For the First Time, Researchers Spy on the Undisturbed Quantum World
Seeing the unseeable.
Update: An earlier version of the post stated the study below is to appear in the journal Nature Photonics this week. This was incorrect and resulted from an erroneous press release. The post has been ammended.
Here’s an unpleasant thought: part of our everyday reality, the most important part arguably, is forever shielded from our observation. There is a door to this realm that is locked and our ability to know what’s inside that door is entirely derived from the effects those insides have on everything else. This is the world of the very, very small, existing on scales so minute that any effort to observe it with the tools of our macroscopic world results in a disruption of the particle world. That's an uncomfortable notion.
We can and have, nonetheless, created an absolutely amazing model/understanding of the quantum world (the world of the very, very small), but to actually watch it in any conventional sense of “watching” an electron in orbit is to disrupt the electron. Think about what it is to observe something: it involves a physical entity interacting with that something. For example, a photon (light particle) making a trip from the observer to the electron/atom/very small thing or the other way around.
That photon necessarily has an interaction with the very small thing being observed, changing it. So now we’re not observing what we wanted to observe, but instead this brand new thing that is a product of the observed and the observation. This is the basic problem of the “standard quantum limit”: looking without touching. Now, as detailed in a paper posted to the arVix pre-print servier, researchers at the Institute of Photonic Sciences (ICFO) claim to have beat the limit, seen the unseeable.
Their achievement has a cool name: a “quantum non-demolition measurement.” This just means what I described above. The team looked at a cloud of atoms, observing the whole thing down to its constituent electrons, without disruption anything. Basically, they found a way to pick the lock for the first time in history, granting humans access to the quantum world in a way never before possible. It’s a pretty big deal. Among other things, that lock-picking could be taken fairly literally, as it implies the possibility of eavesdropping on a quantum communication system.
The process sounds fairly simple, conceptually speaking. Basically, you take two measurements of the system to be observed using pulses of light, and, within their QNDM technique, the effects cancel. “A first measurement gives us information reflecting the action of the first light pulse,” says the IFCO’s Dr. Robert Sewell. “A second measurement, taken with photons in a complementary state from the first, cancels the influence of the preliminary pulse, allowing us to observe the original characteristics of the object."
There are actually two achievements that made this lock-picking possible. The first is transferring all of the measurement noise from the observations into one variable, which could then be set aside. The second: being able to measure correlations among different atoms in one go of it, rather than needing a whole collection of measurements. So, this is a highly practical accomplishment. "This experiment provides rigorous proof of the effectiveness of quantum physics for measuring delicate objects," Sewell adds.
This is also philosophically interesting. The guts of atom are a key part of arguments about scientific realism. It’s a debate that’s been raging primarily in the years since the quantum universe came to be known, along with the notion that there is a substrate of particles with different and strange rules that we can’t observe directly. But they govern reality.
On the one hand, realists argue that correct models and theories about a thing make the thing real, while anti-realists maintain that only the models and theories, as tools, are actually real. And the unobserved remains unreal until observed. Here’s a bit from a University of Michigan backpage:
The central issue is this: Do scientific theories and hypotheses refer to real but unobservable entities, forces, and relations? Or should we interpret theories and hypotheses as convenient systems through which to summarize the empirical regularities of observable entities and processes, with the apparent reference to unobservables as simply a facon de parler with no greater significance than the imagined can opener in the classic joke about the economist and the accountant?
You can probably guess which of those is more favored with science, particularly given science’s long history of validating theories, eventually, through observation--and given science’s history of using aspects of the quantum world as real-world utility. After all, there is nothing particularly elusive about an atomic explosion in a major urban center, is there? Anyhow, you probably shouldn’t spend too much time worrying about that whole debate as more and more theory tumbles into the realm of observation (gravity or dark matter, for example). Just imagine it isn’t real.
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