Scientists have gained new insights into the collective behavior of atoms when they interact with light. A study by researchers at the University of Warsaw and Emory University has explored how the interplay between atoms can amplify their collective light emission. This research, along with related work from Purdue University, delves into the phenomena of superradiance and subradiance, where atomic ensembles emit light at rates faster or slower than they would individually. These findings have significant implications for the development of quantum technologies, including quantum memory, entanglement generation, and quantum sensing.
The core of this research lies in understanding how the interactions between atoms in a dense ensemble alter their collective response to light. When atoms are close together, they can influence each other’s emission of photons, leading to cooperative effects. This collective behavior is not just a simple sum of individual atomic actions; instead, it gives rise to new, emergent properties. The Purdue University team, for example, studied a dense atomic ensemble coupled to a nanophotonic microring resonator, a tiny device that guides light in a loop. By trapping cold atoms in a small volume, they observed how the atoms collectively couple to both the guided light in the resonator and to the unguided light emitted into free space. This dual interaction is key to controlling and understanding the collective emission process.
Superradiance and Subradiance
At the heart of collective atomic light emission are two opposing phenomena: superradiance and subradiance. Superradiance is the enhancement of the emission rate, where the atoms in an ensemble emit light much faster than a single atom would. This is a result of the atoms’ dipoles oscillating in phase, leading to a more intense, coherent burst of light. Subradiance, on the other hand, is the suppression of the emission rate, where the collective emission is slower than that of a single atom. This occurs when the atomic dipoles are out of phase, leading to destructive interference that traps the light within the ensemble for a longer period.
The ability to control these phenomena is crucial for various quantum applications. Superradiance, with its rapid release of photons, is valuable for creating high-intensity light sources and for fast quantum communication. Subradiance, by contrast, is ideal for quantum memory, as it allows for the storage of quantum information for extended periods. The Purdue study demonstrated the ability to selectively induce both superradiance and subradiance by manipulating the state of the atomic ensemble.
Controlling Emission with Nanophotonics
The use of nanophotonic devices, such as the microring resonator in the Purdue experiment, provides a powerful tool for controlling collective emission. These devices can confine light to very small volumes, enhancing the interaction between light and atoms. In the microring resonator, the light circulates as a “whispering-gallery mode,” and the atoms are coupled to the evanescent field of this mode. By adjusting the distance of the atoms from the resonator, the researchers could tune the strength of this interaction.
This precise control allowed the researchers to study the collective emission into two distinct channels: the guided mode of the resonator and the unguided modes of free space. They found that they could achieve superradiant emission into the resonator while simultaneously observing either superradiant or subradiant emission into free space, depending on the initial state of the atoms. This selective collective emission is a significant step forward in engineering atom-light interfaces for specific quantum tasks.
Experimental Setup and Methodology
The experimental setup at Purdue University involved laser-cooling and trapping up to 60 cesium atoms in a microtrap positioned above the nanophotonic microring resonator. The atoms were prepared in a specific spin-polarized ground state and were excited by resonant laser pulses sent through a bus waveguide coupled to the microring. The emitted photons were then detected by a single-photon counting module, allowing the researchers to measure the decay rate of the collective emission.
To distinguish between the different collective emission regimes, the researchers prepared the atomic ensemble in two distinct states: a steady state (SS) and a timed-Dicke state (TDS). The steady state was achieved by using a long excitation pulse, while the timed-Dicke state was created with a short pulse. These two states exhibit different collective emission properties. The timed-Dicke state, characterized by a phase-correlated spin-wave-like excitation, is expected to be superradiant, while the steady state is expected to show subradiant characteristics.
Theoretical Modeling and Results
The experimental results were supported by a theoretical model that calculated the dynamics of the atomic dipoles interacting with both the resonator’s guided mode and the free-space radiation modes. The model predicted that the steady state would be primarily populated with subradiant eigenstates, while the timed-Dicke state would be dominated by superradiant eigenstates. These predictions were consistent with the experimental observations.
The researchers were able to measure the decay rates of the collective emission and found that the timed-Dicke state decayed faster than the single-atom rate, a clear signature of superradiance. In contrast, the steady state showed a decay rate slower than the single-atom rate, indicating subradiance. This demonstrates the ability to selectively control the collective emission by preparing the atoms in different initial states.
Implications for Quantum Technologies
The ability to control collective light emission from atomic ensembles has far-reaching implications for the development of quantum technologies. The enhanced atom-light interfaces demonstrated in these studies are crucial for applications in quantum computing, communication, and sensing.
Quantum Memory and Communication
Subradiant states, with their long lifetimes, are ideal for storing quantum information in quantum memories. By trapping light in an atomic ensemble, it is possible to create a long-lived quantum memory that can be read out on demand. Superradiant states, on the other hand, can be used to create bright, single-photon sources, which are essential for quantum communication protocols. The ability to switch between these two regimes provides a versatile toolkit for building complex quantum networks.
Quantum Sensing and Metrology
The enhanced sensitivity of atomic ensembles to light can also be harnessed for quantum sensing and metrology. The collective effects can be used to create more precise atomic clocks and sensors for measuring magnetic fields or other physical quantities. The work on collective emission in dense atomic ensembles opens up new avenues for improving the performance of these devices.
Future Directions
While the recent studies have provided significant insights into collective light emission, there are still many open questions to be explored. One area of future research is the investigation of collective emission in ordered arrays of atoms. Theoretical work suggests that ordered arrays could lead to even greater enhancements in atom-light coupling and could enable new functionalities, such as directional photon emission.
Another promising direction is the exploration of more complex nanophotonic structures to control the interaction between atoms and light. By engineering the properties of the photonic environment, it may be possible to achieve even greater control over the collective emission process. These advancements will continue to push the boundaries of quantum science and technology, paving the way for new discoveries and applications.
