164 research outputs found
Low frequency Rabi spectroscopy for a dissipative two-level system
We have analyzed the interaction of a dissipative two level quantum system
with high and low frequency excitation. The system is continuously and
simultaneously irradiated by these two waves. If the frequency of the first
signal is close to the level separation the response of the system exhibits
undamped low frequency oscillations whose amplitude has a clear resonance at
the Rabi frequency with the width being dependent on the damping rates of the
system. The method can be useful for low frequency Rabi spectroscopy in various
physical systems which are described by a two level Hamiltonian, such as nuclei
spins in NMR, double well quantum dots, superconducting flux and charge qubits,
etc. As the examples, the application of the method to a nuclear spin and to
the readout of a flux qubit are briefly discussed.Comment: 4 pages, 3 figures, the figures are modifie
Radio-Frequency Method for Investigation of Quantum Properties of Superconducting Structures
We implement the impedance measurement technique (IMT) for characterization
of interferometer-type superconducting qubits. In the framework of this method,
the interferometer loop is inductively coupled to a high-quality tank circuit.
We show that the IMT is a powerful tool to study a response of externally
controlled two-level system to different types of excitations. Conclusive
information about qubits is obtained from the read-out of the tank properties.Comment: 10 pages, 10 figures;to be published in Fizika Nizkikh Temperatur
(Low Temperature Physics); v3: minor polishing; fina
Realization of a classical counterpart of a scalable design for adiabatic quantum computation
We implement a classical counterpart of a scalable design for adiabatic
quantum computation. The key element of this design is a coupler providing
controllable coupling between two bistable elements (in our case
superconducting rings with a single Josephson junction playing the role of a
classical counterpart of superconducting flux qubits) The coupler is also a
superconducting ring with a single Josephson junction that operates in the
non-hysteretic mode. The flux coupling between two bistable rings can be
controlled by changing the magnetic flux through the coupler. Thereby, the
coupling can be tuned from ferromagnetic trough zero to to anti-ferromagnetic.Comment: 3 pages, 3 figures v2: extended discussion experimental result
Coexistence of multi-photon processes and longitudinal couplings in superconducting flux qubits
In contrast to natural atoms, the potential energies for superconducting flux
qubit (SFQ) circuits can be artificially controlled. When the inversion
symmetry of the potential energy is broken, we find that the multi-photon
processes can coexist in the multi-level SFQ circuits. Moreover, there are not
only transverse but also longitudinal couplings between the external magnetic
fields and the SFQs when the inversion symmetry of potential energy is broken.
The longitudinal coupling would induce some new phenomena in the SFQs. Here we
will show how the longitudinal coupling can result in the coexistence of
multi-photon processes in a two-level system formed by a SFQ circuit. We also
show that the SFQs can become transparent to the transverse coupling fields
when the longitudinal coupling fields satisfy the certain conditions. We
further show that the quantum Zeno effect can also be induced by the
longitudinal coupling in the SFQs. Finally we clarify why the longitudinal
coupling can induce coexistence and disappearance of single- and two-photon
processes for a driven SFQ, which is coupled to a single-mode quantized field.Comment: 11 pages, 6 figure
Evidence for entangled states of two coupled flux qubits
We have studied the low-frequency magnetic susceptibility of two inductively
coupled flux qubits using the impedance measurement technique (IMT), through
their influence on the resonant properties of a weakly coupled high-quality
tank circuit. In a single qubit, an IMT dip in the tank's current--voltage
phase angle at the level anticrossing yields the amplitude of coherent flux
tunneling. For two qubits, the difference (IMT deficit) between the sum of
single-qubit dips and the dip amplitude when both qubits are at degeneracy
shows that the system is in a mixture of entangled states (a necessary
condition for entanglement). The dependence on temperature and relative bias
between the qubits allows one to determine all the parameters of the effective
Hamiltonian and equilibrium density matrix, and confirms the formation of
entangled eigenstates.Comment: 4 pages, 4 figures, final versio
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