19 research outputs found
Direct Josephson coupling between superconducting flux qubits
We have demonstrated strong antiferromagnetic coupling between two
three-junction flux qubits based on a shared Josephson junction, and therefore
not limited by the small inductances of the qubit loops. The coupling sign and
magnitude were measured by coupling the system to a high-quality
superconducting tank circuit. Design modifications allowing to continuously
tune the coupling strength and/or make the coupling ferromagnetic are
discussed.Comment: REVTeX 4, 4 pages, 5 figures; v2: completely rewritten, added
finite-temperature results and proposals for ferromagnetic galvanic couplin
Metastable Voltage States of Coupled Josephson Junctions
We investigate a chain of capacitively coupled Josephson junctions in the
regime where the charging energy dominates over the Josephson coupling,
exploiting the analogy between this system and a multi-dimensional crystal. We
find that the current-voltage characteristic of the current-driven chain has a
staircase shape, beginning with an (insulating) non-zero voltage plateau at
small currents. This behavior differs qualitatively from that of a single
junction, which should show Bloch oscillations with vanishing dc voltage. The
simplest system where this effect can be observed consists of three grains
connected by two junctions. The theory explains the results of recent
experiments on Josephson junction arrays.Comment: 5 pages, 4 figures include
Four-qubit device with mixed couplings
We present the first experimental results on a device with more than two
superconducting qubits. The circuit consists of four three-junction flux
qubits, with simultaneous ferro- and antiferromagnetic coupling implemented
using shared Josephson junctions. Its response, which is dominated by the
ground state, is characterized using low-frequency impedance measurement with a
superconducting tank circuit coupled to the qubits. The results are found to be
in excellent agreement with the quantum-mechanical predictions.Comment: REVTeX 4, 5pp., 7 EPS figure files. N.B.: "Alec" is my first, and
"Maassen van den Brink" my family name. v2: final published version, with
changed title, different sample micrograph, and several clarification
Low-frequency characterization of quantum tunneling in flux qubits
We propose to investigate flux qubits by the impedance measurement technique
(IMT), currently used to determine the current--phase relation in Josephson
junctions. We analyze in detail the case of a high-quality tank circuit coupled
to a persistent-current qubit, to which IMT was successfully applied in the
classical regime. It is shown that low-frequency IMT can give considerable
information about the level anticrossing, in particular the value of the
tunneling amplitude. An interesting difference exists between applying the ac
bias directly to the tank and indirectly via the qubit. In the latter case, a
convenient way to find the degeneracy point in situ is described. Our design
only involves existing technology, and its noise tolerance is quantitatively
estimated to be realistic.Comment: 6 pages, 11 figures, to appear in Phys.Rev.
Theory of weak continuous measurements in a strongly driven quantum bit
Continuous spectroscopic measurements of a strongly driven superconducting
qubit by means of a high-quality tank circuit (a linear detector) are under
study. Output functions of the detector, namely, a spectrum of voltage
fluctuations and an impedance, are expressed in terms of the qubit spectrum and
magnetic susceptibility. The nonequilibrium spectrum of the current
fluctuations in the qubit loop and the linear response function of the driven
qubit coupled to a heat bath are calculated with Bloch-Redfield and rotating
wave approximations. Backaction effects of the qubit on the tank and the tank
on the qubit are analyzed quantitatively. We show that the voltage spectrum of
the tank provides detailed information about a frequency and a decay rate of
Rabi oscillations in the qubit. It is found that both an efficiency of
spectroscopic measurement and measurement-induced decoherence of the qubit
demonstrate a resonant behaviour as the Rabi frequency approaches the resonant
frequency of the tank. We determine conditions when the spectroscopic
observation of the Rabi oscillations in the flux qubit with the tank circuit
can be considered as a weak continuous quantum measurement.Comment: 28 page
Low-frequency measurement of the tunneling amplitude in a flux qubit
We have observed signatures of resonant tunneling in an Al three-junction
qubit, inductively coupled to a Nb LC tank circuit. The resonant properties of
the tank oscillator are sensitive to the effective susceptibility (or
inductance) of the qubit, which changes drastically as its flux states pass
through degeneracy. The tunneling amplitude is estimated from the data. We find
good agreement with the theoretical predictions in the regime of their
validity.Comment: REVTeX4, 3pp., 3 EPS figures. v2: new sample, textual clarifications.
v3: minor polishing; final, to appear in PRB Rapid
A Fully Quantum Mechanical Model of a SQUID Ring Coupled to an Electromagnetic Field
A quantum system comprising of a monochromatic electromagnetic field coupled
to a SQUID ring with sinusoidal non-linearity, is studied. A magnetostatic flux
is also threading the SQUID ring, and is used to control the
coupling between the two systems. It is shown that for special values of
the system is strongly coupled. The time evolution of the system is
studied. It is shown that exchange of energy takes place between the two modes
and that the system becomes entangled. A second quasi-classical model that
treats the electromagnetic field classically is also studied. A comparison
between the fully quantum mechanical model with the electromagnetic field
initially in a coherent state and the quasi-classical model, is made.Comment: 7 pages, 9 figures. Uploaded as implementing a policy of arXiving old
paper