Researchers have achieved storage times of up to 180 μs of single photons heralding quantum information within a Pr3+:Y2⁢SiO5 rare-earth ion doped crystal. This demonstration combines an atomic frequency comb spin-wave protocol with a XY4 spin-rephasing sequence, a technical approach previously elusive for absorptive quantum memories.

The work represents a step toward scalable long-distance quantum repeater links, enabling coordination between distant nodes in a future quantum internet. “Researchers demonstrate a new quantum memory device that will enable faraway nodes in a network to coordinate in time,” highlighting a key ingredient for distributed quantum computing, sensing, and secure communications.

Telecom-Heralded Single-Photon Storage Achieves 180 Microsecond Duration

The demonstrated storage of telecom-heralded single photons for up to 180 μs within a Pr3+:Y2⁢SiO5 crystal represents a new benchmark for absorptive solid-state quantum memories. The experimental setup utilized a cavity-enhanced spontaneous parametric down-conversion source and a quantum memory maintained at 3 Kelvin within a closed-cycle cryostat, enabling precise control over the quantum states. Measurements revealed cross-correlation values as high as 4.6(4) between the heralding photons and the stored signal photons, confirming the successful preservation of quantum information during the storage period.

This level of correlation signifies a robust storage process, minimizing decoherence and maintaining the integrity of the quantum state. The researchers also observed long-lived atomic frequency comb photon echoes in the classical regime, sustaining storage for approximately 3 ms, a duration exceeding that of previously reported spin-wave atomic frequency comb experiments.

This extended classical storage capability validates the underlying principles and allows for enhanced quantum storage performance. The system’s design circumvents the need for quantum frequency conversion, a complex and often lossy process, by directly utilizing moderate photon bandwidths of 2.5 MHz. This simplification streamlines the implementation of the quantum memory and improves overall efficiency.

The observed output autocorrelation remained below the single-photon bound of 0.5 for storage times under 180 microseconds, indicating the maintenance of non-classical light characteristics. “This demonstrates the ability of our system to maintain the quantum character of input single photons at storage times that are relevant for metropolitan-scale fiber links and, as such, represents an important step toward the implementation of realistic quantum networks,” the researchers write.

While the single-photon storage memory preparation sequence was optimized to minimize CP noise and maximize cross-correlations, it did so at the expense of storage efficiency. However, embedding the quantum memory within an impedance-matched cavity has previously demonstrated single-photon storage efficiencies of up to 18% using spin-wave atomic frequency comb techniques, offering a potential pathway to address this limitation.

Pr3+:Y2⁢SiO5 Crystal Enables Long-Lived Quantum Memory

This approach directly addresses a long-standing challenge in the field of quantum memory, namely the maintenance of non-classical light characteristics during storage. By generating photons directly within the narrow spectral modes of the cavity, the team achieved a biphoton linewidth of 2. 5 MHz, aligning with the 4. 6 MHz bandwidth suitable for atomic frequency comb storage in the rare-earth ion doped crystal. Filtering the idler field with a narrowband filter cavity further refined the generated state, contributing to the observed coherence.

This meticulous control over the photon generation process is critical for maintaining the integrity of the quantum information encoded within the stored photons, and represents a departure from previous emissive memory schemes. Measurements of the output autocorrelation revealed values below the single-photon bound of 0.

The observed values only reached the classical threshold of 1 at longer storage durations, confirming the successful storage of single photons with maintained quantum character. The ability to maintain this quantum character is essential for building reliable and efficient quantum networks capable of transmitting information over significant distances.

The team’s approach also allows for moderate photon bandwidth, a significant advantage over previous demonstrations that often relied on sub-MHz spectrally narrow photons, which are difficult to generate with deterministic or parametric sources. the absence of a quantum frequency conversion stage streamlines the experimental setup and reduces potential sources of noise and loss.

The researchers emphasize that this result “sets the current state of the art for single-photon storage in spin-rephased absorptive solid-state quantum memories and represents a milestone toward the accomplishment of long-distance quantum network links.” The long-lived storage achieved in this work, coupled with the compatibility with telecom wavelengths, positions this technology as a promising candidate for realizing practical quantum communication networks.

Atomic Frequency Comb Protocol Extends Storage to Milliseconds

This achievement surpasses previous demonstrations utilizing similar spin-wave atomic frequency comb techniques, extending storage capabilities significantly. This broadening, previously limiting storage times, was cancelled by inserting a pair of identical pulses between control pulses with a precise center-to-center temporal separation, as detailed in their published work. 6(4). This compatibility with telecom wavelengths is essential for integration with existing fiber-optic infrastructure, enabling potential long-distance quantum communication.

