On the detection of finite-frequency current fluctuations

We consider a measurement of finite-frequency current fluctuations, using a resonance circuit as a model for the detector. We arrive at an expression for the measurable response in terms of the current-current correlators which differs from the standard (symmetrized) formula. The possibility of detection of vacuum fluctuations is discussed. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 3, 280–284 (10 February 1997) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Nonlinear transport through NS junctions due to imperfect Andreev reflection

We investigate a normal-metal-superconductor (point) contact in the limit where the number of conducting channels in the metallic wire is reduced to few channels. As the effective Fermi energy drops below the gap energy, a conducting band with a width twice the Fermi energy is formed. Depending on the mode of operation, the conduction band can be further squeezed, leading to various nonlinear effects in the current-voltage characteristics such as current saturation, a N-shaped negative differential resistance, bistability, and hysteresis. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 7, 572–577 (10 October 1998) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Postselective measurement of the electronic entanglement in the system of two Mach-Zehnder interferometers with coulomb interaction

The parameters at which the maximally entangled two-particle state appears in the system of two ideal electronic Mach-Zehnder interferometers are known. In this work, the operation of nonideal devices with finite scattering in splitters and with reflection in the region of the Coulomb Interaction has been considered. It has been shown that, under the condition of postselection of experimental results, these factors can increase the observed Bell parameter to values exceeding Cirel’son’s (or Tsirelson’s) bound equal to 2√2 up to the mathematical limit equal to 4. A simple postselective measurement scheme providing B = 4 has been described. Although the results of such measurements are not as fundamental as the observation of the violation of Bell’s inequality, they can indirectly indicate the existence of an entangled state in the system. The measurement system is more stable against fluctuations of the phase than a system without postselection. Furthermore, it has been found that the proposed system is optimal for investigation of cross correlations between interferometers in the dc regime (beyond the paradigm of the violation of Bell’s inequality) because they cannot be generated by coordinated fluctuations of Aharonov-Bohm phases.     

Electronic entanglement in the vicinity of a superconductor

A weakly biased normal-metal-superconductor junction is considered as a potential device injecting entangled pairs of quasi-particles into a normal-metal lead. The two-particle states arise from Cooper pairs decaying into the normal lead and are characterized by entangled spin- and orbital degrees of freedom. The separation of the entangled quasi-particles is achieved with a fork geometry and normal leads containing spin- or energy-selective filters. This solid state entangler is characterized by noise cross-correlations which are identical to the noise in one lead, a signature consistent with entanglement. A connection to Bell-type experiments is envisioned (cond-mat/0009193). Received 20 September 2001     

Quantum and classical binomial distributions for the charge transmitted through coherent conductor

We discuss controversial results for the statistics of charge transport through coherent conductors. Two distribution functions for the charge transmitted was obtained previously, one actually coincides with classical binomial distribution, the other is different, and we call it here quantum binomial distribution. We show that high-order charge correlators, determined by the either distribution functions, all can be measured in different setups. The high-order current correlators, starting with the third order, reveal (missed in previous studies) special oscillating frequency dependence on the scale of the inverted time flight from the obstacle to the measuring point. Depending on setup, the oscillating terms give substantially different contributions.     

Use of small-scale quantum computers in cryptography with many-qubit entangled states

We propose a new cryptographic protocol. It is suggested to encode information in ordinary binary form into many-qubit entangled states with the help of a quantum computer. A state of qubits (realized, e.g., with photons) is transmitted through a quantum channel to the addressee, who applies a quantum computer tuned to realize the inverse unitary-transformation decoding of the message. Different ways of eavesdropping are considered, and an estimate of the time needed for determining the secret unitary transformation is given. It is shown that using even small quantum computers can serve as a basis for very efficient cryptographic protocols. For a suggested cryptographic protocol, the time scale on which communication can be considered secure is exponential in the number of qubits in the entangled states and in the number of gates used to construct the quantum network.