One of the reasons quantum mechanics is hard to understand is because it's so complicated. Objects at the quantum level behave counterintuitively compared to what can be observed at the macro-scale. However, recently, researchers have announced possibly the largest objects yet observed being governed by quantum physics.
According to Erwin Schrödinger, macro-scale objects exhibiting quantum behavior sounds absurd. Schrödinger devised the thought experiment commonly known as "Schrödinger's cat" to highlight how preposterous this particular interpretation of quantum physics is, as in one scenario, an atom having a 50-50 chance of decaying would eventually result in a cat being both alive and dead simultaneously. But since then, experiments have indeed shown particles like electrons and photons existing in multiple states simultaneously.
While there is technically no size limit in quantum physics, there are two important conditions that must be met to observe quantum behavior. The first is isolation: an isolated object can behave as quantum mechanics allows. If an object isn’t isolated, things like other matter and radiation restrict it to classical laws of physics. The second condition is energy. To exhibit quantum behavior, an object must have more energy than its environment. Light in a vacuum is a great subject for quantum study because it meets these two requirements easily. As objects get bigger and more complex, these two conditions become harder to meet.
But it is possible to meet them, and in April 2018, Nature published a paper detailing the largest objects yet to exhibit quantum entanglement. Entangled objects are somehow connected over a distance, and what happens to one has an immediate effect on the other. Using a carefully applied superconducting electrical circuit, two vibrating circular membranes were made to oscillate so similarly they could be considered in an entangled state. Although these drum-like membranes are just about the width of a human hair, they contain trillions of atoms, making them the largest objects entangled yet. Studying how entanglement might look on a larger scale has applications in many fields of research, from developing super-sensitive measurement techniques to detecting gravitational waves.