Alec Maassen van den Brink

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.

Sign and magnitude tunable coupler for superconducting flux qubits

We experimentally confirm the functionality of a coupling element for flux-based superconducting qubits, with a coupling strength

Four-Qubit Device with Mixed Couplings

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WKB analysis of the Regge-Wheeler equation down in the frequency plane

whose sign and magnitude can be tuned {\it in situ}. To measure the effective

Josephson-phase qubit without tunneling

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Mediated tunable coupling of flux qubits

, the groundstate of a coupled two-qubit system has been mapped as a function of the local magnetic fields applied to each qubit. The state of the system is determined by directly reading out the individual qubits while tunneling is suppressed. These measurements demonstrate that

Second quantization of open systems using quasinormal modes

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Jordan blocks and generalized bi-orthogonal bases: realizations in open wave systems

can be tuned from antiferromagnetic through zero to ferromagnetic.

SUSY transformations for quasinormal modes of open systems

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.

Unconventional gravitational excitation of a Schwarzschild black hole

The Regge–Wheeler equation for black-hole gravitational waves is analyzed for large negative imaginary frequencies, leading to a calculation of the cut strength for waves outgoing to infinity. In the—limited—region of overlap, the results agree well with numerical findings [Leung et al., Class. Quantum Grav. 20, L217 (2003)]. Requiring these waves to be outgoing into the horizon as well subsequently yields an analytic formula for the highly damped Schwarzschild quasinormal modes, including the leading correction. Just as in the WKB quantization of, e.g., the harmonic oscillator, solutions in different regions of space have to be joined through a connection formula, valid near the boundary between them where WKB breaks down. For the oscillator, this boundary is given by the classical turning points; fascinatingly, the connection here involves an expansion around the black-hole singularity r=0.