Bringing quanta out of the cold

When it comes to working with atoms, or other quantum systems, some physicists prefer to keep them cold. That generally works well, but Earth is a warm place.

“We live in a heat bath,” joked Molly May, a JILA graduate student in the Raschke group.

Despite their small size, atoms have the ability to contain and carry information which makes them a promising platform for quantum sensing, metrology, and information processing. However, artificial atoms in the form of quantum dots can also encode information, and the information in those quantum dots can be controlled with light, May explained, making them attractive for these new applications.

But that gets tricky. Quantum dots and atoms tend to lose the information rapidly to their surroundings, so one has to work quickly and precisely. That typically means working in extremely cold temperatures. Extremely cold temperatures isolate the atoms, preventing them from interacting with their environment—which makes them easier to work with and control.

And if controlling and working with atoms is restricted to cold temperatures, that makes it harder to take advantage of atoms’ potential applications outside the lab, added JILA Fellow Markus Raschke. Now an advance from the Raschke group at JILA and their collaborators at the University of Maryland has brought quantum dots out of the cold, allowing them to work at room temperatures with precision and control.

Writing on a nanoscale

The Raschke group works with quantum dots that are about 8 nanometers across, more than 1000 times smaller than a human hair, which is big compared to atoms.

Writing information on a quantum dot involves changing its energy level from its ground state to an excited state. And photons—tiny packets of energy that make up light—can do that when they have a strong interaction (or are coupled) with the dot or atom.

But compared to the quantum dots or atoms, light—especially visible light—has a very long wavelength, almost a hundred times larger than the quantum dot. As a result, it has a very weak interaction with the quantum dot. Light needs to shine on the quantum dot for a long time to excite its energy level–i.e. write information on it.

However, over that time, the movement and coupling of the atom knocks its excitation out of phase, hitting other atoms and molecules, or simply internal processes. That phase decoherence causes the atom to lose its information to the surrounding environment.

There are two ways to work around this. One option is to work in high vacuum and at ultra-cold temperatures, usually around a few micro-Kelvin, to reduce the chances of this phase decoherent interaction. That requires complex cooling or refrigeration techniques in a lab, which isn’t always ideal.

The other option is to squeeze your light into a space smaller than its wavelength, ideally comparable to the size of the quantum dot. Using nanoscale antennas, e.g., made by metallic nanostructures, you can create an optical nano-cavity, squeezing the light into a smaller space. But those cavities are static. You either hit the dot or you don’t, May explained.

“Even when you get lucky and find a quantum dot, you cannot tune the cavity or control the interaction,” she added.

Some like it hot

To work at room temperature and control the quantum state, May looked to optical atomic force microscopy (AFM), a technique already used at JILA that uses a 10 nanometer needle-like gold tip, as a tool to compress the light into the tiny space needed to get strong interaction with the quantum dot.

When she shines light on the gold tip, it focuses it into a narrow region at the tip apex, creating an optical nanocavity with the quantum dot sample positioned into the near-field of the tip apex – one small enough to interact with that tiny dot.

“It works like the antenna in your phone, just for light. The tip nano-cavity collects the light that we shine on it and focuses it to a tiny spot,” May said.

The AFM now provides the advantage that the tip can move around. So May can position the tip near the quantum dot with atomic precision. Strong coupling sets in at very short distances between the tip and the quantum dot. When that happens, the tip and the dot exchange information really well, May explained, tossing photons back and forth like ping-pong ball.

“As we come closer and closer to the quantum dot, we see that the coupling kind of kicks in.” May said. “I was actually impressed by how well it works. And, that really hasn't been done before. We can really optimize the interaction.”

Seeing the results, Raschke was surprised. “At first we almost did not believe the results and thought something had gone wrong,” he said. “It was truly exhilarating when we saw the signal for the first time and we're convinced it is real.”

That strong-coupling effect using the AFM tip lets May and her fellow researchers tune, control, and image the interaction with the quantum dot in real time and space. Pull the tip back, by a distance of even just one atom diameter, and the signal gets weaker. Push it closer, and the signal gets stronger.

Free from the fridge

Being able to write and read the energy level of a single quantum dot or atom using light is an enabling step for several emerging quantum technologies, like metrology and sensing tools. And breaking out of the ultra-cold regimes could help bring these tools out of the lab for wider applications.

“Often, quantum physics and tailored light-matter interactions happen in dilution refrigerators which create ultracold, ultrahigh vacuum, very pristine conditions,” May said. “And it's attractive to think about moving away from that, because if we want to use these technologies more broadly, it's so much cheaper to be able to just do it in your lab or your office or your living room.”

It won’t get rid of the need for research and technologies at low temperature because of other important virtues, Raschke clarified. But this finding complements and expands quantum science and technology into new regimes at room temperature. But it also serves as a new tool to provide fundamental new insight into how quantum information is lost in interacting environments, which can help optimize quantum systems more broadly.

“This is exploratory research for broadening quantum technologies, creating a wider range of platforms and broader accessibility. We are bringing quantum technologies into more real world applications, which is really exciting,” he said.

Their paper was published in Science Advances on July 12, 2019.

Written by Rebecca Jacobson