This level of correlation is vital for maintaining the integrity of quantum states during transmission and processing, a key requirement for practical quantum networks. A comb-shaped absorption profile with a periodicity of a periodicity of was prepared within the inhomogeneously broadened optical transition through spectral hole burning, allowing for storage at a predetermined time. The team imprinted this profile onto the optical transition using a spectral window of 4. 6 MHz, constrained by the excited state hyperfine splitting.

This precise spectral manipulation enabled the creation of collective excitations within the atomic ensemble, which were then mapped into and out of a long-lived hyperfine state using bright control pulses. From the decay of the signal, they extracted a spin inhomogeneous broadening value of 15. 8(1) kHz, consistent with previously published data. The ability to maintain these nonclassical correlations for extended periods addresses a critical challenge in quantum repeater designs.

In typical metropolitan fiber-optical links, a communication time of this order corresponds to approximately 25 kilometers; however, the probabilistic nature of entanglement creation over multiple links demands significantly longer storage times, typically tens of milliseconds. “Long storage times on the hundreds of microseconds to milliseconds scale have already been achieved in several types of emissive QMs and exploited in demonstrations of long-distance entanglement distribution,” the researchers note.

Their approach, utilizing a Pr3+:Y2⁢SiO5 rare-earth ion doped crystal, avoids the need for quantum frequency conversion, simplifying implementation and improving overall efficiency. The researchers emphasize that their technique allows for ultralong spin coherence times, observed in multiple rare-earth ion crystal species ranging from hundreds of milliseconds to several hours. This extended coherence is important for enabling the storage of classical light for up to one hour, single-photon-level pulses for up to 42 seconds, and, now, nonclassical correlations between collective spin excitations and single photons for one millisecond.

While previous demonstrations utilized emissive schemes, this work represents an advancement by achieving storage without quantum frequency conversion. “In the classical regime, storage of bright pulses for several milliseconds is achieved, the longest reported to date for spin-wave AFC in,” they report, highlighting the current performance of their system.

XY4 Spin-Rephasing Sequence Mitigates Decoherence

The implementation of a four-pulse XY4 sequence significantly bolsters spin coherence against imperfections in radio frequency (rf) pulses, a critical factor in maintaining quantum information storage. Unlike simpler pulse schemes, this XY4 sequence demonstrates robustness to errors in pulse area, ensuring successful maintenance of spin coherence even with less-than-ideal rf control. This resilience is particularly important as achieving perfect rf pulse calibration can be experimentally challenging, and deviations can quickly degrade the stored quantum state.

The team observed a strong suppression of triple coincidence events at zero delay, registering a value of, confirming the high purity of heralded single photons emitted by the cavity-enhanced spontaneous parametric down-conversion (cSPDC) source. This advance builds upon absorptive rare-earth ion doped crystal (REIC) quantum memories, which offer advantages including massive multiplexing capabilities, high storage efficiency, and compatibility with photonic integration.

These crystals benefit from narrowband photon-pair sources and the potential for long-lived storage within the hyperfine nuclear spin state, achieved by mitigating decoherence through spin-rephasing sequences. This combination allowed for nonclassical correlations between heralding photons and stored signal photons for up to 180 μs, with measured cross-correlation values reaching 4.6(4). The observed storage times translate to an equivalent fiber-link distance exceeding 30 kilometers, a substantial step toward practical long-distance quantum communication networks.

While storage times of up to approximately 3 ms have been achieved for classical storage and up to 180 μs for nonclassical correlations, further improvements are anticipated. They note that increasing the number of rf pulses, as in an XY8 sequence, can enhance pulse error resilience but also introduces additional noise, necessitating careful calibration. The researchers acknowledge that maximizing storage efficiency often requires minimizing correlated photon pair (CP) noise, a trade-off they have actively addressed in their experimental design.

Scalable Quantum Repeater Links Advance with Absorptive Memories

This achievement addresses a core challenge in quantum networking: maintaining the fragile quantum state of information over extended distances and durations. Most quantum network designs rely on creating entanglement between quantum memories heralded by single photons traveling through optical fibers; the stored excitation within the memory must persist for at least the round-trip travel time of the heralding photon. In typical metropolitan fiber networks operating at telecom wavelengths, this communication time is approximately 25 microseconds for a 25 kilometer link.

Analysis of storage efficiency as a function of time reveals a storage time limited by spin dephasing. The implementation of spin-rephasing sequences for single-photon storage opens the possibility of achieving ultralong storage times, important for long-distance quantum network links. The combination of high-efficiency storage and the potential for multiplexing establishes quantum memories as a leading candidate for building scalable quantum networks.

👉 More information
🗞 Long-Lived Telecom-Heralded Single-Photon Storage in an Absorptive Spin-Rephased Quantum Memory
✍️ Alberto E. Rodríguez-Moldes, Félicien Appas, Jonathan Hänni, Jelena V. Rakonjac, Samuele Grandi and Hugues de Riedmatten
🧠 DOI: http://link.aps.org/doi/10.1103/ftkb-pkvp

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