Physicists have successfully utilized Doppler-shifted nuclear resonant absorbers to create a nuclear frequency comb, facilitating a quantum memory in the challenging X-ray region.
Light serves as a powerful medium for transmitting information, playing a crucial role in both traditional communication technologies and the rapidly evolving field of quantum applications, such as quantum networking and computing. However, managing light signals poses significantly greater challenges compared to handling standard electronic signals.
An international research team, featuring Dr. Olga Kocharovskaya, a renowned professor in the Department of Physics and Astronomy at Texas A&M University, has revealed an innovative method for storing and retrieving X-ray pulses at the single photon level. This idea was initially put forth in earlier theoretical research by Kocharovskaya’s team, and it holds promise for future X-ray quantum technologies.
The project, spearheaded by Professor Dr. Ralf Röhlsberger from the Helmholtz Institute Jena, was conducted using synchrotron facilities PETRA III at the German Electron Synchroton (DESY) in Hamburg and the European Synchrotron Radiation Facility in France. This work resulted in the first realization of quantum memory within the hard X-ray spectrum, as published in the journal Science Advances.
“Quantum memory is crucial for a quantum network, enabling the storage and retrieval of quantum information,” stated Kocharovskaya, a member of the Texas A&M Institute for Quantum Science and Engineering. “While photons are speedy and reliable carriers of quantum information, it’s challenging to keep them stable when their information may be needed later. A useful approach is to embed this information in a quasi-stationary medium, such as polarization or spin waves with extended coherence time, allowing it to be released again via the original photons.”
Kocharovskaya explained that although several quantum memory protocols have been created, they primarily focus on optical photons and atomic ensembles. By utilizing nuclear ensembles instead, the potential for longer memory durations increases, which can be achieved even in high-density solid-state conditions and at room temperature. This enhancement in memory time stems from the nuclear transitions’ lower susceptibility to external influences, attributed to their smaller nuclei sizes. When combined with a precise focus on high-frequency photons, these methods could pave the way for the development of compact solid-state quantum memories that last longer and cover a wider range.
“Transitioning directly from optical/atomic to X-ray/nuclear protocols poses significant challenges, sometimes rendering it impossible,” noted Dr. Xiwen Zhang, a postdoctoral researcher in Kocharovskaya’s group who was involved in the experiment and co-authored the research paper. “Thus, we proposed a new protocol based on earlier ideas.”
Zhang explained that the essence of their new protocol is straightforward concerning quantum principles. Essentially, a group of moving nuclear absorbers constructs a frequency comb within the absorption spectrum due to the Doppler frequency shift resulting from their motion. A brief pulse that matches the comb’s spectrum, absorbed by these nuclear targets, will be re-emitted with a delay reflective of the inverse Doppler shift, resulting from constructive interference among the various spectral components.
“In our current experiment, we successfully implemented this concept with one stationary and six synchronously moving absorbers, creating a seven-teeth frequency comb,” Zhang elaborated.
Zhang mentioned that the lifetime of nuclear coherence is the key factor that limits the maximum storage duration for this type of quantum memory. For instance, opting for longer-lived isomers instead of the iron 57 isotope used in the current research could yield longer memory periods.
Nevertheless, he emphasizes that operating at the single-photon level without information loss qualifies this nuclear frequency comb method as a quantum memory, marking a first in the realm of X-ray energies. The team’s future ambitions include the on-demand release of stored photon wave packets, which might result in the entanglement of different hard X-ray photons—an essential component for quantum information processing. Additionally, this research illustrates the possibility of adapting optical quantum technologies for short wavelength applications, which inherently experience less “noise” due to the averaging effects of fluctuations across numerous high-frequency oscillations.
Kocharovskaya expressed excitement about the intriguing challenges ahead and indicated that she and her team are eager to continue exploring the capabilities of their adjustable, robust, and highly flexible platform to propel advancements in quantum optics at X-ray energies in the near future.
