Broadband waveguide quantum memory for entangled photons (1009.0490v2)

Abstract: The reversible transfer of quantum states of light in and out of matter constitutes an important building block for future applications of quantum communication: it allows synchronizing quantum information, and enables one to build quantum repeaters and quantum networks. Much effort has been devoted worldwide over the past years to develop memories suitable for the storage of quantum states. Of central importance to this task is the preservation of entanglement, a quantum mechanical phenomenon whose counter intuitive properties have occupied philosophers, physicists and computer scientists since the early days of quantum physics. Here we report, for the first time, the reversible transfer of photon-photon entanglement into entanglement between a photon and collective atomic excitation in a solid-state device. Towards this end, we employ a thulium-doped lithium niobate waveguide in conjunction with a photon-echo quantum memory protocol, and increase the spectral acceptance from the current maximum of 100 MHz to 5 GHz. The entanglement-preserving nature of our storage device is assessed by comparing the amount of entanglement contained in the detected photon pairs before and after the reversible transfer, showing, within statistical error, a perfect mapping process. Our integrated, broadband quantum memory complements the family of robust, integrated lithium niobate devices. It renders frequency matching of light with matter interfaces in advanced applications of quantum communication trivial and institutes several key properties in the quest to unleash the full potential of quantum communication.

• The paper demonstrates a 5 GHz quantum memory that overcomes prior 100 MHz bandwidth limits using a thulium-doped lithium niobate waveguide.
• It employs a photon-echo protocol with atomic frequency combs to map and retrieve time-bin entangled qubits with fidelity near 1.
• The results highlight potential for scalable quantum networks and telecom integration, despite a system efficiency of approximately 0.2%.

Overview of "Broadband waveguide quantum memory for entangled photons"

The paper, authored by Saglamyurek et al., presents a significant advancement in the field of quantum communication, focusing on the reversible transfer of photon-photon entanglement into matter using a solid-state device. This research confronts the ongoing challenge in quantum communication technologies: the necessity for reliable quantum memories capable of storing entangled states of light while preserving their quantum properties.

Key Contributions

The authors successfully demonstrate a broadband quantum memory that extends the spectral acceptance bandwidth significantly beyond previous implementations. They achieve a bandwidth of 5 GHz by employing a thulium-doped lithium niobate waveguide, supplemented with a photon-echo quantum memory protocol based on atomic frequency combs (AFC). This contrasts with the previous maximum of 100 MHz, representing a substantial improvement in the capability to store entangled photon states.

In terms of experimental innovation, they map entanglement from photon pairs generated via spontaneous parametric down-conversion (SPDC) into a collective atomic excitation and back, without detectable loss of entanglement. The results include a fidelity close to 1 and measures of concurrence and entanglement of formation that support the device's efficacy in preserving quantum states through the storage process.

Experimental Implementation

The experiment uses an optical setup to generate time-bin entangled qubits, with photon pairs centered around 795 nm and 1532 nm. These photons are directed towards a quantum memory and qubit analyzers to assess the input and output states' entanglement fidelity. Critical for their success is the use of a Ti:Tm:LiNbO3 waveguide, which is cooled to 3 K and leverages controlled de- and rephasing through the application of electric fields and magnetic control. Specifically, the AFC utilized presents a 5 GHz wide comb, creating conditions suitable for broadband storage of entangled states.

Numerical Results and Analysis

The reported values, such as a fidelity of 0.95 ± 0.03 and concurrence and entanglement of formation indices consistently exceeding zero, confirm the statistical robustness of the mapping process. Furthermore, the experimental setup's system efficiency is limited to ~0.2%, attributed to fiber-coupler losses and mode overlap imperfections, which the authors suggest can be significantly improved with further optimization.

Implications and Future Directions

This research suggests practical implications for the construction of scalable quantum networks by overcoming significant barriers in the fidelity and bandwidth of quantum memories. The demonstrated compatibility with existing telecom infrastructures highlights the potential for integration into current technologies.

Future research may focus on reducing the complexity and enhancing the efficiency of solid-state quantum memories. The exploration of other rare-earth ions in lithium niobate could yield diverse wavelength storage capabilities, with the potential for extended coherence times and broader bandwidth efficiency. Additionally, experimental optimization might stem from improving mode overlap, leveraging phase-matching operations, and potentially employing spin-wave storage to encode vast amounts of quantum information over prolonged timescales.

In conclusion, this work substantiates a substantial step forward in achieving reliable quantum memories for entangled photons, enhancing both theoretical and practical foundations for quantum communication systems.