64 research outputs found
Period-doubling-bifurcation readout for a Josephson qubit
We propose a threshold detector with an operation principle, based on a
parametric period-doubling bifurcation in an externally pumped nonlinear
resonance circuit. The ac-driven resonance circuit includes a dc-current-biased
Josephson junction ensuring parametric frequency conversion (period-doubling
bifurcation) due to its quadratic nonlinearity. A sharp onset of oscillations
at the half-frequency of the drive allows for detection of small variations of
an effective inductance and, therefore, the read-out of the quantum state of a
coupled Josephson qubit. The bifurcation characteristics of this circuit are
compared with those of the conventional Josephson bifurcation amplifier, and
its possible advantages are discussed.Comment: 6 page
Numerical analysis of a three-wave-mixing Josephson traveling-wave parametric amplifier with engineered dispersion loadings
The recently proposed Josephson traveling-wave parametric amplifier (JTWPA)
based on a ladder transmission line consisting of radio-frequency SQUIDs and
exploiting three-wave mixing (3WM), has great potential in achieving both a
gain of 20 dB and a flat bandwidth of at least 4 GHz. To realize this concept
in practical amplifiers we model the advanced JTWPA circuit with periodic
modulation of the circuit parameters (engineered dispersion loadings), which
allow the basic mixing process, i.e., , where
, , and are the signal, the pump, and the idler
frequencies, respectively, and efficiently suppress propagation of unwanted
higher tones including , , , etc. The engineered
dispersion loadings allow achieving sufficiently wide dB-bandwidth from
GHz to GHz combined with a reasonably small ripple (~dB) in the
gain-versus-frequency dependence
Two-junction superconductor-normal metal single-electron trap in a combined on-chip RC environment
Dissipative properties of the electromagnetic environment as well as on-chip
RC filtering are shown to suppress random state switchings in the two-junction
superconductor(S) - normal metal(N) electron trap. In our experiments, a local
high-ohmic resistor increased the hold time of the trap by up to two orders of
magnitude. A strong effect of on-chip noise filtering was observed for
different on-chip geometries. The obtained results are promising for
realization of the current standard on the basis of the S-N hybrid turnstile.Comment: 4 pages 3 figures LT2
Single-charge escape processes through a hybrid turnstile in a dissipative environment
We have investigated the static, charge-trapping properties of a hybrid
superconductor---normal metal electron turnstile embedded into a high-ohmic
environment. The device includes a local Cr resistor on one side of the
turnstile, and a superconducting trapping island on the other side. The
electron hold times, t ~ 2-20s, in our two-junction circuit are comparable with
those of typical multi-junction, N >= 4, normal-metal single-electron tunneling
devices. A semi-phenomenological model of the environmental activation of
tunneling is applied for the analysis of the switching statistics. The
experimental results are promising for electrical metrology.Comment: Submitted to New Journal of Physics 201
Nonideal quantum detectors in Bayesian formalism
The Bayesian formalism for a continuous measurement of solid-state qubits is
derived for a model which takes into account several factors of the detector
nonideality. In particular, we consider additional classical output and
backaction noises (with finite correlation), together with quantum-limited
output and backaction noises, and take into account possible asymmetry of the
detector coupling. The formalism is first derived for a single qubit and then
generalized to the measurement of entangled qubits.Comment: 10 page
Numerical analysis of the radio-frequency single-electron transistor operation
We have analyzed numerically the response and noise-limited charge
sensitivity of a radio-frequency single-electron transistor (RF-SET) in a
non-superconducting state using the orthodox theory. In particular, we have
studied the performance dependence on the quality factor Q of the tank circuit
for Q both below and above the value corresponding to the impedance matching
between the coaxial cable and SET.Comment: 14 page
Selective quantum evolution of a qubit state due to continuous measurement
We consider a two-level quantum system (qubit) which is continuously measured
by a detector. The information provided by the detector is taken into account
to describe the evolution during a particular realization of measurement
process. We discuss the Bayesian formalism for such ``selective'' evolution of
an individual qubit and apply it to several solid-state setups. In particular,
we show how to suppress the qubit decoherence using continuous measurement and
the feedback loop.Comment: 15 pages (including 9 figures
Measurement of b Quark Fragmentation Fractions in p-pbar Collisions at sqrt(s)=1.8 TeV
We have studied the production of B hadrons in 1.8-TeV p-pbar collisions. We
present measurements of the fragmentation fractions, f_u, f_d, f_s and
f_baryon, of produced b quarks that yield B^+, B^0, B^0_s and Lambda_b hadrons.
Reconstruction of several electron-charm final states yields
f_s/(f_u+f_d)=0.213+/-0.068 and f_baryon/(f_u+f_d)=0.118+/-0.042, assuming
f_u=f_d. If all B hadrons produced in p-pbar collisions cascade to one of these
four hadrons, we determine f_u=f_d=0.375+/-0.023, f_s=0.160+/-0.044 and
f_baryon=0.090+/-0.029. If we do not assume f_u=f_d, we find
f_d/f_u=0.84+/-0.16.Comment: 16 pages, 1 figure. Submitted to Physical Review Letter
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