 Once upon a time, I worked with a postdoc who shaped my views of mathematical physics, research, and life. Each week, I’d email him a PDF of the calculations and insights I’d accrued. He’d respond along the lines of, “Thanks so much for your notes. They look great! I think they’re mostly correct; there are just a few details that might need fixing.” My postdoc would point out the “details” over espresso, at a café table by a window. “Are you familiar with…?” he’d begin, and pull out of his back pocket some bit of math I’d never heard of. My calculations appeared to crumble like biscotti.

Some of the math involved CPTP maps. “CPTP” stands for a phrase little more enlightening than the acronym: “completely positive trace-preserving”. CPTP maps represent processes undergone by quantum systems. Imagine preparing some system—an electron, a photon, a superconductor, etc.—in a state I’ll call “\$latex rho\$”. Imagine turning on a magnetic field, or coupling one electron to another, or letting the superconductor sit untouched. A CPTP map, labeled as \$latex mathcal{E}\$, represents every such evolution.

“Trace-preserving” means the following: Imagine that, instead of switching on the magnetic field, you measured some property of \$latex rho\$. If your measurement device (your photodetector, spectrometer, etc.) worked perfectly, you’d read out one of several possible numbers. Let \$latex p_i\$ denote the probability that you read out the \$latex i^{rm{th}}\$ possible number. Because your device outputs some number, the probabilities sum to one: \$latex sum_i p_i = 1\$.  We say that \$latex rho\$ “has trace one.” But you don’t measure \$latex rho\$; you switch on the magnetic field. \$latex rho\$ undergoes the process \$latex mathcal{E}\$, becoming a quantum state \$latex mathcal{E(rho)}\$. Imagine that, after the process ended, you measured a property of \$latex mathcal{E(rho)}\$. If your measurement device worked perfectly, you’d read out one of several possible numbers. Let \$latex q_a\$ denote the probability that you read out the \$latex a^{rm{th}}\$ possible number. The probabilities sum to one: \$latex sum_a q_a =1\$. \$latex mathcal{E(rho)}\$ “has trace one”, so the map \$latex mathcal{E}\$ is “trace preserving”.

Now that we understand trace preservation, we can understand positivity. The probabilities \$latex p_i\$ are positive (actually, nonnegative) because they lie between zero and one. Since the \$latex p_i\$ characterize a crucial aspect of \$latex rho\$, we call \$latex rho\$ “positive” (though we should call \$latex rho\$ “nonnegative”). \$latex mathcal{E}\$ turns the positive \$latex rho\$ into the positive \$latex mathcal{E(rho)}\$. Since \$latex mathcal{E}\$ maps positive objects to positive objects, we call \$latex mathcal{E}\$ “positive”. \$latex mathcal{E}\$ also satisfies a stronger condition, so we call such maps “completely positive.”**

So I called my postdoc. “It’s almost right,” he’d repeat, nudging aside his espresso and pulling out a pencil. We’d patch the holes in my calculations. We might rewrite my conclusions, strengthen my assumptions, or prove another lemma. Always, we salvaged cargo. Always, I learned.

I no longer email weekly updates to a postdoc. But I apply what I learned at that café table, about entanglement and monotones and complete positivity. “It’s almost right,” I tell myself when a hole yawns in my calculations and a week’s work appears to fly out the window. “I have to fix a few details.”

Am I certain? No. But I remain positive.

*Experts: “Trace-preserving” means \$latex rm{Tr}(rho) =1 Rightarrow rm{Tr}(mathcal{E}(rho)) = 1\$.

To understand what “completely positive” means, imagine that our quantum system interacts with an environment. For example, suppose the system consists of photons in a box. If the box leaks, the photons interact with the electromagnetic field outside the box. Suppose the system-and-environment composite begins in a state defined on a Hilbert space . acts on the system’s part of state. Let denote the identity operation that maps every possible environment state to itself. Suppose that changes the system’s state while preserves the environment’s state. The system-and-environment composite ends up in the state . This state is positive, so we call “completely positive”: 