HUMOR - Heisenberg Uncertainty Measured with Opto-mechanical Resonators

A minimal observable length, at least as small as the Planck length (10-35 m) is a common feature of theories that aim to merge quantum physics and gravity. HUMOR is the first experiment devoted to the design and development of a completely new way of probing space-time at extremely low dimensions.

The Science

The HUMOR experiment is based on very refined micro-mechanical oscillators, built with micro-lithography on silicon wafers (techniques similar to those that are used to build computer processors), of nanometric or micrometric thickness (thousandths or millionths of a millimeter). The shape of the oscillators is designed to best isolate them from the external environment. These are then cooled down to a few degrees above absolute zero, to limit heat induced vibrations. This way, a very high purity of the oscillation is achieved: when excited, they can vibrate more than a million times before the oscillation amplitude decreases significantly. The movement is measured with laser beams and low noise electrostatic sensors, with sensitivity to the displacement comparable to the size of the atomic nucleus. The aim is to measure how stable the oscillation period remains during the motion, even at relatively large amplitudes. Several theories that seek to unify, in a unitary description, general relativity and quantum mechanics, result in an anticipation of a change in its period when the oscillator explores in its motion larger regions of space, as if the return spring turned rigid. The experiment can measure changes even in the period of some part in a billion, before arriving at amplitudes such that the very structure of matter (the material, crystalline silicon and silicon nitride, which oscillators are built of) is stressed to the point of responding abnormally and thus masking any effects attributable to changes in quantum mechanics due to gravity. At any rate, results improve the previous upper limits to quantum gravity effects (namely, to the parameter that quantifies to what extent traditional quantum mechanics should be deformed) by many orders of magnitude. For example, the limits of the hydrogen atom, obtained with precision spectroscopy, were over 20 orders of magnitude less stringent. As a matter of fact, HUMOR is the first experiment designed specifically to study possible effects of quantum gravity, and has entered a region in which results are starting to be significant. The next challenge is to further cool an oscillator, down to less than a thousandth of a degree above absolute zero, using laser light. At this temperature, the behavior of the oscillator is markedly quantum-like (i.e., it shows features that cannot be explained by classical physics, such as the inability to be fully localized). It will thus be possible to highlight in the most direct manner any anomalies due to effects of quantum gravity. At the same time, researchers will be able to study the boundary between classical physics, normally used to describe the behavior of mechanical objects, and quantum physics, which dominates the universe at the microscopic scale of atomic distances (less than a billionth of a meter) and investigate whether, as some theories suggest, gravity plays a key role in this transition.


• Involved external institutions: CNR-IMEM (Italy), CNR-INO (Italy), FBK-CMM (Italy), European Laboratory for Non-Linear Spectroscopy - LENS (Italy), Università di Firenze – Dipartimento di Fisica e Astronomia (Italy), Università di Trento – Dipartimento di Fisica (Italy), Università di Camerino – Dipartimento di Fisica (Italy), Delft University of Technology – EKL laboratory, Delft (The Netherlands).
• INFN groups: Firenze, Perugia, TIFPA
• Principal Investigator: Francesco Marin (Universita’ di Firenze, INFN-FI, LENS)
• INFN Project: CSN II
• Duration: 6 years (begin of 2013 – end of 2018)


• Local responsible for TIFPA: Enrico Serra
• Involved TIFPA people: Michele Bonaldi, Antonio Lorenzo Borrielli, Giovanni Andrea Prodi



HUMOR Upper limits to the deformed commutator


HUMOR Experiment at a glance


HUMOR Computer-aided design drawings and SEM details of two optomechanical oscillators, with resonant frequency of the main mode at 117 kHz and 150 kHz