Researchers Develop Hybrid Memory Using Gradient Echo and EIT for Reversible Light-atom Mapping

Researchers are continually seeking ways to improve quantum memories, essential components for building future quantum networks, and a team led by Stanisław Kurzyna, Mateusz Mazelanik, and Wojciech Wasilewski from the University of Warsaw now presents a novel hybrid approach. Their work demonstrates a quantum memory that combines two distinct methods, Gradient Echo Memory and Electromagnetically Induced Transparency, to reversibly map information between light and atomic coherence. This innovative combination allows for flexible conversion between different spectro-temporal modes of light, offering a versatile tool that could significantly enhance the interoperability of quantum networks. Furthermore, the protocol opens exciting possibilities for fundamental studies of atomic coherence, including investigations into Rydberg polaritons and the mapping of single Rydberg excitations and ionic impurities.

By leveraging the complementary strengths of GEM and EIT, the team realises time-to-frequency and frequency-to-time conversion mechanisms for spectro-temporal modes. This approach allows for manipulation of quantum information stored within the atomic ensemble, offering potential advantages for quantum information processing and communication. This approach leverages the complementary strengths of each protocol to perform both time-to-frequency and frequency-to-time conversions, offering a versatile tool for advanced applications like quantum networks. The core of the system relies on manipulating the interaction between light and atoms within an ensemble, enabling precise control over the storage and retrieval of optical information. To implement this hybrid memory, scientists utilize the GEM protocol, employing a magnetic field gradient to spatially encode the frequency components of an incoming optical signal within the atomic cloud.

The frequency shift at any given point within the cloud is directly proportional to its position and the strength of the applied gradient, effectively creating a spatial map of the input signal’s frequency spectrum. This storage mechanism relies on the principle of inhomogeneous broadening, where the magnetic field induces variations in the atomic energy levels, allowing for frequency-selective storage. To read out the stored information, the magnetic field gradient is reversed, unwinding the phase acquired during storage and reconstructing the original optical pulse, now mirrored in time. Complementing the GEM protocol, researchers harnessed the power of EIT to achieve a different form of mapping.

By exploiting the extremely high electric susceptibility under two-photon resonance, they dramatically slowed the group velocity of light propagating through the atomic medium, creating a “slow-light polariton”. This spatial compression effectively maps the temporal profile of the input pulse onto the position within the memory. Stopping the light completely is achieved by switching off the coupling beam, halting the pulse within the atomic ensemble. The group velocity, and therefore the degree of spatial compression, is precisely controlled by parameters like optical depth and the Rabi frequency of the coupling beam.

By combining these two procedures, storing light using GEM and reading it out using EIT, or vice versa, scientists demonstrate the ability to perform frequency-to-time and time-to-frequency mapping. Numerical simulations confirm the efficacy of this approach, demonstrating the faithful reconstruction of the initial pulse after storage and retrieval, and validating the precise control over both temporal and spatial encoding of optical information. This innovative methodology opens new avenues for manipulating and processing quantum information with unprecedented precision and flexibility.

Light and Atomic Coherence Reversibly Mapped

Researchers have demonstrated a novel hybrid memory system that seamlessly integrates gradient echo memory (GEM) and electromagnetically induced transparency (EIT) protocols, achieving reversible mapping between light and atomic coherence. This breakthrough allows for the conversion of spectro-temporal modes, effectively translating information between time and frequency domains, and offering a versatile tool for enhancing network interoperability. The system utilizes the complementary strengths of both GEM and EIT to manipulate light propagation within an atomic cloud of rubidium-87 atoms, achieving an optical depth of 80 with an ensemble temperature of 80 μK. Experiments reveal that by storing a pulse in GEM and reading it out with EIT, or vice versa, the system can precisely control the timing of light signals based on their frequency.

When the bandwidth of the stored pulse is smaller than the memory bandwidth, different frequencies experience distinct delays due to variations in propagation distance through the atomic medium, resulting in a clear separation of signals in time. Conversely, when the pulse bandwidth matches the memory bandwidth, the system stores light across the entire cloud, minimizing frequency-dependent delays and preserving signal integrity. These findings are corroborated by numerical simulations that closely match the experimental data. To further demonstrate the system’s capabilities, researchers successfully reversed the storage and readout protocols, storing light using EIT and reading it out with GEM.

This confirms the reversibility of the process and highlights the potential for dynamic control of light signals. By manipulating the storage time and applying magnetic gradients, the team demonstrated that the width of a pulse in the time domain during EIT storage directly corresponds to its width in the frequency domain during GEM readout. Data collected from 200 experimental sequences, coherently averaged to preserve phase information, confirm the precision of this mapping. Specifically, the team successfully separated two frequencies by 1 MHz, demonstrating the system’s ability to resolve and manipulate closely spaced signals. By leveraging the complementary properties of these techniques, the team successfully converted spectro-temporal modes of light, effectively mapping frequency to time and vice versa. This capability offers potential benefits for applications such as enhancing interoperability in communication networks and implementing optical time-of-flight spectroscopy. The experiments confirmed theoretically predicted coherence mapping, including the storage of multi-frequency pulses and the reversal of the mapping process, demonstrating control over both spectral and temporal characteristics of the stored light.

While the current experimental parameters successfully realize the protocol, the authors acknowledge that increasing the density of the atomic ensemble and improving the memory bandwidth would enhance efficiency and delay times. Future work may focus on utilizing this hybrid memory for the tomography of Rydberg polariton propagation and mapping single Rydberg impurities, potentially without the need for camera-based detection. Data supporting the findings has been deposited in a public repository to ensure reproducibility.