159 research outputs found
Form factor of the process \gamma^*\gamma^* -->\pi^o for small Virtuality of One of the Photons and QCD Sum Rules
We extend the QCD sum rule analysis of the \gamma^*\gamma^* -->\pi^o form
factor into the region where one of the photons has small virtuality: q^2 <<
Q^2 > 1 GeV^2. In this kinematics, one should perform an additional
factorization of short- and long-distance contributions. The extra
long-distance sensitivity of the three-point amplitude is described by
two-point correlators (bilocals), and the low-momentum dependence of the
correlators involving composite operators of two lowest twists is extracted
from auxiliary QCD sum rules. Our estimates for the form factor at q^2=0, Q^2 >
1 1 GeV^2 are in good agreement with existing experimental data.Comment: 6 pages, uses epsf.sty, 1 eps figur
Quantum coherence control of solid-state charge qubit by means of a suboptimal feedback algorithm
The quantum coherence control of a solid-state charge qubit is studied by
using a suboptimal continuous feedback algorithm within the Bayesian feedback
scheme. For the coherent Rabi oscillation, the present algorithm suggests a
simple bang-bang control protocol, in which the control parameter is modulated
between two values. For the coherence protection of idle state, the present
approach is applicable to arbitrary states, including those lying on the
equator of the Bloch sphere which are out of control in the previous Markovian
feedback scheme.Comment: 5 pages, 4 figure
Continuous weak measurement and feedback control of a solid-state charge qubit: physical unravelling of non-Lindblad master equation
Conventional quantum trajectory theory developed in quantum optics is largely
based on the physical unravelling of Lindbald-type master equation, which
constitutes the theoretical basis of continuous quantum measurement and
feedback control. In this work, in the context of continuous quantum
measurement and feedback control of a solid-state charge qubit, we present a
physical unravelling scheme of non-Lindblad type master equation.
Self-consistency and numerical efficiency are well demonstrated. In particular,
the control effect is manifested in the detector noise spectrum, and the effect
of measurement voltage is discussed.Comment: 8 pages, 5 figure
Phonitons as a sound-based analogue of cavity quantum electrodynamics
A quantum mechanical superposition of a long-lived, localized phonon and a
matter excitation is described. We identify a realization in strained silicon:
a low-lying donor transition (P or Li) driven solely by acoustic phonons at
wavelengths where high-Q phonon cavities can be built. This phonon-matter
resonance is shown to enter the strongly coupled regime where the "vacuum" Rabi
frequency exceeds the spontaneous phonon emission into non-cavity modes, phonon
leakage from the cavity, and phonon anharmonicity and scattering. We introduce
a micropillar distributed Bragg reflector Si/Ge cavity, where Q=10^5-10^6 and
mode volumes V<=25*lambda^3 are reachable. These results indicate that single
or many-body devices based on these systems are experimentally realizable.Comment: Published PRL version. Note that the previous arXiv version has more
commentary, figures, etc. Also see http://research.tahan.com
Quantum feedback control of a superconducting qubit: Persistent Rabi oscillations
The act of measurement bridges the quantum and classical worlds by projecting
a superposition of possible states into a single, albeit probabilistic,
outcome. The time-scale of this "instantaneous" process can be stretched using
weak measurements so that it takes the form of a gradual random walk towards a
final state. Remarkably, the interim measurement record is sufficient to
continuously track and steer the quantum state using feedback. We monitor the
dynamics of a resonantly driven quantum two-level system -- a superconducting
quantum bit --using a near-noiseless parametric amplifier. The high-fidelity
measurement output is used to actively stabilize the phase of Rabi
oscillations, enabling them to persist indefinitely. This new functionality
shows promise for fighting decoherence and defines a path for continuous
quantum error correction.Comment: Manuscript: 5 Pages and 3 figures ; Supplementary Information: 9
pages and 3 figure
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