The precise measurement of time delays and colour differences is the core of many modern technologies, including spectroscopy and radar. Research conducted by John Donohue, Senior Manager of Scientific Outreach at the Institute for Quantum Computing (IQC), is using quantum-inspired techniques to achieve a new level of precision of measurement.
Donohue was working as a postdoctoral fellow in Professor Christine Silberhorn's Integrated Quantum Optics group at Paderborn University in Germany from 2017 to 2018. The research team, along with collaborators from Palacky University and the Complutense University of Madrid, have demonstrated a new method for measuring and characterizing pulses of light in time with improved precision.
The key to their technique is a homegrown waveguide device called a quantum pulse gate, developed at Paderborn University. This small chip contains and guides light like an optical fibre but has some rather grand capabilities beyond that.
When a laser beam is focused to a small point, it quickly expands back to its original size. By trapping it in a waveguide, laser beams and photons can be tightly focused down to a few millionths of a metre wide and kept that way for a few centimetres.
"Effects that might be difficult to explore by focusing into crystals and other optics become much more efficient and easier to explore (with the waveguides)," Donohue says.
Where physically measuring the distance between two objects is not possible, like stars in space, we must infer it in an indirect way. One way to infer and estimate how far apart two objects or signals are is by using an optical timing measurement to detect pulses of light. However, this can be incredibly difficult to do if those pulses don't share coherence.
"If two pulses are so close together that they bleed into each other, I can't easily separate which is which. I'd need a ton of information to precisely answer the simple problem of how much time is between them,” Donohue says. “In quantum language, to get more information, we need to measure more photons. So how can we get this information while measuring as few photons as possible?”
Donohue knows that with a different set of measurements, inspired by quantum information research, he can get rid of this problem. This will allow him to accurately obtain information through the projection of specific modal shapes.
Taking inspiration from work done with spatial optics, the group applied this thinking to measurement in time. Rather than being interested in how far apart two objects are, they were interested in measuring when two events occur relative to each other.
This is where the waveguide devices proved paramount.
The device is capable of decomposing pulses by their shape, rather than their time of arrival. Instead of asking when the pulse arrives, the device asks the pulse which specific superposition of times it could have arrived at. By decomposing the pulse into these shapes, properties such as the time difference and relative intensities of the pulses can be estimated efficiently.
In their experimental demonstration, the research group focused on how precisely they were able to estimate these parameters per photon measured. They found that their technique was a significant improvement over the existing best possible standard measurement.
Donohue hopes that these tools can be used and applied to quantum information, such as to measure pulse-shape entanglement between photons and study quantum communication.
"While these tools are very useful in classical optics, studying them with an eye on quantum information opens many new doors."
Achieving the Ultimate Quantum Timing Resolution was published in PRX Quantum January 4, 2021.