Researchers have demonstrated electromagnetically induced transparency (EIT) in an ultracold sodium atom gas, achieving light pulse propagation speeds of 17 m/s, which is 20 million times slower than the speed of light in a vacuum. The experiment involved cooling sodium atoms to nanokelvin temperatures using laser and evaporative cooling, resulting in a high atomic density and enabling the quantum interference effects that control the optical properties of the medium. The EIT effect was achieved by using a coupling laser beam at a right angle to a probe laser beam, creating a quantum interference that allows light pulses to pass through an otherwise opaque medium. The study reports an inferred nonlinear refractive index of 0.18 cm²/W and highlights the exceptional optical nonlinearities of the system, which have potential applications in quantum optics. The experiment involved cooling a sodium atom cloud to temperatures below the Bose-Einstein condensation (BEC) transition temperature, resulting in a high-density atomic cloud. The atoms were then subjected to a coupling laser beam tuned to a specific transition, creating a quantum interference that allowed the probe laser beam to pass through the cloud with minimal absorption. The results show that the light speed decreases with increasing atom density, and the lowest light speed of 17 m/s was achieved in a cloud with a high condensate fraction. The study also discusses the implications of the results for quantum optics, including the potential for optical switching at the single photon level and the compression of light pulses in the z-direction. The research provides insights into the behavior of light in ultracold atomic gases and has potential applications in quantum information processing and nonlinear optics.Researchers have demonstrated electromagnetically induced transparency (EIT) in an ultracold sodium atom gas, achieving light pulse propagation speeds of 17 m/s, which is 20 million times slower than the speed of light in a vacuum. The experiment involved cooling sodium atoms to nanokelvin temperatures using laser and evaporative cooling, resulting in a high atomic density and enabling the quantum interference effects that control the optical properties of the medium. The EIT effect was achieved by using a coupling laser beam at a right angle to a probe laser beam, creating a quantum interference that allows light pulses to pass through an otherwise opaque medium. The study reports an inferred nonlinear refractive index of 0.18 cm²/W and highlights the exceptional optical nonlinearities of the system, which have potential applications in quantum optics. The experiment involved cooling a sodium atom cloud to temperatures below the Bose-Einstein condensation (BEC) transition temperature, resulting in a high-density atomic cloud. The atoms were then subjected to a coupling laser beam tuned to a specific transition, creating a quantum interference that allowed the probe laser beam to pass through the cloud with minimal absorption. The results show that the light speed decreases with increasing atom density, and the lowest light speed of 17 m/s was achieved in a cloud with a high condensate fraction. The study also discusses the implications of the results for quantum optics, including the potential for optical switching at the single photon level and the compression of light pulses in the z-direction. The research provides insights into the behavior of light in ultracold atomic gases and has potential applications in quantum information processing and nonlinear optics.