We experimentally demonstrate a complete, end-to-end, quantum key distribution system using a continuous wave laser and standard optical components. Our implementation encodes random bits as weak Gaussian modulations onto the phase and amplitude quadratures of the laser beam. We process data from the quantum channel using a post-selection procedure and subsequently apply information reconciliation and privacy amplification procedures to generate an absolutely secure secret key. The maximum information that an eavesdropper may have obtained about this secret key, from the quantum channel and classical communications, is bounded to below one bit. Under the assumption of individual Gaussian eavesdropping attacks, we achieve a secret key generation rate of 25 Mbits/s for a lossless channel and 1 kbit/s for 90% channel loss, per 17 MHz of detected bandwidth.
We demonstrate a multipartite protocol that utilizes entanglement to securely distribute and reconstruct a quantum state. A secret quantum state is encoded into a tripartite entangled state and distributed to three players. By collaborating together, a majority of the players can reconstruct the state, whilst the remaining player obtains nothing. This (2,3) threshold quantum state sharing scheme is characterized in terms of fidelity (<i>F</i>), signal transfer (<i>T</i>) and reconstruction noise (<i>V</i>). We demonstrate a fidelity averaged over all reconstruction permutations of 0.73 ± 0.04, a level achievable only using quantum resources.
We present an experimental scheme to perform continuous variable (2,3) threshold quantum secret sharing on the quadratures amplitudes of bright light beams. It requires a pair of entangled light beams and an electro-optic feedforward loop for the reconstruction of the secret. We examine the efficacy of quantum secret sharing in terms of fidelity, as well as the signal transfer coefficients and the conditional variances of the reconstructed output state. We show that, in the ideal limit, perfect secret reconstruction is possible. We discuss two different definitions of quantum secret sharing: the sharing of a quantum secret and the sharing of a classical secret with quantum resources.
We present methods of transforming the standard quadrature amplitude squeezing of a continuous-wave light beam to its Stokes parameters and transverse spatial modes statistics. These two states of light are called polarization squeezing and spatial squeezing, respectively. We present experimental results of the quadrature amplitude, polarization and spatial squeezing obtained with a common experimental setup based on optical parametric amplifiers. The transformations from quadrature amplitude to polarization and spatial squeezing are achieved with only simple linear optics.