Physicists can better study the quantum behaviour of objects on the atomic scale.
Erwin Schrödinger was an interesting man. Not only did he conceive a most imaginative way to (theoretically) kill a cat, he was in a constant state of superposition between monogamy and not. He shared a household with one wife and one mistress. (Although he got into trouble at Oxford for this unconventional lifestyle, it didn’t pose a problem in largely Catholic Dublin.) Just like the chemist Albert Hofmann, who tried LSD (lysergic acid diethylamide) on himself first, Schrödinger might have pondered how it would feel for a person to be in a genuine state of quantum superposition. Or even how a cat might feel.
In principle, quantum mechanics would certainly allow for Schrödinger, or any of us, to enter a state of quantum superposition. That is, according to quantum theory, a large object could be in two quantum states at the same time. It is not just for subatomic particles.
Everyday experience, of course, indicates that big objects behave classically. In special labs and with a lot of effort, we can observe the quantum properties of photons or electrons. But even the best labs and greatest efforts are yet to find them in anything approaching the size of a cat.
Could they be found? The question is more than head-in-the-clouds philosophy. One of the most important experimental questions in quantum physics is whether or not there is a point or boundary at which the quantum world ends and the classical world begins.
A straightforward approach to clarifying this question is to experimentally verify the quantum properties of ever-larger macroscopic objects. Scientists find these properties in subatomic particles when they confirm that the particles sometimes behave as a wave, with characteristic peaks and dips. Likewise, lab set-ups based on the principle of quantum interference, using many mirrors, lasers and lenses, have successfully found wave behaviour in macromolecules that are more than 800 atoms in size.
Other techniques could go larger. Called atom interferometers, they probe atomic matter waves in the way that conventional interferometers measure light waves. Specifically, they divide the atomic matter wave into two separate wave packets, and recombine them at the end. The sensitivity of these devices is related to how far apart they can perform this spatial separation. Until now, the best atomic interferometers could put the wave packets about 1 centimetre apart.
In this issue, physicists demonstrate an astonishing advance in this regard. They show quantum interference of atomic wave packets that are separated by 54 centimetres. Although this does not mean that we have an actual cat in a state of quantum superposition, at least a cat could now comfortably take a nap between the two branches of a superposed quantum state. (No cats were harmed in the course of these experiments.)
Making huge molecules parade their wave nature and constructing atom interferometers that can separate wave packets by half a metre are extraordinary experimental achievements. And the technology coming from these experiments has many practical implications: atom interferometers splendidly measure acceleration, which means that they could find uses in navigation. And they would make excellent detectors for gravitational waves, because they are not sensitive to seismic noise.
Schrödinger was more of a philosopher than an engineer, so it is plausible that he would not have taken that much interest in the practical ramifications of his theory. However, he would surely have clapped his hands at the prospect that experimenters could one day induce large objects to have quantum properties. And there are plenty of proposals for how to ramp up the size of objects with proven quantum behaviour: a microscopic mirror in a quantum superposition, created through interaction with a photon, would involve about 1014 atoms. And, letting their imaginations run wild, researchers have proposed a method to do the same with small biological structures such as viruses.
To be clear, science is not close to putting a person or a cat into quantum superposition. Many say that, because of the way large objects interact with the environment, we will never be able to measure a person’s quantum behaviour. But it’s Christmas, so indulge us. If we could, and if we could be aware of such a superposition state, then how would we feel? Because ‘feeling’ would amount to measuring the wave function of the object, and because measuring causes the wave function to collapse, it should really feel like, well, nothing — or perhaps just a grin.
Related links in Nature Research
Quantum superposition at the half-metre scale 2015-Dec-23
Quantum physics: Entanglement beyond identical ions 2015-Dec-16
Measuring entanglement entropy in a quantum many-body system 2015-Dec-02
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Quantum leap. Nature 528, 435–436 (2015). https://doi.org/10.1038/528435b