Very special diamonds are being experimented on at TU Wien (Vienna): Their crystal lattice is not perfectly regular; it contains numerous defects. In places where one would usually expect two neighboring carbon atoms, there is one nitrogen atom and an empty place without any atom. Microwaves can be used to switch these defects between two different states – a higher energy state and a lower energy state. This makes them an interesting tool for various quantum technologies, such as novel quantum sensors or devices for quantum computers.

Now it has been possible to control these defects so precisely that they can be used to trigger a spectacular effect: All defects are brought into the high-energy state, in which they remain for some time, until the release of all the energy is triggered with a tiny microwave pulse and all defects simultaneously change to the low-energy state - similar to a snowfield on which a tiny snowball triggers an avalanche and the entire mass of snow thunders down into the valley at the same time.

Atomic spins and microwaves

"The defects in the diamond have a spin – an angular momentum that points either up or down. These are the two possible states they can be in," says Wenzel Kersten, first author of the current publication, who is currently working on his dissertation in the research group of Prof. Jörg Schmiedmayer (Vienna Center for Quantum Science and Technology / TU Wien).

With the help of a magnetic field, one can create an energy difference between these two states. For example, the state "spin up" may correspond to a higher energy than "spin down". In this case, most atoms will be in the "spin down" state. They normally move towards the low energy state, like a ball in a bowl that normally rolls downward.

But with some clever engineering tricks, it is possible to create a so-called "inversion" – the defects all settle into the higher-energy state. "First, microwave radiation is used to bring the spins into the desired state, then the external magnetic field is changed in such a way that the spins remain frozen in this state," explains Prof. Stefan Rotter (Institute for Theoretical Physics, TU Wien), who led the theoretical part of the research.

Such an "inversion" is unstable. In principle, the atoms could spontaneously change their state – similar to a balancing broomstick, which in principle can spontaneously tip over in any direction. But the research team was able to show: Extremely precise control, made possible by chip technology developed at TU Wien, can keep the atoms' spins stable for about 20 milliseconds. "By quantum physics standards, that's a huge amount of time. That's about a hundred thousand times as long as it takes to create this high-energy state or to discharge it. That's like having a cell phone battery that is charged within an hour and then holds all of its energy for ten years," says Jörg Schmiedmayer.

Tiny cause - big effect

During this time, however, it is possible to bring about the change of state in a targeted manner – all it takes is a very weak trigger, such as a microwave pulse of minimal intensity. "It causes an atom to change its spin, which makes neighboring atoms change their spin too, creating an avalanche effect. All the energy is released, in the form of a microwave pulse that is about a hundred billion times stronger than the one used to trigger the effect originally," explains Stefan Rotter. "In proportion, that is as if a single snowflake were to trigger a snow slab weighing several hundred tons."

This offers many interesting possibilities: It is possible to amplify weak electromagnetic pulses in this way, for example; the effect could be used for special sensors or to create a kind of "quantum battery" in which a certain amount of energy can be stored at the quantum level and released in a targeted manner.

Original publication

W. Kersten et al., Triggered Superradiance and Spin Inversion Storage in a Hybrid Quantum System, Phys. Rev. Lett. 131, 043601, opens an external URL in a new window

Contact

Prof. Jörg Schmiedmayer
Vienna Center for Quantum Science and Technology (VCQ)
TU Wien
+43 1 58801 141888
schmiedmayer@AtomChip.org

Dipl.-Ing. Wenzel Kersten
Vienna Center for Quantum Science and Technology (VCQ)
TU Wien
+43 1 58801 141867
wenzel.kersten@tuwien.ac.at

Prof. Stefan Rotter
Institute for Theoretical Physics
TU Wien
Wiedner Hauptstraße 8–10, 1040 Wien
+43 1 58801 13618
stefan.rotter@tuwien.ac.at