Dmitry Uskov

Optimization of quantum interferometric metrological sensors in the presence of photon loss

We optimize two-mode entangled number states of light in the presence of loss in order to maximize the extraction of the available phase information in an interferometer. Our approach optimizes over the entire available input Hilbert space with no constraints, other than fixed total initial photon number. We optimize to maximize the Fisher information, which is equivalent to minimizing the phase uncertainty. We find that in the limit of zero loss, the optimal state is the maximally path-entangled so-called N00N state, for small loss, the optimal state gradually deviates from the N00N state, and in the limit of large loss, the optimal state converges to a generalized two-mode coherent state, with a finite total number of photons. The results provide a general protocol for optimizing the performance of a quantum optical interferometer in the presence of photon loss, with applications to quantum imaging, metrology, sensing, and information processing.

Maximal success probabilities of linear-optical quantum gates

Numerical optimization is used to design linear-optical devices that implement a desired quantum gate with perfect fidelity, while maximizing the success rate. For the two-qubit controlled-sign [or controlled NOT (CNOT)] gate, we provide numerical evidence that the maximum success rate is

General linear-optical quantum state generation scheme: Applications to maximally path-entangled states

S=2/27

Strong localization of positive charge in DNA induced by its interaction with environment

using two unentangled ancilla resources; interestingly, additional ancilla resources do not increase the success rate. For the three-qubit Toffoli gate, we show that perfect fidelity is obtained with only three unentangled ancilla photons\char22{}less than in any existing scheme\char22{}with a maximum

Optical absorption spectra and monomer interaction in polymers: Investigation of exciton coupling in DNA hairpins

S=0.003\text{ }40

Geometric phase for SU(N) through fiber bundles and unitary integration

. This compares well to

Four-level and two-qubit systems, subalgebras, and unitary integration

S={(2/27)}^{2}/2\ensuremath{\approx}0.002\text{ }74

Generic two-qubit photonic gates implemented by number-resolving photodetection

, obtainable by combining two CNOT gates and a passive quantum filter [T. C. Ralph, K. J. Resch, and A. Gilchrist, Phys. Rev. A 75, 022313 (2007)]. The general optimization approach can easily be applied to other areas of interest, such as quantum error correction, cryptography, and metrology [M. M. Wilde and D. B. Uskov, Phys. Rev. A 79, 022305 (2009); G. A. Durkin and J. P. Dowling, Phys. Rev. Lett. 99, 070801 (2007)].

Optimal Encoding Capacity of a Linear Optical Quantum Channel

We introduce schemes for linear-optical quantum state generation. A quantum state generator is a device that prepares a desired quantum state using product inputs from photon sources, linear-optical networks, and postselection using photon counters. We show that this device can be concisely described in terms of polynomial equations and unitary constraints. We illustrate the power of this language by applying the Grobner-basis technique along with the notion of vacuum extensions to solve the problem of how to construct a quantum state generator analytically for any desired state, and use methods of convex optimization to identify bounds to success probabilities. In particular, we disprove a conjecture concerning the preparation of the maximally path-entangled n ,0 + 0, nNOON state by providing a counterexample using these methods, and we derive a new upper bound on the resources required for NOON-state generation.

Resource-efficient generation of linear cluster states by linear optics with postselection

We investigate a quantum state of positive charge in DNA. A quantum state of electron hole is determined by the competition of the pi-stacking interaction b sharing a charge between different base pairs and the interaction lambda with the local environment which attempts to trap charge. To determine which interaction dominates, we investigate charge quantum states in various (GC)(n) sequences choosing DNA parameters that satisfy experimental data for the balance of charge transfer rates G(+) <--> G(n)(+), n = 2, 3. We show that experimental data can be consistent with theory only assuming b<<lambda, meaning that charge is typically localized within the single G site. Consequently, as follows from our modeling consideration, any DNA duplex including the one consisting of identical base pairs cannot be considered as a molecular conductor. Our theory can be verified experimentally, measuring balance of charge transfer reactions G(+) <--> G(n)(+), n > or = 4 and comparing the experimental results with our predictions.