A quantum watch can tell time without counting it. This stopwatch-like method of observing quantum experiments is a deceptively simple 鈥� and always accurate 鈥� way to measure the passage of time.
at Uppsala University in Sweden and her colleagues based the clock on a type of experiment called a pump-probe experiment. In these tests, a 鈥減ump鈥� laser pulse is sent into a cloud of atoms, raising them to higher energy levels, and then another, less powerful 鈥減robe鈥� pulse is used to measure the effect of the pump.
These experiments are crucial for many applications in materials science, notably the development of solar panels, but it can be difficult to measure how much time passes between the pump and the probe. This quantum watch, made with helium atoms, solves that problem.
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鈥淚t鈥檚 a completely new way of measuring time,鈥� says, also at Uppsala University and part of Berholts鈥檚 team. 鈥淲ith other clocks, you try to improve timekeeping by making it more and more complex. But in our case, we actually go the complete opposite direction 鈥� we use basically the simplest possible structure that could tell time.鈥�
The researchers first fired a laser beam into a cloud of helium atoms. This put the atoms into a superposition of quantum states, meaning that they were in multiple energy levels at once. These energy levels interact with one another 鈥� like how, in the famous double-slit experiment, a single photon can go through two openings at the same time 鈥� and create an interference pattern that changes over time.
When the researchers measured that interference pattern for as little as 1.7 trillionths of a second, they could compare it to simulations of the interference and find the unique slice of time where the patterns matched, telling them precisely how long the helium atoms had been in the superposition. The non-repeating nature of the interference pattern made it easy to prove without a doubt that the timing was accurate.
The benefit of this is that, unlike with a traditional clock, there is no need to measure exactly when the atoms were put into a superposition. 鈥淚f you鈥檙e using a counter, you have to define zero. You start counting at some point,鈥� says Berholts. 鈥淭he benefit of this is that you don鈥檛 have to start the clock 鈥� you just look at the interference structure and say 鈥榦kay, it鈥檚 been 4 nanoseconds.鈥欌��
鈥淸This is] a very elegant experiment that will be very useful in what they call pump-probe experiments,鈥� says at the University of Queensland in Australia. It won鈥檛 be useful in measuring time more generally, he says, but in experiments where all that needs to be measured is a delay between two times, it could be extraordinarily accurate.
These experiments are so ubiquitous when studying small-scale systems that change quickly that an improvement in accuracy could be crucial. This could allow researchers to make extraordinarily fast measurements of systems that change over time, such as a single molecule falling apart, quantum interactions between light and matter or a material being exposed to a magnetic field.
Physical Review Research
