Ph.D., Joseph Fourier University, Grenoble, France, 2000
Following a BSc. from UAIC-Iasi, Romania Dr. Chiorescu obtained his PhD. on molecular quantum magnets at the Joseph Fourier University, Grenoble, France (PhD. 2000). He then carried out a 3-year postdoctoral research program at the Delft University of Technology, Netherlands with results in the superconducting quantum computing area. A one year postdoc at Michigan State University followed, where he worked on a high-frequency setup to study spin torque devices. Since 2005, he has been an Assistant Professor at FSU & NHMFL and Principal Investigator of the group "Quantum Spin Dynamics and Nanotechnologies". The research activities are done both in the FSU campus (nanofabrication facilities) and NHMFL (lab location).
Dr. Chiorescu's research interests lie in the area of quantum effects at nanoscopic scale. His group's main research direction focuses on quantum computation and involves studies of spin dynamics in two possible cases: magnetic molecules and superconducting flux qubits. Both systems are two level systems characterized by measurable magnetic moments and consequently the physics and the necessary equipment are relatively similar. Temperatures as low as 10 mK are suitable to explore the lowest energy levels of a quantum system. Control over the energy levels will be achieved by combining high frequency electromagnetic fields (f<50GHz) with static magnetic fields. Their experimental laboratory is located in the C-wing of the National High Magnetic Field Laboratory. The central piece is a dilution refrigerator with high cooling power which together with a superconducting vector magnet allows low temperature studies of magnetic samples in complex magnetic field configurations. The setup is attached to a table fitted with vibration isolators and lowered below the ground level. An important part of the research is done in the nanofabrication facilities located in the FSU campus, at Martech institute.
In the recent years the field of molecular magnetism has known a remarkable growth. Right from the beginning these systems were recognized as potential nanosized structures for information storage. Aside from the technological potential, these structures are test-beds for quantum mechanics at macroscopic scale. Each molecule is formed of magnetic ions strongly coupled by exchange such that the ground state of the molecule is described as a single spin. Usually these systems show an anisotropy barrier against spin's free rotation. However, the first studies on such systems have shown the existence of the quantum tunneling of the molecular spin. This was possible because, though in crystalline form, the molecules are relatively well isolated from each other (wherefrom the name of Single Molecular Magnets-SMM). Thus the quantum physics of their spin is not strongly affected by the collective nature of the sample. Also, the moderate size of the spin S (from ½ to 10-20) combined with large values of the anisotropy barrier ensures energy splitting between the 2S+1 levels larger than kT in a typical dilution refrigerator. At present, literally hundreds of magnetic molecules can be synthesized in chemistry labs around the world, in well defined crystals.
The molecular magnets have demonstrated key quantum effects - like quantum tunneling of the magnetization and Berry phase interference - originating from their mastered structure. Numerous EPR and magnetostatic studies revealed important details of the tunneling process. Nevertheless, very little is known about the dynamics of a spin during tunneling and if it can be brought into a coherent regime (i.e. coherent magnetic resonance in a tunneling window). Such experiments are needed in order to prove the potential implementation of the magnetic molecules as quantum bits. It is worth noting that a natural advantage of the molecular spins comes from the easiness of constructing a large number of identical qubits by chemical methods. Studies done up to now indicate relatively short coherent times in molecular systems, of the order of nanoseconds, in comparison with other qubit implementations like the superconducting systems. A way to restrain the effects of the thermal bath is to operate in the phonon bottleneck regime. The goal of our research program is to achieve coherent control of the total spin of a molecular magnet. We propose to use microwave pulses in resonance with the energy levels of the molecule (qubit) to induce Rabi rotations. Also, we will investigate a second method consisting of applying fast, non-adiabatic variations of the external bias of a qubit. Quantum oscillations between the two levels of the qubit should be observable in both cases.
Superconducting flux qubits
As solid-state qubits, superconducting systems are ahead of the field at the present time. They have mesoscopic sizes and can be elaborated in clean rooms on tiny chips. They show relatively large coherence times, of the order of microseconds and are generally described by a two-level Hamiltonian. Thus, they can also be seen as man-made spins that can be manipulated by an external bias. Quantum oscillations have been observed in the Cooper pair box (charge) qubit using the charge degree of freedom and conditional gate operations using such two coupled charge qubits have been presented. In addition, a qubit based on the phase degree of freedom in a Josephson junction was demonstrated and a hybrid charge-phase qubit was proven. The PI of the QSD group performed experiments in the area of superconducting quantum computation during a postdoc at TU Delft, Netherlands where he worked on the so-called flux qubit consisting of three Josephson junctions arranged in a superconducting loop. Single-qubit operations are currently achieved in these systems. Significant progress still has to be done to increase the coherence time though the main decoherence mechanisms are relatively well studied. A different approach to realize quantum gates with flux qubits could result from and uses the qubit-squid entanglement. The flux qubit is a two-level system, which is coupled to the SQUID measurement system that detects the quantum states. Simultaneously, the SQUID is an effective inductance which in parallel with an external shunt capacitance acts as a harmonic oscillator. At TU Delft, we achieved generation and control of the entangled state by performing Rabi oscillations between coupled |qubit, oscillator> states. We plan to achieve complex manipulation of entangled states such as demonstrated in ion/atom-trap experiments or cavity quantum electrodynamics but with superconducting devices.