A low-noise atomic quantum memory in the Paschen-Back regime

Quantum networks have been proposed to overcome current limitations in quantum communication and computing. Through entanglement of adjacent network nodes they could allow the creation of unconditionally secure links [1] and... [ view full abstract ]

Quantum networks have been proposed to overcome current limitations in quantum communication and computing. Through entanglement of adjacent network nodes they could allow the creation of unconditionally secure links [1] and be used to create linear optical quantum computation networks [2].

A promising path to realize these networks is the heterogeneous quantum node approach [3]. Each node consists of separate, and thus individually optimizable, physical systems to generate and store single photons. We have recently demonstrated a quantum memory in warm Rb vapor with on-demand storage and retrieval compatible with photons emitted by quantum dots [4]. Using laser pulses on the single-photon level with a 660MHz linewidth, we achieved an end-to-end efficiency of 3.4(3)% for a storage time of 50ns. Quantum memories operating at such high bandwidth are crucial for applications in computing.

  We are working to improve our memory by engineering the atomic energy levels with an applied tesla-order magnetic field. In doing so, we enter the Paschen-Back regime, where the hyperfine Hamiltonian is a perturbation to the magnetic field Hamiltonian, and the atomic electron and nuclear spins decouple. Separation of ground state hyperfine sublevels larger than the optical linewidth in the warm vapor allows us to optically address each individual sublevel, and we can form a near-ideal atomic three-level system without laser cooling. We should therefore avoid loss channels such as single photon absorption, and completely suppress four-wave mixing, which is a limiting noise source in conventional high-bandwidth atomic memories.

Figure: Energy levels of the Rb D2 line at 0.8T. In the Paschen-Back regime m_J and m_I are ‘good’ quantum numbers. Optical transitions with Δm_I ≠ 0 have low probabilities, since they would require a nuclear spin flip, thus creating four isolated manifolds. By applying a CW signal and control laser of suitable polarization to the indicated transitions (see figure) we observed EIT. This constitutes a first step towards a memory in the Paschen-Back regime.

[1] L.-M. Duan et al., Nature 414, 413(2001).

[2] E. Knill et al., Nature 409, 46(2001).

[3] N. Sangouard et al., PRA 76, 050301(R)(2007).

[4] J. Wolters et al., PRL 119, 060502(2017).