There is an art to making good measurements. In the ideal case, you want a really sensitive measuring device so that you can measure the smallest of changes to the signal you are interested in. But what you find is that most of the sensitivity is wasted, because in addition to signal, there is always noise. And for most "on the bleeding edge" experiments, the noise is huge compared to the signal.
In classical physics, there is a seemingly magical machine, called a lock-in amplifier, that extracts signal from the deepest and darkest pits of noise. But it seemed that the way quantum mechanics worked would make a quantum lock-in relatively useless. It turns out that, with a bit of clever thinking, you can make a quantum lock-in, and the resulting measurements have even more exquisite sensitivity.
Getting a lock on the lock-in amplifier
So what is this lock-in amplifier that you speak of? Basically, if you already know something about the signal that you want to extract, then you can use that to get the rest. Imagine that I have a very weak light emitter sitting in a very bright room and I want to know how bright it is. The bright light in the room is the noise and I want to see my very weak emitter. There are several possibilities that are, on the surface, very different, but they're actually all the same thing.
In this first case, I know what color to look for. Say it emits a very pure form of blue light—I design a filter that only passes that specific color of blue and look into the room through that filter. I will, with any luck, see my signal. That's because, for any given signal, the total amount of noise is nearly always much greater than the signal, but the amount of noise just in the region of interest around the signal is usually smaller than the signal. So any filter that is specific enough will allow you to see the signal.
How do you filter the signal? Typically, it's very small and changes slowly, putting it right in the worst of the noise. The first step is to shift the signal to a higher frequency, which we do by multiplying it with some pure oscillating source. Now we are looking for very small changes on a high frequency signal, where it is still drowned in the surrounding noise.
To extract the signal, we multiply it again, this time with a second oscillator with the same frequency. This has two effects. First, the oscillator can be very strong, so it acts as a sort of amplifier, making the signal stronger. The second is that we can now just look at the part of the signal that's changing very slowly. All the noise, apart from the tiny component that happened to have the same frequency and phase as the oscillator, gets eliminated.
The key to the success of the lock-in is the ability to multiply electronic signals together. That might not seem very difficult, but in fact, it's a nonlinear operation. For everything apart from semiconductor-based electronics, nonlinear operations are inefficient and difficult; for quantum mechanics, they appear to be impossible, because, on the face of it, quantum mechanics is entirely linear.
The linearity, however, comes about because there is a lot hidden in a single mathematical symbol (H, for the record). That symbol represents all the interactions of a quantum system with its environment. For most purposes, you choose your interactions so that they eliminate as much nonlinearity as possible.
What the team of Israeli researchers realized was that if you choose the environment correctly—in this case, a series of radio frequency pulses and laser light fields—then you can do the equivalent of a multiplication. The key is that you have to choose your oscillators so that they do not commute with the signal—commute is the mathematical term that refers to the fact that certain pairs of quantities, like position and momentum, cannot both be measured to arbitrary precision. Once you have a multiplication, the war is won and lock-in amplification nirvana is within your reach.
In a bit more detail, the researchers used a single strontium ion as their sensor. The ion is placed into a superposition of two different spin states (the spin state is the orientation of the ion's magnetic field). In response to the noise in the surrounding magnetic field, the probability of finding the spin in either up or down oscillates randomly a very small amount. The researchers added a tiny oscillating signal to that noise, and then went through the lock-in process.
Recall that we need two multiplication processes. To do this, the researchers chose to modulate a spin state with a series of radio frequency pulses that invert the probabilities of spin up and spin down. This process was then applied twice—once to shift the measurement away from the noise and a second time to retrieve the signal, which is the spin state. Therefore any particular measurement is random, and you must repeat this many times to calculate the probability of spin up and spin down to retrieve the actual signal.
To test this, they tuned their lock-in right through the noisiest place in the spectrum: the 50Hz line, where the stray fields from the electrical mains are the bane of every physicist's existence. They used these lines to quantify just how good their system is. Then, just to show off, they used the same system to measure the frequency shift of a laser as it tuned through one of the resonances of the strontium ion.
Just how good is this thing? Try 13pT/Hz1/2 good. Basically, that says that at 1Hz you can see changes that are on the order of 10-12T. For reference, the Earth's magnetic field is 1 million times stronger than this. But the important part of this research is not the numbers—experiments always start off crappy and get better, so this will improve until it hits the quantum limit—it's the fact that this is a generic instrument. It can probe anything that interacts electromagnetically. Sure, it is a rather complicated instrument, but no more so than the atomic clocks that we shoot into space at regular intervals, so it will, I think, find itself in widespread use in a fairly short time.