Last Update: October 2000

MQC

Collaboration:

> Roma 1, Napoli

Experiment site: Roma

1. Goal of the experiment

> The MQC experiment [the acronym stands for Macroscopic Quantum Coherence) aims to study the behavior of a ``macroscopic`` system to verify if the predictions given by the standard Quantum Mechanics Theory are still valid or if the Macrorealistic view should be used to describe its time evolution. This kind of experiment has been done on a microscopic system in 1972, when A. Aspect found a violation of the Bell inequalities on a couple of polarized photons, showing the non-locality of the microscopic system.

> The natural extension to larger systems (the Schrodinger`s cat problem) was raised in 1980 by A.J. Leggett, but remained a "gedanken" experiment for more than 10 years. The experiment proposed by the Rome Group (G. Diambrini Palazzi and Carlo Cosmelli) has the goal of investigating the behavior of a two-state macroscopic system realized by means of a rf SQUID. With this system, cooled at 10 mK, we should measure Rabi oscillations between two equivalent states composed by a number of the order of 1010 pairs of electrons. The appearance of Rabi oscillations, and subsequent measurements on the system time evolution, should falsify for the first time the realistic description of a macroscopic system. It should be noted that the realization of a two level system showing Rabi oscillations is also the first step towards the implementation of a Quantum Computer, being the SQUID a qu-bit that could be easily manipulated and controlled.

2. Physics achievements

> The main experimental problem is the realization of a very low dissipative system, where the decoherence time is long enough to observe the rf SQUID`s Rabi oscillations. In this year we have tested an isolated rf SQUID cooled below 25mK observing Energy Level Quantization (ELQ) in the quantum regime. The appearance of ELQ is a clear signature of very low dissipation and with this system we have measured for the first time the intrinsic dissipation of a macroscopic system in the quantum regime (Phys. Rev. Lett.,82, 5357 (1999)). We have also tested the operation of a new type of fast laser-driven superconducting trigger giving the start time for the time measurements. We designed and tested a hysteretic dc SQUID to be used in the final experiment to realize non-invasive measurements on the flux states of the rf SQUID.

> Extensive studies have been done on the problem of isolating the system from any kind of external high frequency noise that, if not filtered, would destroy any quantum effect on the system.

3. INFN contribution to the experiment

> - Financial support:132 MLit.

4. Number of publications in refereed journals: 7

5. Number of talks to conferences: 5

6. Number of undergraduate thesis: 1

7. Leadership role in the experiment:

> - The national coordinator is G. Diambrini Palazzi from University of Rome, La Sapienza

> - The Spokesperson is C. Cosmelli from University of Rome, La Sapienza.

> - The Rome group plays a natural leading role within the collaboration for what concerns the final measurements and the data analysis on SQUID devices.

8. Innovative instruments

The MQC experiment has a particular technological significance:

> - in developing very low dissipative quantum systems.

> - in developing fast superconducting laser-driven trigger system.

> - in realizing fully integrated superconducting systems working in the quantum regime.

9. Competing experiments

> At present there is only the group of J. Lukens at Stony Brook (USA), trying to realize the same kind of measurements. The Lukens’s group at present is working on spectroscopic measurements, while we are planning time measurements. The MQC group is the only group planning to realize (through hysteretic dc-SQUID readout) the non-invasive measurements necessary to the complete set of measurements.

10. International Committee which has reviewed the experiment.

The International Committee nominated by the Commissione Scientifica II, INFN, was formed by:

J. Clarke (University of Berkeley, Berkeley, CA, USA)

A. Barone (University of Naples Politecnico, Napoli, Italy)

A. J. Leggett (University of Illinois at Urbana-Champaign, ILL, USA)

G. C. Ghirardi (University of Trieste and SISSA, Trieste, Italy)