E. Il’ichev

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 tanks 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.

Anomalous Periodicity of the Current-Phase Relationship of Grain-Boundary Josephson Junctions in High-Tc Superconductors

The current-phase relation (CPR) for asymmetric 45\ifmmode^\circ\else\textdegree\fi{} Josephson junctions between two d-wave superconductors has been predicted to exhibit an anomalous periodicity. We have used the single-junction interferometer to investigate the CPR for these kinds of junctions in

Low noise, low power consumption high electron mobility transistors amplifier, for temperatures below 1 K

{\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}

Radio-frequency based monitoring of small supercurrents

thin films. A remarkable amplitude of the

Characterization of superconducting structures designed for qubit realizations

\ensuremath{\pi}

Consistency of ground state and spectroscopic measurements on flux qubits

-periodical component of the CPR has been experimentally found, providing an additional source of evidence for the d-wave symmetry of the pairing state of the cuprates.

Radio-frequency method for investigation of quantum properties of superconducting structures

Low noise three-stage pseudomorphic high electron mobility field-effect transistors amplifier were designed for the temperature range below 1 K. A minimum noise temperature TN≈100 mK was measured at an ambient temperature of about 380 mK at frequencies between 1 and 4 MHz for a source resistance of 10 kΩ. The gain of the amplifier was 50 at a power consumption of about 200 μW. The noise parameters of the amplifier are stable to within 30%, for a power consumption in the range of 100–300 μW. Minimum voltage spectral noise density of the amplifier with respect to the input is about 200 pV/Hz1/2 and the corner frequency of the 1/f noise is close to 300 kHz.

Quantum behavior of a flux qubit coupled to a resonator

We consider the applicability of the established rf readout technique, which allows to obtain the supercurrent-phase relation of a Josephson element from impedance measurements in the phase-biased regime. In experiments on Nb-based single and double tunnel junctions, we demonstrate that this method holds even if the Josephson coupling energy is smaller than the thermal energy. Compared with conventional current–voltage measurements, we evaluate the rf technique to be favorable in particular for investigations of small supercurrents in low-capacitance Josephson elements.

Reading out the state inductively and microwave spectroscopy of an interferometer-type charge qubit

We implement the impedance measurement technique in order to characterize superconducting structures designed for applications in quantum computing. We report an experimental study of the circuit consisting of three small Josephson junctions incorporated in a low-inductance superconducting loop. Measurements of the circuit response to an applied weak ac bias as a function of the external magnetic flux φe yield complete information on the properties of the circuit. We found that the system displays two metastable states. From experimental data, we have determined the magnetic field dependent Josephson energy U(φe) of these states, the potential barrier ΔU between these states, as well as the difference of the critical currents of the junction in the loop.

Method for direct observation of coherent quantum oscillations in a superconducting phase qubit

We compare the results of ground state and spectroscopic measurements carried out on superconducting flux qubits which are effective two-level quantum systems. For a single qubit and for two coupled qubits we show excellent agreement between the parameters of the pseudospin Hamiltonian found using both methods. We argue that by making use of the ground state measurements the Hamiltonian of N coupled flux qubits can be reconstructed as well at temperatures smaller than the energy level separation. Such a reconstruction of a many-qubit Hamiltonian can be useful for future quantum information processing devices.