One of the surprising features predicted by quantum mechanics is what is called "entanglement" between objects in different places, which Albert Einstein famously called "spooky action at a distance", and felt was so contrary to common-sense (and incompatible with his theory of gravity) that an attempt to demonstrate it experimentally would surely show that quantum theory was not correct. But when it eventually become possible to test it experimentally, quantum mechanics passed the test. In recent years it has become apparent that while Einstein's opposition to quantum theory was wrong, the property of "entanglement" that he identified as a prediction of quantum theory is not just a curiosity that philosophers can debate, but perhaps its central ingredient. It lies at the heart of the recent discoveries of "topological quantum matter" (the work for which Haldane shared the 2016 Nobel Prize for Physics), and is now viewed as a "resource" or fuel that could drive future powerful "quantum computers". Einstein felt the introduction of the "cosmological constant" in his theory of gravity was his "biggest mistake", but today it seems likely that it is related to the recently discovered "dark energy". If his disbelief in entanglement was his "second biggest mistake", it, like the first, has been a very fruitful one indeed.