[ home ] [ biography ] [ research ] [ publications ] [ download ]



Postdoc 2:

Currently working on spintronic devices at Michigan State University, Department of Physics and Astronomy, experimental CMP group, under the supervision of Professors Jack Bass and Bill Pratt.

Postdoc 1:

-- direct link to: description of the flux-qubit experiment
-- see also I.Chiorescu, Y. Nakamura, C.J.P.M. Harmans, J.E. Mooij, "Quantum dynamics of a superconducting flux-qubit", published online in Science: 13 February 2003 10.1126/science.1081045.
-- visit the research web page of our group.

Our flux qubit consists of three Josephson junctions arranged in a superconducting loop threaded by an externally applied magnetic flux. Varying the flux bias controls the energy level separation of this effectively two-level system. The qubit can be engineered such that the two lowest eigenstates are energetically well separated from the higher ones.

[ flux-qubit ]
Flux-qubit (SEM picture - top) and the measured Rabi oscillations (bottom)
Click here for an animation showing the switching probability v. the microwave pulse length (the Rabi oscillations).

[ flux-qubit ]
[ qubit-squid entanglement ]
AFM picture of the flux-qubit

To fabricate the samples, we use the cleanroom facilities of the Delft Institute for MicroElectronics and Submicron Technology (DIMES). The fabrication included sample design, e-beam lithography and various chemical treatments of the sample. A typical sample contains Josephson junctions made of aluminium using the shadow evaporation technique, on-chip gold or platinium resistors, on-chip coplanar waveguides and aluminium shunt capacitors realized by strong plasma oxidation. The metals evaporation is done in high vacuum. The measurements are done in a dilution refrigerator using low noise circuitry and sample shielding.

Recently, we observed coherent time evolution between the two quantum states of our qubit. By resonant microwave pulses we manipulated the superposition of the two states carrying opposite macroscopic persistent currents. Readout by means of switching-event measurements of a SQUID revealed quantum-state oscillations with high fidelity. Under strong microwave driving we were able to induce hundreds of coherent oscillations. Pulsed measurements yielded a relaxation time of 900 ns and a free-induction dephasing time of 20 ns. Our results are very promising for the future of the solid-state quantum computing.

PhD. thesis

-- see also bibliography (download)
-- visit the research web page of the nanomagnetism group in Grenoble.

My thesis involved both numerical and especially experimental work regarding the quantum tunneling of the magnetic moment in molecular magnets. More precisely, I was interested in both differences and similarities between high spin and low spin molecular magnets (see results on the 1/2 spin molecular complex V15 or on Mn12 with S=10). Also, see a new Hall magnetometer (web page).

[ Mn12 and V15 magnetic molecules]
The Mn12 (left) and the V15 (right) magnetic molecules

The molecular magnets are formed of exchanged decoupled large magnetic molecules arranged within a crystal. The samples studied were supplied by various chemistry groups. All the molecules in a crystal have the same total spin, same anisotropy and the same controlled orientation, giving a well defined mesoscopic magnet which amplifies, at macroscopic scales, the quantum properties of a single molecule in its given environment. We studied different molecular systems such as Mn12, Fe8, V15, Mn16, Cr12 and others, among which some show spectacular phenomena.

In the high spin Mn12 molecular magnets my work was oriented towards experiments on the quantum tunneling and numerical calculations (matrix diagonalization). We showed the tunneling of the Mn12 magnetic moment in between the bottom of the two wells (ground state tunneling) in the very low temperature regime. At higher temperatures, we studied how the thermal activation influences the quantum relaxation of the molecular magnetic moment. In the low spin V15 molecular magnet the work was focused on the coupling between the total molecular spin 1/2 and phonons. This system is a two level system (spin up - spin down states) without a barrier between the levels. The spin is coupled to the environmental degrees of freedom (phonons, nuclear spins) leading to irreversibility in the spin reversal. It should be also mention here that, due to a large splitting in zero field, the dephasing arising from the nuclear spins bath is not necessarily destroying the coherent superposition of the spin up and the spin down states. This is why the V15 system could be a future candidate for the study of the macroscopic quantum coherence.