Light-Matter interface detection of the full correlations distribution of quantum many-body systems

The last fifty years have witnessed tremendous advances in science and technology with a huge impact on society and economy leading to a new information revolution in analogy with the industrial one. Although electronic devices have reached an incredible level of complexity, control and miniaturisation, information processing relies on the same classical principles enunciated by mathematicians in the 1930s (Turing, Church, von Neumann). In the 1980s, visionary ideas from theoretical physicists, including R. P. Feynman and D. Deutsch, and later from computer scientists such as P. Shor, combining concepts from quantum mechanics led to another revolution of information technology: the birth of quantum information theory. In the classical world, a bit, the smallest unit of information, can assume values 0 or 1 corresponding roughly to an electrical circuit being open or closed. In the quantum world, instead, one deals with quantum bits or qubits, embodied for example by an electron spin or a photon polarisation. These qubits can assume the two values 0 and 1 as in the classical case but they can also be prepared in a superposition of the two values simultaneously. This, apparently shocking, property has been verified in numerous experiments and is responsible for the amazing speed-up of certain tasks like integer numbers factorisation with quantum computers, i.e. devices that process qubits in analogy with traditional computers.

So far quantum computers have only been realised with a small number of qubits-no more than ten-with trapped ions or neutral atoms, photons but also solid state devices. Large scale quantum computers are therefore expected to be realised only in a few decades.
However special purposes quantum computers, called quantum simulators are currently being produced in laboratories working with atoms at temperatures one billionth above the absolute zero (ultracold). Such experiments aim at reproducing, with a controlled environment, the physics of hard to access quantum materials, for example a high-temperature superconductor, thus allowing scientists to probe its properties and test models and theories.

A big open question for quantum simulators with ultracold atoms is how, once the sample is prepared in a quantum state, to detect its features. Several techniques are being used based on imaging through a high resolution optical microscope or on scattering of laser light off the sample. In this project we propose the use of a beam of polarised light to probe arrays of neutral atoms. As a consequence of the light-atoms interaction, the light polarisation rotates depending on the state of the atoms. Therefore the outgoing pulse of light, that can be measured, gives information about the state of the atoms.
The advantage of this scheme is that one can perform the measurement without destroying the atomic samples as in other proposals. The outcomes of this project will shed light on the intimate structure of the quantum state of many qubits embodied by atoms trapped by electromagnetic fields. For this reason, it is expected to have a strong impact not only in quantum information theory, but also in atomic physics, in statistical mechanics and in the condensed matter physics.

Qubits have another peculiarity compared to their classical counterpart: one can correlate the state of one qubit with that of another one in such a way that if one performs a measurement of the two qubits the outcomes always coincide. This phenomenon called entanglement is at the basis of quantum information applications like quantum teleportation. Another goal of this project is a proposal to entangle two of these ultracold atomic samples thus creating entanglement between two separated massive objects composed of hundreds of atoms. The scheme we propose can be implemented in the next generation of experiments with ultracold atoms.

The aim of this research is to probe degenerate quantum gases at very low temperatures using light. Thus the central topic of this proposal is intrinsically multi-disciplinary and I expect an impact for industries and for the general public as explained below.

Basic research on quantum technologies and in particular on light-matter interfaces, which is addressed in this project, are likely to have applications in the next few years for imaging and metrology and in the long term for quantum sensors, i.e. devices that, thanks to quantum correlations, are capable of achieving a better sensitivity than with traditional, "classical" devices. This will have strong impact for manufacturing nanoscopic apparatuses, navigation systems and magnetometers.

More concretely, this research will provide cutting edge training for the PDRA who will learn and develop advanced and efficient algorithms for treating many-body quantum systems. These techniques can also be employed for other purposes: quantum chemistry, in which one is able to describe approximately a hundred coherent molecular orbitals; classical statistical mechanics; simulations of classical stochastic processes for studying non-equilibrium dynamics. To achieve this level of expertise, the PDRA will become more proficient with scientific software for programming, plotting and for graphics design.

The PDRA will also receive training for writing papers and funding applications; communication, through frequent internal seminars and presentations at international events; preparing and designing websites for effective dissemination and publicity; engaging with popular press. Some of this training will be supported by specific courses at Queen's University and will contribute to the PDRA's professional development.

As there is always a continuos need to engage the public about fundamental research, especially for theoretical and less applied disciplines such as quantum theory, public engagement forms an important aspect in our proposal. I plan a rich outreach activity which includes:
1) Secondary schools work experience placements, in which a few pupils come to visit our department for a few days and discuss with members of staff their research topics and the academic, undergraduate and postgraduate, experience.
2) Non technical physics seminars for secondary schools.

Apart from this, I will organise a one-evening event orbiting around the topic: "The perception of the quantum reality". Three academics from three different disciplines, theoretical physics, chemistry and philosophy, will give short presentations about their own professional perspectives, followed by a round table with questions and comments from the audience. The aim is twofold: on one hand, to raise awareness in the society of the latest development in quantum science, and, on the other, to trigger a critical discussion of the implications of quantum theory to other subjects like, in this case, chemistry and philosophy.