What is Quantum Cryptography?

Quantum cryptography uses quantum mechanics and properties of particles like photons to establish cryptographic keys between two communicating parties. It relies on the basic principle that altering or measuring a quantum state will disturb the original state, allowing detection of eavesdropping attempts.

How Does it Work?

In Quantum Cryptography, keys are distributed using quantum properties like polarization or phase of single photons. A transmitter, like Alice, encodes the key bits into non-mutually orthogonal states of photons. These photons are transmitted to the receiver, Bob, through optical fiber or free-space. Bob randomly measures the photon states in one of the encoding bases to read the key. Due to the no-cloning theorem of quantum mechanics, any eavesdropping by an adversary like Eve disturbs the quantum states. This allowsAlice and Bob to detect the presence of Eve.

After photons transmission, Alice and Bob disclose over a classical channel which bases they used to encode and measure each bit. They can check for errors caused by eavesdropping and discard bits measured in different bases. The remaining shared bits make up the cryptographic key, which is information theoretically secure.

Quantum Key Distribution Protocols

Two popular quantum key distribution protocols are BB84 and E91. In BB84 protocol introduced by Bennett and Brassard in 1984, Alice encodes each bit in one of four possible states - horizontal/vertical polarization or +45°/−45° polarization. Bob measures randomly in either of the rectilinear or diagonal basis. The E91 protocol by Ekert in 1991 relies on quantum entanglement where Alice and Bob measure properties of pairs of entangled photons created by a central source. Violations of Bell's inequalities detected by Alice and Bob are used to test for eavesdropping.

Implementations and Applications

Working quantum key distribution systems have been demonstrated over distances up to hundreds of kilometers in the lab and field trials. This includes implementations using polarization-encoded single photons and wavelength-division multiplexing over installed fiber infrastructure. Commercial cryptography solutions are also now available from ID Quantique, Quantum Xchange and others providing security for banking, government and enterprise networks.

It enables unconditionally secure communication where security relies on laws of physics rather than computational complexity assumptions. It provides advantages for future quantum networks and long-term secure storage of encrypted data. Integration with classical cryptosystems further enhances security for applications like secure videoconferencing, database encryption, and VPNs. With continued improvements, quantum cryptography promises ubiquitous secure communication enabled by the unique properties of quantum mechanics.

Limitations and Challenges

While theoretically break-proof, limitations remain for widespread use of quantum cryptography. It is currently limited to point-to-point communication rather than large networks due to losses in photons transmission through optical fiber or free-space. Storage, manipulation and measurement of quantum states needed for computing and networking is also technologically challenging. Environmental disturbances and imperfect devices also introduce errors.

Going forward, overcoming losses in photons transmission through better quantum repeaters, improving stability and efficiency of single-photon sources and detectors are active research areas. Development of practical large-scale quantum networks, quantum memories and integration with classical systems will determine when quantum cryptography becomes a commercial solution for many users. Though significant challenges remain, rapid progress indicates quantum technologies will ultimately revolutionize cyber-security.

It leverages fundamental quantum physics to provide unconditional security for encryption currently not achievable through classic cryptanalysis alone. With technological progress it enables a future where secure communication is the norm, not dependent on computational assumptions. Though challenges remain, the promise of quantum cryptography based on physical principles is revolutionizing how we think about cyber and information security.

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