Photon echoes using atomic frequency combs in Pr:YSO - experiment and semiclassical theory

Area of Science:

• Quantum optics and spectroscopy
• Solid-state quantum information processing
• Rare-earth-doped crystalline materials

Background:

• Photon echoes in rare-earth-doped crystals are crucial for developing quantum memories.
• The atomic frequency comb (AFC) protocol offers a method for broadband quantum memory.
• Hyperfine structure in rare-earth ions like Pr³⁺ complicates AFC implementation by altering spectral modulation.

Purpose of the Study:

• To investigate the challenges of implementing the AFC protocol in systems with hyperfine structure.
• To explore the intermediate regime where hyperfine spacing is comparable to comb spacing for quantum memory applications.
• To enhance spectral range for storing quantum information in Pr³⁺-doped crystals.

Main Methods:

• Preparation of broadband AFCs using optical combs with varying tooth spacings (1 MHz to 16 MHz).
• Measurement of transmission spectra and photon echoes for different comb spacings.
• Theoretical prediction using rate equation and density matrix codes, followed by semiclassical theory with a frequency-dependent dielectric function.

Main Results:

• Broadband AFCs were successfully prepared and characterized in the challenging intermediate hyperfine regime.
• Photon echo measurements provided insights into the effects of hyperfine structure on AFC protocols.
• Theoretical models (rate equation/density matrix and semiclassical) offered good, albeit distinct, accounts of the observed photon echoes.

Conclusions:

• The hyperfine structure of Pr³⁺ presents significant challenges for broadband quantum memory using the AFC protocol.
• Operating in the intermediate hyperfine/comb spacing regime is complex but potentially beneficial for broader spectral storage.
• Discrepancies between theoretical models highlight the need for further refinement in understanding these complex quantum systems.