Ben Lerner’s novel Leaving the Atocha Station follows the American poet Adam Gordon during a year in Madrid. He never quite gets around to learning Spanish, but since he has a good ear for the musicality of language he kind of manages to go with the flow and broadly understand what’s happening around him. It’s very funny and well described, anyone who ever lived abroad will get a good laugh.

I realised the other day that Adam’s feeling of fleeting in and out of understanding, very much applies to my own process of trying to get a grip of quantum mechanics. I’m reading book after book and mostly I just let the new concepts wash over me without conscious effort to make sense. It works surprisingly well. Feels a bit like what it must be for a child to pick up their first language: bath in it until the meaning and nuance of expressions have become self-evident. Or like when cartographers get their bearing by means of triangulation; once you’ve seen something from enough different angles, you start to get a feeling for its place in the bigger landscape.

It’s bound to happen however, that you stumble over a word or an expression that resists comprehension, even when its surroundings have become familiar. Quantum discord has turned out to be such a bête noire for me.

The notion was introduced some twenty years ago, by physicists who needed a way to describe how measurements of parts of a quantum system will propagate and affect the whole. While that sounds a lot like entanglement, quantum discord is distinctly different.

Different how, I cannot seem to wrap my head around.

I started out thinking that discord was simply describing a system’s gradual transition towards entanglement. This would make sense since entangled states also always exhibit quantum discord.

It’s not accurate however, quantum discord isn’t just a ‘lesser form of entanglement’, but a unique property of its own, which captures all types of quantum correlations, including those that don’t fit into the standard frame of entanglement.

Up until this point I was only aware that there was different modalities of entanglement, but not that entanglement was just one of many different types of quantum correlations.

So I end up having to explore what other types of correlations there are. Turns out that apart from entanglement there are also Einstein-Podolsky-Rosen Steering, Quantum Coherence, Bell Nonlocality and Contextuality.

So there are a number of different types of quantum correlations, and the idea of quantum discord seems to be to quantify them, specifically those that don’t fit into the categories of entanglement or nonlocality.

When forced to simplify, some experts explain it as a measurement of a certain system’s “quantumness” (I know you think there is no such word dear spell-checker, we’re in uncharted territory here, just try to relax into it).

In the end, I’m back to the explanation where I first encountered the term. In John Gribbin’s book Quantum Computing from Colossus to Qubits, quantum discord is described as a way to make mixed and pure qubits interact with each other, meaning it’s a way to not just deal with noise in a quantum system, but to actually use noise as an advantage. Gribbin notes, however, that while we have mathematical formulations for quantum discord, the intuitive understanding can be elusive.

I guess sometimes we simply have to accept the limits of our understanding.