Laser Spectroscopy Lab.


Our current research interests are:

1.     Realization of a CNOT gate using 7Li atom trapped in a 1D optical lattice

We demonstrated in 2014 that broadening of a ground hyperfine transition for an alkali-metal atom in an optical trap due to differential ac Stark shift could be eliminated by using a “magic” polarization. For 7Li, we obtained 0.6-Hz linewidth and coherence time close to 1 s. Using the long coherence time and similarity of 7Li to 9Be+ we are trying to realize a 7Li-CNOT gate following the protocol of Wineland’s experiment 20 years ago.


2.     New cooling schemes for a zero-entropy system of 87Rb atoms in a 1D optical lattice

We started a new experiment to cool 87Rb atoms to 3D ground vibrational state in a 1D optical lattice. We also aim to have one and only one atom at each site over 100 consecutive sites so that they constitutes a zero-entropy system well suited for initial state for quantum information processings. Our approach is based on evaporative cooling using selective removal of high-energy atoms and motion-selective coherent population trapping, which is analogous to the well-known velocity-selective CPT cooling scheme.


3.     Atom-interferometer Gyroscope using a slow atomic beam

Using 87Rb atom from a low-velocity-intense source, we are constructing an atom interferometer gyroscope. This work is supported by Agency of Defense Development of Korea.


4.     Precise displacement-measurement using an optoelectronic oscillator

It is a common practice to measure a small displacement using a Michelson interferometer. When power change due to a displacement is measured, it is called a homodyne detection. When two frequencies are used to produce a beating signal, a displacement can appear as a phase shift, and it is called a heterodyne detection. In our experiment we combined a Michelson interferometer with an optoelectronic oscillator so that a displacement produces a frequency shift.


During last 20 years our research has been focused on an optical dipole trap. Some of the representative results are:

1.     Analogous Zeeman effect:

With proper detuning and polarization, an ac Stark shift of a ground-state alkali-metal atom takes the form of a pure Zeeman shift. We used it to demonstrate (i) an optical trap that behaves like a magnetic trap, (ii) optical Stern-Gerlach effect, where light intensity gradient played the role of a static magnetic field gradient of the landmark experiment, and (iii) optical Faraday effect.


2.     Magic wavelength for cesium D2 transition in an optical trap

We pointed out that at 935 nm, differential ac Stark shift for Cs D2 transition disappears due to the three-level structure of 6S-6P-5D. We experimentally demonstrated it in 2003.


3.     Magic polarization for lithium ground hyperfine transition in an optical trap

We pointed out that with a proper polarization, ac Stark shift induced by a vector polarizability could be tuned to eliminate differential shift by a scalar polarizability in 2007. This scheme works best with lithium due to its small fine structure. We demonstrated a sub-Hz linewidth and a coherence time approaching 1 s using 7Li in an optical trap in 2014.


4.     Transformation of a 1D optical lattice to a traveling-wave trap

We formed an optical lattice in a Fabry-Perot cavity. By producing sidebands at plus and minus one free spectral range away from the carrier, we could average out the axial intensity gradient at central region of the lattice.


Welcome to Laser Spectroscopy Laboratory at Department of Physics, Korea University. LSL is an experimental atomic physics group led by Prof. Donghyun Cho since 1994.


Prof. Asan Science Bldg. 409 (tel. 02-3290-3102)

Lab. Asan Science Bldg. B105 (tel. 02-923-2865)