologies. Quantum key distribution (QKD) and quantum teleportation are the two important components. They make use of the entanglement of the states wherein a (quantum) system of two states, one each at the sender and the receiver are created such that manipulation of any of them will get reflected at the other system. It is one of the weird and not so intuitive behaviors of quantum systems. The other important concept used in quantum communication is the measurement of the state. Measurement alters the state. For example, if we read a RAM any number of times, the bit pattern remains the same unless we change the data, or the memory goes bad! This is not true in the quantum world. Once the state is measured or “read”, it can change!

Equally weird is the teleportation wherein it is possible to transfer the information from the sender to receiver without actually throwing the data over the channel. [1] As it is different from normal communication, it is several orders faster than the conventional communication system. It is worth to note that the transfer is the information rather than the signals.

In QKD an entangled system is created between the quantum systems (Qbits) of the sender and the receiver The key to be transferred I to the receiver is sent over the quantum channel. Whenever an intruder intercepts the key, the entanglement breaks down and is immediately made known to both the sender and the receiver. Alternatively, a successful transfer of the key leads to subsequent usage of the same for encryption of the data over normal communication channels. Here the role of the quantum channel is to securely transfer the key for the encryption of the data that follows.

The architecture

Most of the implementations used today are based on the BB84 QKD system [2,3]. A simple QKD system is detailed here. It consists of the following components

• Random number generator. The key for data encryption (actually a super set of the key; some bits would get discarded as we see here later) is not stored but generated as a random sequence of bits. Quantum Random number generator is often used to generate the bits independent of the previous history or the environment so that no known algorithm should be able to predict the next bit
• Random basis generator: The random bits generated by the previous module are mapped at random to rectilinear or diagonal basis. The rectilinear basis comprises vertical and horizontal polarization states corresponding to the bits 0 and 1. The diagonal bases are formed from a 45 degree or 135 degree polarization state.
• Photon pupariation: polarized (single) photons are generated based on the choice of the basis in the previous module. Subsequently the photon is put on the channel- the dark fiber channel or quantum channel. The basis, state and time of transmission are noted. Open-air as well as satellite channels have been tried.
• Detector: In the receiver, basis functions are selected at random for the measurement over the photon. Accordingly, the polarization states are derived that may be right or wrong based on the coincidence between the sender and receiver basis functions. The decoded polarization, basis function and the time are noted. All this data gets exchanged over public channels. The bits corresponding to non-coincidence of basis functions are discarded. What remains would be the Key for encrypting the data that follows over classical or normal channels. If an intruder measures or alters the state of single photons, it results in errors. These errors are detected by setting appropriate thresholds. Some errors are tolerable in a long sequence of the key. i.e., even if a few bits are compromised, we can still go ahead with the key.

The challenge ahead

While quantum communication technology is attractive, providing a fool proof mechanism to transfer data, it throws open a set of challenges. The technical challenges include [4] design of high SNR single photon detectors, transfer of information across the repeaters (without destroying the states), interface (and integration) with the classical infrastructure, quantum noise etc.

The operational challenges include the setup of quantum channels, last mile connectivity, distribution of the load etc. The channels can be fiber (dark channel) or satellite or open air that requires a large number of repeaters or trusted nodes

Adoption of the technology has to happen at a large scale. Standardization bodies play a crucial role here in bridging the gap between the theoretical concepts and deployable implementations. Still there are white spaces in the standards due to the emerging use cases, advancements in the technologies and region-specific customizations that need to be addressed. Open Source implementation of these standards encourages the industry for a wider adoption along with differentiating features.

Quantum teleportation, although very attractive, is still in its infancy. The catch is, it works only after the entangled pair is created. In addition, transfer of large data such as a video from a Rover on Mars is a distant dream. However, control signals (such as camera or Rover control) can be easily teleported once an entangled pair is created, it should be possible to steer a Rover on Mars in the direction of choice in real time rather than waiting over several minutes.

Future directions

Depending on the degree of penetration, quantum communication can enable a variety of use cases. Finance industry has already started leveraging the technology in providing secure transactions such as the one with ATMs. [5].

Apart from finance, QKD is the most sought-after technology in the defense industry. Secure communication over short distances is the critical requirement here.

The cable industry distributing the content can use the Quantum channel for the secure transfer of the Key. Subsequently, the content gets decrypted with the key and distributed further.

Quantum internet is not too far from a large-scale deployment. Before that, the interfacing issues and relay problems are to be resolved

References

• https://www.nature.com/articles/s41598-022-06853-w
• C. H. Bennett and G. Brassard. Quantum cryptography: Public key distribution and coin tossing. Proceedings of IEEE International Conference on Computers, Systems and Signal Processing, vol 175,1984
• https://www.uts.edu.au/sites/default/files/2020-11/Zhang__A_quantum_random_number_generator_integrated_into_a_transmitter_of_BB84_QKD_system.pdf
• https://quantlr.com/quantum/the-5-most-significant-challenges-facing-quantum-cryptography/
• https://www.sktelecom.com/en/press/press_detail.do?idx=1459