Louis-Philippe Lamoureux

Fiber-Optics Implementation of the Deutsch-Jozsa and Bernstein-Vazirani Quantum Algorithms with Three Qubits

We report on a fiber-optics implementation of the Deutsch-Jozsa and Bernstein-Vazirani quantum algorithms for 8-point functions. The measured visibility of the 8-path interferometer is about 97.5%. Potential applications of our setup to quantum communication or cryptographic protocols using several qubits are discussed.

Reduced randomness in quantum cryptography with sequences of qubits encoded in the same basis

Time coding quantum key distribution with coherent faint pulses is experimentally demonstrated. A measured 3.3% quantum bit error rate and a relative contrast loss of 8.4% allow a 0.49 bit/pulse advantage to Bob.We consider the cloning of sequences of qubits prepared in the states used in the BB84 or six-state quantum cryptography protocol, and show that the single-qubit fidelity is unaffected even if entire sequences of qubits are prepared in the same basis. This result is only valid provided that the sequences are much shorter than the total key. It is of great importance for practical quantum cryptosystems because it reduces the need for high-speed random number generation without impairing on the security against finite-size cloning attacks.

Experimental error filtration for quantum communication over highly noisy channels.

Error filtration is a method for encoding the quantum state of a single particle into a higher dimensional Hilbert space in such a way that it becomes less sensitive to phase noise. We experimentally demonstrate this method by distributing a secret key over an optical fiber whose noise level otherwise precludes secure quantum key distribution. By filtering out the phase noise, a bit error rate of 15.3% +/- 0.1%, which is beyond the security limit, can be reduced to 10.6% +/- 0.1%, thereby guaranteeing the cryptographic security.

Fiber optical implementation of the Deutsch-Josza algorithm

A robust method for manipulating multidimensional quantum information encoded in time bins in optical fibers is demonstrated. This is illustrated by implementing the Deutsch-Josza algorithm, but this also applies to the Bemstein-Vazirani algorithm and, with slight modifications, to other quantum computation problems (such as the distributed Deutsch-Josza problem) and to multidimensional quantum cryptography.