18 research outputs found
How Do Schr\"odinger's Cats Die?
Recent experiments with superconducting qubits are motivated by the goal of
fabricating a quantum computer, but at the same time they illuminate the more
fundamental aspects of quantum mechanics. In this paper we analyze the physics
of switching current measurements from the point of view of macroscopic quantum
mechanics.Comment: 4 figures, 12 page
Nonlocal appearance of a macroscopic angular momentum
We discuss a type of measurement in which a macroscopically large angular
momentum (spin) is "created" nonlocally by the measurement of just a few atoms
from a double Fock state. This procedure apparently leads to a blatant
nonconservation of a macroscopic variable - the local angular momentum. We
argue that while this gedankenexperiment provides a striking illustration of
several counter-intuitive features of quantum mechanics, it does not imply a
non-local violation of the conservation of angular momentum.Comment: 10 pages, 1 figur
Localization of the relative phase via measurements
When two independently-prepared Bose-Einstein condensates are released from
their corresponding traps, the absorbtion image of the overlapping clouds
presents an interference pattern. Here we analyze a model introduced by
Javanainen and Yoo (J. Javanainen and S. M. Yoo, Phys. Rev. Lett. 76, 161
(1996)), who considered two atomic condensates described by plane waves
propagating in opposite directions. We present an analytical argument for the
measurement-induced breaking of the relative phase symmetry in this system,
demonstrating how the phase gets localized after a large enough number of
detection events.Comment: 8 pages, 1 figur
Quantum-enhanced magnetometry by phase estimation algorithms with a single artificial atom
Phase estimation algorithms are key protocols in quantum information processing. Besides applications in quantum computing, they can also be employed in metrology as they allow for fast extraction of information stored in the quantum state of a system. Here, we implement two suitably modified phase estimation procedures, the Kitaev and the semiclassical Fourier-transform algorithms, using an artificial atom realized with a superconducting transmon circuit. We demonstrate that both algorithms yield a flux sensitivity exceeding the classical shot-noise limit of the device, allowing one to approach the Heisenberg limit. Our experiment paves the way for the use of superconducting qubits as metrological devices which are potentially able to outperform the best existing flux sensors with a sensitivity enhanced by few orders of magnitude
Electronic and thermal sequential transport in metallic and superconducting two-junction arrays
The description of transport phenomena in devices consisting of arrays of
tunnel junctions, and the experimental confirmation of these predictions is one
of the great successes of mesoscopic physics. The aim of this paper is to give
a self-consistent review of sequential transport processes in such devices,
based on the so-called "orthodox" model. We calculate numerically the
current-voltage (I-V) curves, the conductance versus bias voltage (G-V) curves,
and the associated thermal transport in symmetric and asymmetric two-junction
arrays such as Coulomb-blockade thermometers (CBTs),
superconducting-insulator-normal-insulator-superconducting (SINIS) structures,
and superconducting single-electron transistors (SETs). We investigate the
behavior of these systems at the singularity-matching bias points, the
dependence of microrefrigeration effects on the charging energy of the island,
and the effect of a finite superconducting gap on Coulomb-blockade thermometry.Comment: 23 pages, 12 figures; Berlin (ISBN: 978-3-642-12069-5
Reduction and Emergence in Bose-Einstein Condensates
A closer look at some proposed Gedanken-experiments on BECs promises to shed
light on several aspects of reduction and emergence in physics. These include
the relations between classical descriptions and different quantum treatments
of macroscopic systems, and the emergence of new properties and even new
objects as a result of spontaneous symmetry breaking
Optimal superadiabatic population transfer and gates by dynamical phase corrections
In many quantum technologies adiabatic processes are used for coherent quantum state operations, offering inherent robustness to errors in the control parameters. The main limitation is the long operation time resulting from the requirement of adiabaticity. The superadiabatic method allows for faster operation, by applying counterdiabatic driving that corrects for excitations resulting from the violation of the adiabatic condition. In this article we show how to construct the counterdiabatic Hamiltonian in a system with forbidden transitions by using two-photon processes and how to correct for the resulting time-dependent ac-Stark shifts in order to enable population transfer with unit fidelity. We further demonstrate that superadiabatic stimulated Raman passage can realize a robust unitary NOT-gate between the ground state and the second excited state of a three-level system. The results can be readily applied to a three-level transmon with the ladder energy level structure