Dipole blockade without dipole-dipole interaction

The dipole blockade phenomenon is a direct consequence of strong dipole-dipole interaction, where only single atom can be excited because the doubly excited state is shifted out of resonance. The corresponding two-body entanglement with non-zero concurrence induced by the dipole blockade effect is an important resource for quantum information processing. Here, we propose a novel physical mechanism for realizing dipole blockade without the dipole-dipole interaction, where two qubits coupled to a cavity, are driven by a coherent field. By suitably chosen placements of the qubits in the cavity and by adjusting the relative decay strengths of the qubits and cavity field, we kill many unwanted excitation pathways. This leads to dipole blockade. In addition, we show that these two qubits are strongly entangled over a broad regime of the system parameters. We show that a strong signature of this dipole blockade is the bunching property of the cavity photons which thus provides a possible measurement of the dipole blockade. We present dynamical features of the dipole blockade without dipole-dipole interaction. The proposal presented in this work can be realized not only in traditional cavity QED, but also in non-cavity topological photonics involving edge modes

Breaking the energy-symmetry blockade in magneto-optical rotation

The magneto-optical polarization rotation effect has prolific applications in various research areas spanning the scientific spectrum including space and interstellar research, nano-technology and material science, biomedical imaging, and sub-atomic particle research. In nonlinear magneto-optical rotation (NMOR), the intensity of a linearly-polarized probe field affects the rotation of its own polarization plane while propagating in a magnetized medium. However, typical NMOR signals of conventional single-beam $\Lambda-$scheme atomic magnetometers are peculiarly small, requiring sophisticated magnetic shielding under complex operational conditions. Here, we show the presence of an energy-symmetry blockade that undermines the NMOR effect in conventional single-beam $\Lambda-$scheme atomic magnetometers. We further demonstrate, both experimentally and theoretically, an inelastic wave-mixing technique that breaks this NMOR blockade, resulting in more than five orders of magnitude ($>$300,000-fold) NMOR optical signal power spectral density enhancement never before seen with conventional single-beam $\Lambda-$scheme atomic magnetometers. This new technique, demonstrated with substantially reduced light intensities, may lead to many applications, especially in the field of bio-magnetism and high-resolution low-field magnetic imaging