Achieving Quantum Memory in the Challenging X-ray Range

Light is an exceptional medium for information transmission, utilized for classical communication technologies and increasingly for quantum networking and computing. However, processing these light signals is considerably more complex compared to conventional electronic signals. An international team of researchers, including Dr. Olga Kocharovskaya from Texas A&M University, has achieved a breakthrough in storing and releasing X-ray pulses at the single-photon level. This concept, initially theorized by Kocharovskaya's team, shows promise for future X-ray quantum technologies.

The research was spearheaded by Helmholtz Institute Jena's Dr. Ralf Röhlsberger and executed at the PETRA III synchrotron sources in Hamburg and the European Synchrotron Radiation Facility in France. This work marks the inaugural realization of quantum memory in the hard X-ray spectrum, with findings published in Science Advances.

According to Kocharovskaya, quantum memory is a crucial component of quantum networks, allowing storage and retrieval of quantum information. Although photons serve as rapid and durable carriers of quantum information, their stationary holding is challenging. A practical method involves imprinting the information into a quasi-stationary medium, such as a long-coherence-time polarization or spin wave, which can later re-emit the original photons.

Established protocols for quantum memories focus on optical photons and atomic ensembles. However, Kocharovskaya notes that using nuclear ensembles offers significantly longer memory times, even at high solid-state densities and room temperatures, due to the lesser sensitivity of nuclear transitions to external field perturbations.

Dr. Xiwen Zhang, another key participant and co-author, explains that extending optical/atomic protocols to X-ray/nuclear scales is typically impossible. Instead, a new protocol was suggested, involving a set of moving nuclear absorbers creating a frequency comb in the absorption spectrum due to Doppler shifts. A short pulse matching this comb is re-emitted with delay, thanks to constructive interference among different spectral components.

This concept was successfully tested with a combination of one stationary and six moving absorbers forming a seven-teeth frequency comb. However, nuclear coherence lifetime remains the limiting factor for maximum storage time. Using longer-lived isotopes than iron-57 could extend memory duration.

Operating at a single-photon level without information loss, this nuclear frequency comb protocol qualifies as the first quantum memory for X-ray energies. The team's further objectives include the on-demand release of stored photon wave packets, paving the way for entanglements between multiple hard X-ray photons—essential for quantum information processing.

Their research underscores the potential of expanding optical quantum technologies to short wavelengths, which are less "noisy" due to averaging fluctuations over many high-frequency oscillations.

Kocharovskaya and her collaborators eagerly anticipate advancing quantum optics at X-ray energies, exploring possibilities with their versatile and robust platform.

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