Quantum cryptography uses quantum properties to encrypt and decrypt data, resisting quantum attacks. Quantum cryptography is the science of exploiting quantum mechanical properties to perform cryptographic tasks. The best known example of quantum cryptography is quantum key distribution (QKD) which offers an information-theoretically secure solution to the key exchange problem. The advantage of quantum cryptography lies in the fact that it allows the completion of various cryptographic tasks that are proven or conjectured to be impossible using only classical (i.e. non-quantum) communication. For example, it is impossible to copy data encoded in a quantum state. If one attempts to read the encoded data, the quantum state will be changed due to wave function collapse (no-cloning theorem). This could be used to detect eavesdropping in QKD.
How does quantum cryptography work?
Quantum cryptography works by following a model that was developed in 1984 by Bennett and Brassard, which is now called BB84. The model assumes there are two parties named Alice and Bob who wish to exchange a message securely. Alice initiates the message by sending Bob a key. The key is a stream of photons that travel in one direction. Each photon represents a single bit of data — either a 0 or 1. However, in addition to their linear travel, these photons are oscillating, or vibrating, in a certain manner. So, before Alice, the sender, initiates the message, the photons travel through a polarizer. The polarizer is a filter that enables certain photons to pass through it with the same vibrations and lets others pass through in a changed state of vibration. The polarized states could be vertical (1 bit), horizontal (0 bit), 45 degrees right (1 bit) or 45 degrees left (0 bit). The transmission has one of two polarizations representing a single bit, either 0 or 1, in either scheme she uses. The photons now travel across optical fiber from the polarizer toward the receiver, Bob. This process uses a beam splitter that reads the polarization of each photon.
Bob receives the photons and randomly chooses one of two possible polarizers to measure each photon’s polarization. He records the results in a table along with the type of polarizer he used for each photon. He then communicates with Alice over a public channel and tells her which type of polarizer he used for each photon, but not the results. Alice then tells him which polarizers were correct and which were wrong. Bob discards the wrong measurements and keeps only the correct ones. The remaining bits form a shared secret key that Alice and Bob can use to encrypt and decrypt their messages using a one-time pad or another symmetric-key algorithm.
However, there is a possibility that an eavesdropper, Eve, might intercept the photons sent by Alice and try to measure their polarization before sending them to Bob. This would disturb the quantum state of the photons and introduce errors in Bob’s measurements. Alice and Bob can detect this by comparing a subset of their bits over the public channel and calculating the error rate. If the error rate is too high, they abort the protocol and start over. Otherwise, they can use error correction and privacy amplification techniques to reduce the errors and increase the security of their key.
What are the advantages of quantum cryptography?
Quantum cryptography has several advantages over classical cryptography:
- It offers unconditional security based on the laws of physics, rather than computational complexity or mathematical assumptions.
- It can detect any attempt of eavesdropping or tampering with the data transmission.
- It can resist attacks from quantum computers that could break some classical cryptographic algorithms.
- It can enable new applications such as device-independent QKD, quantum secret sharing, quantum digital signatures, quantum coin flipping and more.
What are the challenges of quantum cryptography?
Quantum cryptography also faces some challenges and limitations:
- It requires specialized hardware and infrastructure such as single-photon sources, detectors and optical fibers.
- It is susceptible to noise, losses and interference that can degrade the performance and reliability of the system.
- It has limited transmission distance and rate due to attenuation and decoherence of quantum signals.
- It still needs classical cryptography for authentication, error correction and privacy amplification purposes.
- It faces regulatory, standardization and interoperability issues among different vendors and protocols.
Quantum cryptography is a promising technology that can enhance the security and privacy of data communication in the era of quantum computing. It uses quantum properties to encrypt and decrypt data, resisting quantum attacks. It works by sending and measuring photons with different polarizations to generate a secret key that can be used for encryption and decryption. It can detect any eavesdropping or tampering with the data transmission. It has several advantages over classical cryptography, but also some challenges and limitations. Quantum cryptography is still in its infancy and requires further research and development to overcome the technical and practical obstacles and to realize its full potential.