Generation of high-fidelity controlled-not logic gates by coupled superconducting qubits

Building on the previous results of the Weyl chamber steering method, we demonstrate how to generate high-fidelity controlled-not by direct application of certain, physically relevant Hamiltonians with fixed coupling constants containing Rabi terms. Such Hamiltonians are often used to describe two superconducting qubits driven by local rf-pulses. It is found that in order to achieve 100% fidelity in a system with capacitive coupling of strength g one Rabi term suffices. We give the exact values of the physical parameters needed to implement such CNOT. The gate time and all possible Rabi frequencies are found to be t=pi/(2g) and Omega_1/g = sqrt(64n^2-1), n=1,2,3,... . Generation of a perfect CNOT in a system with inductive coupling, characterized by additional constant k, requires the presence of both Rabi terms. The gate time is again t = pi/(2g), but now there is an infinite number of solutions, each of which is valid in a certain range of k and is characterized by a pair of positive integers (n,m). We distinguish two cases, depending on the sign of the coupling constant: (1) the antiferromagnetic case (k>=0); (2) the ferromagnetic case (k<=0). The paper concludes with consideration of fidelity degradation by switching to resonance. Simulation of time-evolution based on the 4th order Magnus expansion reveals characteristics of the gate similar to those found in the exact case, with slightly shorter gate time and shifted values of Rabi frequencies.Comment: 14 pages, 1 figure (select PostScript to view); minor corrections; added Appendix: Derivation of the Hamiltonian for inductively coupled flux qubit

Quantum collisions of finite-size ultrarelativistic nuclei

We show that the boost variable, the conjugate to the coordinate rapidity, which is associated with the center-of-mass motion, encodes the information about the finite size of colliding nuclei in a Lorentz-invariant way. The quasi-elastic forward color-changing scattering between the quantum boost states rapidly grows with the total energy of the collision and leads to an active breakdown of the color coherence at the earliest moments of the collision. The possible physical implications of this result are discussed.Comment: 23 pages, RevTeX. New references and two figures added. Final version accepted for publication in Physical Review

Quasiparticle scattering by quantum phase slips in one-dimensional superfluids

Quantum phase slips (QPS) in narrow superfluid channels generate momentum by unwinding the supercurrent. In a uniform Bose gas, this momentum needs to be absorbed by quasiparticles (phonons). We show that this requirement results in an additional exponential suppression of the QPS rate (compared to the rate of QPS induced by a sharply localized perturbation). In BCS-paired fluids, momentum can be transferred to fermionic quasiparticles, and we find an interesting interplay between quasiparticle scattering on QPS and on disorder.Comment: 4 pages, revtex, no figures; to be published in Phys. Rev. Letter

Quantum Zeno effect in the Cooper-pair transport through a double-island Josephson system

Motivated by recent experiments, we analyze transport of Cooper pairs through a double-island Josephson qubit. At low bias in a certain range of gate voltages coherent superpositions of charge states play a crucial role. Analysis of the evolution of the density matrix allows us to cover a wide range of parameters, incl. situations with degenerate levels, when dissipation strongly affects the coherent eigenstates. At high noise levels the so-called Zeno effect can be observed, which slows down the transport. Our analysis explains certain features of the I-V curves, in particular the visibility and shape of resonant peaks and lines

Dephasing of qubits by transverse low-frequency noise

We analyze the dissipative dynamics of a two-level quantum system subject to low-frequency, e.g. 1/f noise, motivated by recent experiments with superconducting quantum circuits. We show that the effect of transverse linear coupling of the system to low-frequency noise is equivalent to that of quadratic longitudinal coupling. We further find the decay law of quantum coherent oscillations under the influence of both low- and high-frequency fluctuations, in particular, for the case of comparable rates of relaxation and pure dephasing

Information entropic superconducting microcooler

We consider a design for a cyclic microrefrigerator using a superconducting flux qubit. Adiabatic modulation of the flux combined with thermalization can be used to transfer energy from a lower temperature normal metal thin film resistor to another one at higher temperature. The frequency selectivity of photonic heat conduction is achieved by including the hot resistor as part of a high frequency LC resonator and the cold one as part of a low-frequency oscillator while keeping both circuits in the underdamped regime. We discuss the performance of the device in an experimentally realistic setting. This device illustrates the complementarity of information and thermodynamic entropy as the erasure of the quantum bit directly relates to the cooling of the resistor.Comment: 4 pages, 3 figure