Light is an excellent carrier of information used not only for classical communication technologies but increasingly also for quantum applications such as quantum computing. However, processing light signals is far more difficult compared to the more common electronic signals. In experiments at DESY´s brilliant X-ray source PETRA III and at the ESRF, an international team of researchers has demonstrated how X-ray pulses can be stored and released in a novel way that could be applicable for future X-ray quantum technologies. The results are published the journal Science Advances.

In quantum computing, much like in classical computing, various processes must be synchronized to each other. For instance, in a multi-thread process, each thread must wait for the others to finish before the process can continue. This renders a memory device essential, where the quantum information, so called qubits, can be stored and released at predetermined times without any loss of information. In optical quantum computing, qubits are encoded in the various degrees of freedom of photon wave packets, such as their polarization state, number of photons, or waveform. Storing such a wave packet without losing its quantum information is a major challenge for optical quantum applications since photons are notoriously difficult to control.

Typically, the problem is solved by transferring the quantum information to a long-lived state of a matter system. Various protocols have been established that can force the matter system to re-emit the photon wave packet at a predetermined time allowing the qubit to be read out. One particularly appealing protocol uses a “frequency comb” structure. In this method, the absorption spectrum of the matter system features a series of evenly spaced atomic resonances, manifesting as absorption lines that correspond to the teeth of the comb. When a photon wave packet gets absorbed by such a comb structure, it simultaneously excites all resonances. The photon energy is now stored in such resonances and will then be released again. In the case of a frequency comb, the subsequent emission of the wave packet has a remarkable property: The photons are emitted only at moments in time when all transitions radiate in phase so that constructive interference between all transitions occurs. These moments are determined by the energy spacing of the comb lines. Thus, by preparing a certain frequency comb state in the matter system before the arrival of the photon wave packet, the re-emission of the photon wave packet can be well controlled to occur at predetermined time instants (nearly) without any information loss.

This “quantum memory” protocol has been demonstrated for visible light using strong laser sources to create a frequency comb in an atomic absorption spectrum. This gets increasingly more difficult for light with shorter wavelengths because light sources are much weaker at X-ray energies. “In our paper, we overcome this problem by using a novel approach to form a frequency comb,” explains Ralf Röhlsberger from the Helmholtz Institut Jena and DESY, who conceived the experiment. DESY´s researcher Sven Velten lead author of the study and a researcher in the excellence cluster “Centre of Ultrafast Imaging CUI”, adds: “Instead of an atomic transition, we utilize the nuclear transition of the isotope ⁵⁷Fe at an energy of 14.4 kilo electronvolts corresponding to a wavelength of 86 picometres, thus, at X-ray energies”. Nuclear transitions feature extremely narrow energy linewidths. The transition of ⁵⁷Fe, for example, has an energy linewidth of 5 nano electronvolts, which is nearly 13 orders of magnitude smaller than its transition energy. For these narrow transitions, Doppler shifts caused by mechanical motions can substantially shift the transition. Therefore, the research team used mechanical motions to form a frequency comb structure by using multiple moving absorber foils. After excitation of all foils with a short X-ray pulse, they emit an X-ray wave packet in the same direction after a short time. This coherent packet carries all phase information without any loss. The delay time of the release can be adjusted by the speed of the movement of the absorber foils in relation to each other. Thus, the “nuclear frequency comb” allows re-emission of X-ray photon wave packets at well controlled instants of time while preserving the waveform of the wave packet nearly undistorted.

The synchrotron pulses used in these experiments contained at most one photon, which meets the resonance conditions and thus being able to be stored in the absorber foil system. The ability to work on a single-photon level without loss of information qualifies the nuclear frequency comb as a quantum memory – a first for X-ray energies. It highlights the potential for applying quantum technologies at short wavelengths regimes where devices can be more compact and flexible while operating at room temperature. The nuclear frequency comb also allows for the formation of “time-bin” waveforms, a specific type of photonic qubit. Manipulating and controlling X-ray wave packets at the single-photon level opens up intriguing possibilities for technical applications such as excitation and detection of ultra-narrow nuclear transitions, as well as advancing the field quantum optics at X-ray energies.

Reference
Nuclear quantum memory for hard X-ray photon wave packets, Sven Velten, Lars Bocklage, Xiwen Zhang, Kai Schlage, Anjali Panchwanee, Sakshath Sadashivaiah, Ilya Sergeev, Olaf Leupold, Aleksandr I. Chumakov, Olga Kocharovskaya, Ralf Röhlsberger, Science Advances 26 June 2024, DOI: 10.1126/sciadv.adn9825