Often one of the limiting factors in the rate of scientific discovery is the development of instrumentation and signal processing techniques. For instance, nuclear magnetic resonance spectroscopy has been around for a long time but MRI is much newer because it had to wait for the associated technology to catch up. I am very happy to report that the field is officially closed on one particular branch of instrumentation – the ammeter.
The ammeter measures the rate of flow of electrons between two points in an electrical circuit. However, you can also think of current as the accumulation of charge at the end point of your measurement, in which case the minimum charge is one electron and the minimum current depends on how patient you are (e.g., the longer you wait the smaller the current value you measure). Thus, an ammeter that is sensitive to the passing of a single electron is as good as it gets. This is exactly what a group of researchers in Japan have achieved.
The ammeter consists of two quantum dots, which are essentially weak traps for electrons. These quantum dots are coupled together so that the electrons being counted flow from one dot to the next and then out of the system. The actual measurement is made by a second circuit which is placed in proximity to the pair of dots. This circuit has a current flowing through it which can be measured by a normal ammeter. However, the amount of charge in each dot effectively changes the resistance of the secondary circuit, resulting in a measurable change in current. This system is so sensitive that it can detect a single electron moving from one dot to the next, hence it represents the absolute pinnacle in current measurement sensitivity.
If your current job involves building a new ultra sensitive ammeter for Agilent then don't quit just yet. Although this ammeter is incredibly sensitive to low currents, I don't think it scales well to higher currents so it is only good for, well, measuring the properties of quantum dots and other really small and marvelous quantum devices. In fact, this is why the researchers built it; the more interesting (to me anyway) part of the paper was their description of the statistics of electron flow through the system, which looks remarkably like some optical systems.