191 research outputs found
Signal amplification in a qubit-resonator system
We study the dynamics of a qubit-resonator system, when the resonator is
driven by two signals. The interaction of the qubit with the high-amplitude
driving we consider in terms of the qubit dressed states. Interaction of the
dressed qubit with the second probing signal can essentially change the
amplitude of this signal. We calculate the transmission amplitude of the probe
signal through the resonator as a function of the qubit's energy and the
driving frequency detuning. The regions of increase and attenuation of the
transmitted signal are calculated and demonstrated graphically. We present the
influence of the signal parameters on the value of the amplification, and
discuss the values of the qubit-resonator system parameters for an optimal
amplification and attenuation of the weak probe signal.Comment: 7 pages, 8 figure
Quantum behaviour of a flux qubit coupled to a resonator
We present a detailed theoretical analysis for a system of a superconducting
flux qubit coupled to a transmission line resonator. The master equation,
accounting incoherent processes for a weakly populated resonator, is
analytically solved. An electromagnetic wave transmission coefficient through
the system, which provides a tool for probing dressed states of the qubit, is
derived. We also consider a general case for the resonator with more than one
photon population and compare the results with an experiment on the
qubit-resonator system in the intermediate coupling regime, when the coupling
energy is comparable with the qubit relaxation rate.Comment: 16 pages, 6 figure
Weak continuous monitoring of a flux qubit using coplanar waveguide resonator
We study a flux qubit in a coplanar waveguide resonator by measuring
transmission through the system. In our system with the flux qubit decoupled
galvanically from the resonator, the intermediate coupling regime is achieved.
In this regime dispersive readout is possible with weak backaction on the
qubit. The detailed theoretical analysis and simulations give a good agreement
with the experimental data and allow to make the qubit characterization.Comment: 4 pages, 3 figures, to be published in Phys. Rev.
Elastic Mid-Infrared Light Scattering: a Basis for Microscopy of Large-Scale Electrically Active Defects in Semiconducting Materials
A method of the mid-IR-laser microscopy has been proposed for the
investigation of the large-scale electrically and recombination active defects
in semiconductors and non-destructive inspection of semiconductor materials and
structures in the industries of microelectronics and photovoltaics. The basis
for this development was laid with a wide cycle of the investigations on the
low-angle mid-IR-light scattering in semiconductors. The essence of the
technical idea was to apply the dark-field method for spatial filtering of the
scattered light in the scanning mid-IR-laser microscope. This approach enabled
the visualization of large-scale electrically active defects which are the
regions enriched with ionized electrically active centers. The photoexcitation
of excess carriers within a small volume located in the probe mid-IR-laser beam
enabled the visualization of the large-scale recombination-active defects like
those revealed in the optical or electron beam induced current methods. Both
these methods of the scanning mid-IR-laser microscopy are now introduced in
detail in the present paper as well as a summary of techniques used in the
standard method of the lowangle mid-IR-light scattering itself. Besides the
techniques for direct observations, methods for analyses of the defect
composition associated with the mid-IR-laser microscopy are also discussed in
the paper.Comment: 44 pages, 13 figures. A good oldi
Output spectrum of a measuring device at arbitrary voltage and temperature
We calculate the noise spectrum of the electrical current in a quantum point
contact which is used for continuous measurements of a two-level system
(qubit). We generalize the previous results obtained for the regime of high
transport voltages (when is much larger than the qubit's energy level
splitting (we put )) to the case of arbitrary voltages and
temperatures. When the background output spectrum is essentially
asymmetric in frequency, i.e., it is no longer classical. Yet, the spectrum of
the amplified signal, i.e., the two coherent peaks at is still
symmetric. In the emission (negative frequency) part of the spectrum the
coherent peak can be 8 times higher than the background pedestal.
Alternatively, this ratio can be seen in the directly measureable {\it excess}
noise. For and T=0 the coherent peaks do not appear at all. We relate
these results to the properties of linear amplifiers.Comment: 7 pages, 5 figures, the results generalized for arbitrary angle
between the magnetic field and the observed component of the spin, minor
corrections and typo
Spectroscopy of superconducting charge qubits coupled by a Josephson inductance
We have designed and experimentally implemented a circuit of
inductively-coupled superconducting charge qubits, where a Josephson junction
is used as an inductance, and the coupling between the qubits is controlled by
an applied magnetic flux. Spectroscopic measurements on the circuit are in good
agreement with theoretical calculations. We observed anticrossings which
originate from the coupling between the qubit and the plasma mode of the
Josephson junction. Moreover, the size of the anticrossing depends on the
external magnetic flux, which demonstrates the controllability of the coupling.Comment: Accepted for publication in PRB. 11 pages, 7 figure
Single-shot measurement of the Josephson charge qubit
We demonstrate single-shot readout of quantum states of the Josephson charge
qubit. The quantum bits are transformed into and stored as classical bits
(charge quanta) in a dynamic memory cell - a superconducting island. The
transformation of state |1> (differing form state |0> by an extra Cooper pair)
is a result of a controllable quasiparticle tunneling to the island. The charge
is then detected by a conventional single-electron transistor,
electrostatically decoupled from the qubit. We study relaxation dynamics in the
system and obtain the readout efficiency of 87% and 93% for |1> and |0> states,
respectively.Comment: submitted to Rapid Communications of Phys. Rev. B (february 2004
Structured environments in solid state systems: crossover from Gaussian to non-Gaussian behavior
The variety of noise sources typical of the solid state represents the main
limitation toward the realization of controllable and reliable quantum
nanocircuits, as those allowing quantum computation. Such ``structured
environments'' are characterized by a non-monotonous noise spectrum sometimes
showing resonances at selected frequencies. Here we focus on a prototype
structured environment model: a two-state impurity linearly coupled to a
dissipative harmonic bath. We identify the time scale separating Gaussian and
non-Gaussian dynamical regimes of the Spin-Boson impurity. By using a
path-integral approach we show that a qubit interacting with such a structured
bath may probe the variety of environmental dynamical regimes.Comment: 8 pages, 9 figures. Proceedings of the DECONS '06 Conferenc
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