  Relaxation after a global quench is a natural way to probe thermalization in
closed quantum systems. When a system relaxes after the quench, thermal
observables emerge in the absence of constraints, provided long-time averaging
or a large system. We demonstrate a thermalization mechanism based on averaging
the observables over realizations of a global quench protocol that does not
rely on a system's size or long-time evolution. The quench abruptly establishes
all-to-all couplings of random strength in a few-body system and initializes
the dynamics. Shortly after the quench, the observables averaged over
realizations of random couplings become stationary. The average occupation
probabilities of many-body energy states equilibrate toward the Gibbs
distribution with a finite positive or negative absolute temperature that
depends on the initial state's energy, with the negative temperatures occurring
due to the confined spectrum of the system. Running an experiment on an IBM
Quantum computer (IBMQ) for four qubits, we report the utility of the digital
quantum computer for predicting thermal observables and their fluctuations for
positive or negative absolute temperatures. Implementing thermalization on
IBMQ, this result facilitates probing the dynamical emergence of thermal
equilibrium and, consequently, equilibrium properties of matter at finite
temperatures on noisy intermediate-scale quantum hardware.
