On the Shoulders of Giants 2023

The cycle of seminars "On the Shoulders of Giants" is back, dealing with current issues, from quantum gravity to quantum computing. The event, dedicated to physics students, is organized by the Academy of Sciences in collaboration with the Italian Physics Society, the Italian Physics Students Association and the “A. Righi” and is curated by Prof. Luisa Cifarelli.

The objective of the meetings, scheduled for the months of April, May and October, is made explicit in the title of the seminars, which takes up the famous aphorism attributed to Isaac Newton: "If I have seen further, it is because I stood on the shoulders of giants".

The lectures of professors of the Academy and the University, in fact, will provide a broad and clear overview of physics today, which will allow the participants to "look into the distance". Recordings of the first two appointments are available:

- prof Paolo Rossi (UniPI) and dr Rosario Nania (INFN BO) - "History of quanta" and "Revealers of quanta"

- prof Francesco Minardi (UniBO) and dr Gabriella Bettonte (CINECA) - “Q-bits” and “Quantum computing”

Don't miss the next seminar on Thursday 19th October 2023. Prof Elisa Ercolessi (UniBO) and prof Michele Cicoli (UniBO) will talk about "From the first to the second quantum revolution" and "Quantum gravity".

Next meeting Friday 6 October 2023

Tommaso Calarco, Department of Physics and Astronomy University of Bologna:

"Quantum technologies in Europe" (Italian language)

Introduces: Luisa Cifarelli

https://site.unibo.it/accademiascienzebologna/it/agenda/sulle-spalle-dei-giganti-2

The event will take place in person at the Academy of Sciences of Bologna, via Zamboni 31.

The event will also be streamed at the following link: Virtual classroom

Recordings and seminar highlights from April and May 2023 are now available

*Highlights of the first seminar*

Quantum History - Paolo Rossi (UniPi)

The need to introduce the concept of "quantum" into physics goes back more than a century. Prof Paolo Rossi told us about the beginning of this story and how it evolved to the famous Shrodinger equation and beyond.

The first steps were slow and uncertain: the photoelectric effect, the stability of the atom and blackbody radiation were all open questions for which seemingly unrelated solutions were proposed. The photoelectric effect consists of the emission of electrons by a metal struck by electromagnetic radiation. The fundamental characteristic of the phenomenon is that it has a specific frequency threshold for each metal. That is, the phenomenon occurs only if the frequency of the radiation exceeds a certain value.

The stability of the Rutherford atom (a positively charged nucleus around which negative charges orbit) is not allowed because of radiation emission, in the context of classical electromagnetism. In fact, due to the energy loss of the system, the radius of the orbits of the negative charges would decrease until the atom collapses.

The most blackbody-like object that can be made in the laboratory is a hollow body with reflective inner walls on which a small hole is drilled. The intensity of the radiation emitted at a certain frequency is called the "spectrum." Classical theory predicts increasing intensity at higher and higher frequencies, while experimentally it tends to zero.

The photoelectric effect was explained by Albert Einstein, the atom by Niels Bohr and blackbody radiation by Max Planck. Such different physical phenomena have as their common interpretation the quantum nature of the microscopic world: the first and third phenomena are related to the absorption and emission of discrete amounts of energy and the second from the quantization of angular momentum.

These early successes of quantum theory sparked the development of suitable mathematical models and tools for representing physical quantities, particularly the latter resulting in multiples of integers. Matrices, hitherto studied mostly in mathematics, became essential to the equations of quantum physics, including Schrodinger.

Quantum Detectors - Rosario Nania (INFN BO)

Experimentally studying the microscopic quantum world is a complex challenge, Dr. Rosario Nania told us how the experimental apparatus has evolved over the past century.

A distinctive feature of the first detectors was the active participation of the physicist in "watching" with his or her own eyes the traces left by the quanta.

The electron was discovered by Thomson nl 1897 from the image formed in the fluorescent screen of a cathode ray tube. Rutherford's experiment that was discovered in an atomic nucleus in 1911 required counting small flashes of light to measure particle trajectories. The fog chamber used by Anderson in 1932 to discover antimatter (the positron) used a primitive camera to take pictures of the trail of bubbles left by the particle. The visual component of the experiment was thus an integral part of the physicist's work.

Geiger Muller counters (tube with gas and anode path) relegated the counting process to electronics. The evolution of this technology is the wire chambers (1968), which allow the detection of the trajectory of a particle along a coordinate. The accuracy of the latter detectors is a few millimetres, but in modern high-energy physics, an accuracy of a few micrometres is needed to measure particle decay vertices.

This precision is achieved by silicon detectors, which do not use gases like the previous ones but semiconductors. A charged particle passing through the device generates electron-lacuna pairs. These charges are collected by the electrodes, resulting in a signal.

*Highlights of the second seminar*

Francesco Minardi - Qubits, an intricate story.

The newest quantum computers are an extremely complex and developing application area of quantum mechanics. Prof. Francesco Minardi focused his talk on the superposition principle and entanglement.

Without delving into mathematical formalism, on an intuitive level, one can imagine a classical bit as a system with two possible states (on or off). A Qubit, on the other hand, has a state comparable to a point free to move in a sphere, a continuity of points that allows a "hybrid" state. Various technologies are currently being studied to make Qubits, such as superconducting circuits, ions, neutral atoms, and photonic circuits. The major limitations of Qubits are decoherence (the superposition of states tends to decay due to an external measurement), a much higher error rate than classical bits, and difficulty in making Qubits communicate. One advantage of Qubits is massive parallelism: because of the superposition principle, they are able to try multiple paths simultaneously while solving a problem.

In 1935 Einstein wrote with colleagues Boris Podolsky and Nathan Rosen a paper in which the three physicists claimed to have demonstrated that quantum mechanics is incomplete, that is, that there must be a deeper theory underlying it. An incomplete theory has elements of reality (physical quantities that can be measured without perturbing the system) that have no correspondence in it. Entanglement is a phenomenon particularly well suited to test quantum mechanics: if we take two particles that have interacted with each other at least once and then separate them, the operation of the measurement on the first particle will instantaneously affect the other particle at whatever distance it is from the first because of the collapse of the wave function (the states are no longer superposed). Apparently, this mechanism results in the instantaneous propagation of signals, which is not allowed by special (and general) relativity.

Bell's inequalities allow the completeness of a theory to be studied by correlations between experimentally measurable variables, the first to do so were Freeman and Clauser in 1972: Using a system of polarizers they measured the correlation between pairs of photons and concluded that quantum mechanics was a complete theory.

Gabriella Bettonte - Quantum computing.

Dr Gabriella Bettonte showed us the exponential growth of global interest in quantum computing, fueled, however, at times by misinformation about the real expectations of this computing revolution.

In fact, quantum computing outperforms classical algorithms only in a small circle of problems, particularly cryptography, optimization and simulation of quantum systems. Moreover, the study of algorithms for quantum computers has inspired new classical algorithms.

What we can expect in the near future is the use of quantum computers as "accelerators" of classical computers, but not a complete replacement. In Italy, the CINECA computing centre is studying various aspects of quantum computing and will soon have at its disposal Leonardo, one of the most powerful supercomputers in the world.

Do not miss the final 2023 lecture on Thursday 19 October 2023

Prof. Elisa Ercolessi (UniBO) and prof. Michele Cicoli (UniBO) will lecture on "From the first to the second quantum revolution" and "Quantum gravity".