DSpace Collection: Research Papers (TP)
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Characterization of quantum dynamics using quantum error correction
http://hdl.handle.net/2289/6240
Title: Characterization of quantum dynamics using quantum error correction<br/><br/>Authors: Omkar, S.; Srikanth, R.; Banerjee, Subhashish<br/><br/>Abstract: Characterizing noisy quantum processes is important to quantum computation and communication (QCC), since quantum systems are generally open. To date, all methods of characterization of quantum dynamics (CQD), typically implemented by quantum process tomography, are off-line, i.e., QCC and CQD are not concurrent, as they require distinct state preparations. Here we introduce a method, “quantum error correction based characterization of dynamics,” in which the initial state is any element from the code space of a quantum error correcting code that can protect the state from arbitrary errors acting on the subsytem subjected to unknown dynamics. The statistics of stabilizer measurements, with possible unitary preprocessing operations, are used to characterize the noise, while the observed syndrome can be used to correct the noisy state. Our method requires at most 2(4n−1) configurations to characterize arbitrary noise acting on n qubits.<br/><br/>Description: Open Access.Mon, 29 Dec 2014 22:58:59 GMTFirst order transition for the optimal search time of lévy flights with resetting
http://hdl.handle.net/2289/6239
Title: First order transition for the optimal search time of lévy flights with resetting<br/><br/>Authors: Kusmierz, Lukasz; Majumdar, Satya N.; Sabhapandit, Sanjib; Schehr, Gregory<br/><br/>Abstract: We study analytically an intermittent search process in one dimension. There is an immobile target at the origin and a searcher undergoes a discrete time jump process starting at x0≥0, where successive jumps are drawn independently from an arbitrary jump distribution f(η). In addition, with a probability 0≤r<1, the position of the searcher is reset to its initial position x0. The efficiency of the search strategy is characterized by the mean time to find the target, i.e., the mean first passage time (MFPT) to the origin. For arbitrary jump distribution f(η), initial position x0 and resetting probability r, we compute analytically the MFPT. For the heavy-tailed Lévy stable jump distribution characterized by the Lévy index 0<μ<2, we show that, for any given x0, the MFPT has a global minimum in the (μ,r) plane at (μ∗(x0),r∗(x0)). We find a remarkable first-order phase transition as x0 crosses a critical value x∗0 at which the optimal parameters change discontinuously. Our analytical results are in good agreement with numerical simulations.<br/><br/>Description: Open AccessThu, 27 Nov 2014 22:58:59 GMTDriven inelastic Maxwell gases
http://hdl.handle.net/2289/6209
Title: Driven inelastic Maxwell gases<br/><br/>Authors: Prasad, V.V.; Sabhapandit, Sanjib; Dhar, Abhishek<br/><br/>Abstract: We consider the inelastic Maxwell model, which consists of a collection of particles that are characterized by only their velocities and evolving through binary collisions and external driving. At any instant, a particle is equally likely to collide with any of the remaining particles. The system evolves in continuous time with mutual collisions and driving taken to be point processes with rates τ−1c and τ−1w, respectively. The mutual collisions conserve momentum and are inelastic, with a coefficient of restitution r. The velocity change of a particle with velocity v, due to driving, is taken to be Δv=−(1+rw)v+η, where rw∈[−1,1] and η is Gaussian white noise. For rw∈(0,1], this driving mechanism mimics the collision with a randomly moving wall, where rw is the coefficient of restitution. Another special limit of this driving is the so-called Ornstein-Uhlenbeck process given by dvdt=−Γv+η. We show that while the equations for the n-particle velocity distribution functions (n=1,2,...) do not close, the joint evolution equations of the variance and the two-particle velocity correlation functions close. With the exact formula for the variance we find that, for rw≠−1, the system goes to a steady state. Also we obtain the exact tail of the velocity distribution in the steady state. On the other hand, for rw=−1, the system does not have a steady state. Similarly, the system goes to a steady state for the Ornstein-Uhlenbeck driving with Γ≠0, whereas for the purely diffusive driving (Γ=0), the system does not have a steady state.<br/><br/>Description: Open AccessThu, 18 Dec 2014 22:58:59 GMTTorsional instanton effects in quantum gravity
http://hdl.handle.net/2289/6208
Title: Torsional instanton effects in quantum gravity<br/><br/>Authors: Kaul, Romesh K.; Sengupta, Sandipan<br/><br/>Abstract: We show that in the first-order gravity theory coupled to axions the instanton number of the Giddings-Strominger wormhole can be interpreted as the Nieh-Yan topological index. The axion charge of the baby universes is quantized in terms of the Nieh-Yan integers. Tunneling between universes of different Nieh-Yan charges implies a nonperturbative vacuum state. The associated topological vacuum angle can be identified with the Barbero-Immirzi parameter.<br/><br/>Description: Open AccessSun, 28 Dec 2014 22:58:59 GMT