arXiv subjects

J. B. Majer

Publications and source records attributed to J. B. Majer.

5 recordsLinked to original sources

arXiv2003

Spectroscopy on two coupled flux qubits

We have performed spectroscopy measurements on two coupled flux qubits. The qubits are coupled inductively, which results in a $σ_1^zσ_2^z$ interaction. By applying microwave radiation, we observe resonances due to transitions from the ground state to the first two excited states. From the position of these resonances as a function of the magnetic field applied we observe the coupling of the qubits. The coupling strength agrees well with calculations of the mutual inductance.

cond-mat.mes-hall↗

arXiv2003

Quantum ratchet effect for vortices

We have measured a quantum ratchet effect for vortices moving in a quasi-one-dimensional Josephson junction array. In this solid-state device the shape of the vortex potential energy, and consequently the band structure, can be accurately designed. This band structure determines the presence or absence of the quantum ratchet effect, as observed in the presented experiments. In particular, asymmetric structures possessing only one band below the barrier do not exhibit current rectification at low temperatures and bias currents. The quantum nature of transport is also revealed in a universal/non-universal power-law dependence of the measured voltage-current characteristics for samples without/with rectification.

cond-mat.mes-hall↗

arXiv2002

Multi-band quantum ratchets

We investigate directed motion in non-adiabatically rocked ratchet systems sustaining few bands below the barrier. Upon restricting the dynamics to the lowest M bands, the total system-plus-bath Hamiltonian is mapped onto a discrete tight-binding model containing all the information both on the intra- and inter-well tunneling motion. A closed form for the current in the incoherent tunneling regime is obtained. In effective single-band ratchets, no current rectification occurs. We apply our theory to describe rectification effects in vortex quantum ratchets devices. Current reversals upon variation of the ac-field amplitude or frequency are predicted.

cond-mat.mes-hall↗

arXiv2002

Simple Phase Bias for Superconducting Circuits

A phase-bias tool, based on a trapped fluxoid in a ring, is proposed and demonstrated. It can provide arbitrary phase values and is simple to fabricate. The phase bias has been realized in two superconducting quantum interference devices, where the critical current versus magnetic flux is shown to be shifted by a π/2 and π.

cond-mat.mes-hall↗

arXiv2001

Quantum spin chains and Majorana states in arrays of coupled qubits

Several designs of inter-qubit coupling are considered. It is shown that by a combination of Josephson and capacitive coupling one can realize qubit interactions of variable spin content. Qubit arrays are discussed as models of quantum spin chains. In particular, a qubit model of the 1D quantum Ising spin chain is proposed. A realization of unpaired Majorana fermion states in this system is considered. It is shown that Majorana states are represented by spin flip excitations localized on the chain ends. Using unpaired Majorana states in qubit chains for decoherence protected quantum computing is discussed.

cond-mat.mes-hall↗