88 research outputs found
Quasi-Superradiant Soliton State of Matter in Quantum Metamaterials
Strong interaction of a system of quantum emitters (e.g., two-level atoms)
with electromagnetic field induces specific correlations in the system
accompanied by a drastic insrease of emitted radiation (superradiation or
superfluorescence). Despite the fact that since its prediction this phenomenon
was subject to a vigorous experimental and theoretical research, there remain
open question, in particular, concerning the possibility of a first order phase
transition to the superradiant state from the vacuum state. In systems of
natural and charge-based artificial atome this transition is prohibited by
"no-go" theorems. Here we demonstrate numerically a similar transition in a
one-dimensional quantum metamaterial - a chain of artificial atoms (qubits)
strongly interacting with classical electromagnetic fields in a transmission
line. The system switches from vacuum state with zero classical electromagnetic
fields and all qubits being in the ground state to the quasi-superradiant (QS)
phase with one or several magnetic solitons and finite average occupation of
qubit excited states along the transmission line. A quantum metamaterial in the
QS phase circumvents the "no-go" restrictions by considerably decreasing its
total energy relative to the vacuum state by exciting nonlinear electromagnetic
solitons with many nonlinearly coupled electromagnetic modes in the presence of
external magnetic field.Comment: 6 pages, 4 figure
Distinguishing quantum from classical oscillations in a driven phase qubit
Rabi oscillations are coherent transitions in a quantum two-level system
under the influence of a resonant perturbation, with a much lower frequency
dependent on the perturbation amplitude. These serve as one of the signatures
of quantum coherent evolution in mesoscopic systems. It was shown recently [N.
Gronbech-Jensen and M. Cirillo, Phys. Rev. Lett. 95, 067001 (2005)] that in
phase qubits (current-biased Josephson junctions) this effect can be mimicked
by classical oscillations arising due to the anharmonicity of the effective
potential. Nevertheless, we find qualitative differences between the classical
and quantum effect. First, while the quantum Rabi oscillations can be produced
by the subharmonics of the resonant frequency (multiphoton processes), the
classical effect also exists when the system is excited at the overtones.
Second, the shape of the resonance is, in the classical case,
characteristically asymmetric; while quantum resonances are described by
symmetric Lorentzians. Third, the anharmonicity of the potential results in the
negative shift of the resonant frequency in the classical case, in contrast to
the positive Bloch-Siegert shift in the quantum case. We show that in the
relevant range of parameters these features allow to confidently distinguish
the bona fide Rabi oscillations from their classical Doppelganger.Comment: 8 pages, 4 figures; v2: minor corrections, Fig.1 added, introduction
expande
Towards the Heisenberg limit in microwave photon detection by a qubit array
Using an analytically solvable model, we show that a qubit array-based
detector allows to achieve the fundamental Heisenberg limit in detecting single
photons. In case of superconducting qubits, this opens new opportunities for
quantum sensing and communications in the important microwave range.Comment: 6 pages, 3 figure
Is a single photon's wave front observable?
The ultimate goal and the theoretical limit of weak signal detection is the
ability to detect a single photon against a noisy background. [...] In this
paper we show, that a combination of a quantum metamaterial (QMM)-based sensor
matrix and quantum non-demolition (QND) readout of its quantum state allows, in
principle, to detect a single photon in several points, i.e., to observe its
wave front.
Actually, there are a few possible ways of doing this, with at least one
within the reach of current experimental techniques for the microwave range.
The ability to resolve the quantum-limited signal from a remote source against
a much stronger local noise would bring significant advantages to such diverse
fields of activity as, e.g., microwave astronomy and missile defence.
The key components of the proposed method are 1) the entangling interaction
of the incoming photon with the QMM sensor array, which produces the spatially
correlated quantum state of the latter, and 2) the QND readout of the
collective observable (e.g., total magnetic moment), which characterizes this
quantum state. The effects of local noise (e.g., fluctuations affecting the
elements of the matrix) will be suppressed relative to the signal from the
spatially coherent field of (even) a single photon.Comment: 13 pages, 4 figure
A random walker on a ratchet potential: Effect of a non Gaussian noise
We analyze the effect of a colored non Gaussian noise on a model of a random
walker moving along a ratchet potential. Such a model was motivated by the
transport properties of motor proteins, like kinesin and myosin. Previous
studies have been realized assuming white noises. However, for real situations,
in general we could expect that those noises be correlated and non Gaussian.
Among other aspects, in addition to a maximum in the current as the noise
intensity is varied, we have also found another optimal value of the current
when departing from Gaussian behavior. We show the relevant effects that arise
when departing from Gaussian behavior, particularly related to current's
enhancement, and discuss its relevance for both biological and technological
situations.Comment: Submitted to Europ.Phys. J. B (LaTex, 16 pgs, 8 figures
Tunable refraction in a two dimensional quantum metamaterial
In this paper we consider a two-dimensional metamaterial comprising an array
of qubits (two level quantum objects). Here we show that a two-dimensional
quantum metamaterial may be controlled, e.g. via the application of a magnetic
flux, so as to provide controllable refraction of an input signal. Our results
are consistent with a material that could be quantum birefringent (beam
splitter) or not dependent on the application of this control parameter. We
note that quantum metamaterials as proposed here may be fabricated from a
variety of current candidate technologies from superconducting qubits to
quantum dots. Thus the ideas proposed in this work would be readily testable in
existing state of the art laboratories.Comment: 4 pages, 2 figure
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