Superradiant emission from colour centres in diamond - a flash of light from a solid-state hybrid quantum system

Background:

According to the laws of quantum physics, atoms can be in different states. "When an atom absorbs energy, it is in the so-called excited state. When it returns to its lower (ground) energy state, the energy is released again in the form of a photon. This usually happens by accident, at a completely unpredictable time" explains Johannes Majer, a research group leader at the Atomic Institute of the Vienna University of Technology.
However, if several atoms are close together, an interesting quantum effect can occur: one of the atoms emits (spontaneously and randomly) a photon, thereby affecting all other excited atoms in its neighborhood. Many of them also release their excess energy at the same moment, producing an intense flash of light. This phenomenon is known as "superradiance". Unfortunately, this effect is not directly observable with ordinary atoms. Super radiance is only possible if you collect the atoms in an area that is significantly smaller than the wavelength of the photons to be emitted. This means you have to focus the atoms into a space significantly less than 100 nanometers cubed. In this regime the interactions between the atoms are so strong that the super radiant effect may no longer be possible.

Achievement:

One way around this is to use a quantum system: tiny defects built into diamonds. While ordinary diamonds consist of a regular grid of carbon atoms, lattice defects were deliberately incorporated into the diamonds. At certain points, instead of a carbon atom, there is a nitrogen atom, and the adjacent point in the diamond lattice is unoccupied. These lattice-targeted diamonds were made in Japan by Junichi Isoya and his team at the University of Tsukuba. They have succeeded in producing the world's highest concentration of these desired defects without causing other damage. Just like ordinary atoms, these diamond defects can also be excited - but with photons in the microwave range which have a very long wavelength. Our hybrid system has the distinct advantage that we can work with electromagnetic radiation that has a wavelength of a few centimeters. This makes it straightforward to place all the individual NV centers within a one wavelength radius.
Now if you excite all of this NV centers (trillions of them in our case) it usually takes hours for them all to relax their ground (lowest energy) state. However, this happens within a few hundred nanoseconds due to the superradiance effect. The first photon that is sent out spontaneously causes all other defect sites to emit photons as well. We observe a superadiant flash of microwave light.

Experimental details

Our hybrid system consisted of a 3D lumped element resonator coupled to the dense NV ensemble contained in the diamond sample (Fig 1). The resonator confines the magnetic field such that the electron spins are homogeneously coupled to the cavity mode with almost no spatial dependence on the coupling rate. Operating in the fast cavity limit, a microwave pulse excites the NV centers electron spins. This is followed several hundred nanosecond later by a flash of microwave light. At the same time, optical measurements on the ensemble show the sudden decay of all the NV centers (Fig 2). Our theoretical calculation showed good agreement with the experimental results, proving that the observed phenomenon can be well explained by the superradiant effect.

Technical Features

• Superradiance is inherently a nonlinear effect of both the intensity of the emitted microwave pulse and its emission time in terms of the number of electron spins present in the ensemble. This scaling needs to be confirmed. It is extremely difficult however to exactly determine the number of spins in different samples. The diamond lattice however possesses four different crystallographic orientations which results in four different possible directions for the NV center to be in. In the ensemble we have roughly one quarter in each direction and so by applying appropriate magnetic fields we can tune 1,2,3 or 4 orientations into resonance with the resonator meaning we can vary the number of spins from N to 2N to 3N to 4N (even without knowing N exactly). Owing to this breakthrough, the nonlinear scaling was successfully demonstrated for the first time.
• It is critical in any superradiant experiment to know how the state of the electron spin ensemble is evolving in time. We make use of the optical transition of the NV center to perform optical fluorescence measurements. Here a green laser pulse optically excites a specific transition of the ensemble and we measure the scattered red light to determine the population of the ensemble.

Outlook:

Superradiance is based on the same basic principle as the laser - there is the stimulated emission of photons triggered by a photon which strikes energetically excited atoms. Nevertheless, these are two quite different phenomena: In the laser a background of many photons is constantly needed to stimulate new atoms while in superradiance, a single emisstion triggers the flash of light all by itself. The demonstration here is a first step in the development of many potential solid-state quantum technologies including the maser (a laser at microwave wavelength).

Publication information

"Superradiant Emission from Colour Centres in Diamond"
Andreas Angerer, Kirill Streltsov, Thomas Astner, Stefan Putz, Hitoshi Sumiya, Shinobu Onoda, Junichi Isoya, W. J. Munro, Kae Nemoto, Jorg Schmiedmayer, and Johannes Majer
Nature Physics (2018)

Glossary

*1) Superradiance:

Superradiance is the phenomenon where N local emitters interact with a common light field. If the wavelength of the light is larger than the separation of the emitters, then those emitters interact with the light in a collective and coherent fashion resulting in a high intensity light pulse.

*2) Nitrogen vacancy centers:

The negatively charged nitrogen-vacancy center is a paramagnetic point defect centre in diamond which consists of a nitrogen atom replacing a carbon atom next to an adjacent lattice vacancy.

*3) Hybrid Quantum System:

A hybrid Quantum system is a composite device composed of two distinct subsystems which in principle should have better properties than the individual systems alone. In our case, it is an ensemble of electron spins (from negatively charged nitrogen-vacancy centers) coupled to a 3D lumped element microwave resonator.