Is quantum collapse soon a concept? | For your information

The transition from the quantum world to the classical world involves the collapse of several superimposed states of a system, for example an electron, into a single state. Understanding this phenomenon is one of the most difficult challenges facing physicists, yet there is no shortage of it. One theory to explain this collapse, proposed by Roger Penrose and Lajos Deuse in the 1990s, has to do with gravity. To test this, one way is to study the quantum oscillations of a mechanical resonator, which has been placed in a quantum state, and thus structured, thanks to a superconducting qubit. The problem was to tune the frequencies of these two systems to cause resonant coupling between them, in order to use the qubit to generate any quantum state of the mechanical resonator, in particular a superposition of states.

Samuel Degliese of Kastler Laboratory Brussel (CNRS-LKB) and colleagues from CEA, Inria, ENS, etc. achieved this feat by choosing to lower the frequency of the qubit. To do this, they used and optimized fluxonium, a type of qubit whose transition frequency between states is 1,000 times lower than that of typical qubits (generally several gigahertz). The basic component of this device is the Josephson junction, i.e. two superconducting layers separated by an insulator. The fluxonium qubit also includes a superconducting ring made up of hundreds of Josephson junctions, where the two states of the qubit correspond to currents flowing through the ring in a clockwise and counterclockwise direction. The transition frequency is determined by the strength of the magnetic field passing through the loop.

The fluxonium remains isolated from external disturbances, for example thermal and magnetic. To do this, the team used a cooling technique inspired by cold atom systems and adjusted the magnetic field to a very specific value so that current in the circuit flows simultaneously in two opposite directions. These “Schrödinger's cat” states are known for their resilience to magnetic field fluctuations. This method made it possible to reduce the operating frequency of the qubit by ten times. Furthermore, experimental results demonstrate the exceptional sensitivity of qubits, for detecting small quantum fluctuations in a vibrating membrane, for example. Fluoxonium is therefore ready to be used in future experiments to place a microscopic object in a superposition where it will occupy two positions at the same time, thus revealing one of the greatest mysteries of quantum physics!