Quantum cryptography entangling probe

Eavesdropping on a four-state quantum key distribution protocol


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A scientist at the Army Research Laboratory has recently invented a method for secure communication of encrypted data using quantum cryptography. The patented technology is available via patent license agreement to companies that would make, use, or sell it commercially.

QKD provides data encryption by generating random binary sequence keys for two users to share

Quantum key distribution (QKD) is a method for secure data encryption and decryption that relies on quantum mechanics to generate a random binary sequence for two users to share information. QKD then needs to be paired with an encryption algorithm to transmit the data, most commonly a one-time pad. One unique property of QKD is the ability of the two users to detect any interference or eavesdropping by a third party attempting to gain the code sequence or key. This results from the fact that measuring any quantum system simultaneously disturbs it and introduces detectable anomalies.

The original BB84 protocol is perceived as the most practical and robust QKD protocol. Using the Vernam cipher, the key can be used to encode a message which can be securely transmitted over an open communication line and then decoded, using the shared secret key at the receiver.

At a fundamental level, the potential security of the key rests on the fact that nonorthogonal photon polarization measurement operators do not commute, and this results in quantum uncertainty in the measurement of those states by an eavesdropping probe. Attack approaches include coherent collective attacks in which the eavesdropper entangles a separate probe with each transmitted photon and measures all probes together as one system, and also coherent joint attacks in which a single probe is entangled with the entire set of carrier photons.

An ARL scientist has developed a new method of eavesdropping on a four-state QKD protocol. The new method uses a quantum cryptographic entangling probe to obtain information from a transmitted signal. The transmitter sends a signal photon through a quantum channel (an optical pathway or airspace). The quantum cryptographic entangling probe links a probe photon with the transmitted signal photon to produce a gated probe photon and a gated signal photon. The gated signal photon is relayed to the receiver. The gated probe photon is used by the quantum cryptographic entangling probe to determine the state that will most likely be measured by the receiver in response to receiving the gated signal photon.

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