02 Research
Quantum Interface
Since it is hard to realize all quantum information processes in a single quantum system, interfaces between various quantum systems like photons, atoms, ions, quantum dots, and Josephson junctions are becoming more important. We are focusing on a photon-atom quantum interface based on an atom-cavity system. Above all, we are working on enhancing the efficiency of deterministic single photon generation. A single photon is an ideal carrier of quantum information, playing an important role in quantum optics research and quantum information science like quantum communication and quantum computation. The atom-cavity system has an advantage in high directionality of radiated photons. We realized a single atom-cavity system by trapping a single rubidium atom in a Fabry-Perot cavity and excited the atom with 10 MHz nanosecond π-ulse. As a result, we achieved the repetition rate of 1.7×106 per second which is the highest record among the atom-cavity systems based single photon sources. Efficiency of the single photon generation is defined as the probability of a single photon radiation per a single pumping pulse, and 17% was achieved in this experiment. We are working on making the efficiency of single photon generation over 50%.
Schematic of single photon generation using an atom-cavity system. When an atom comes into the cavity mode, it is excited by π-pulse laser. The excited atom radiates single photon by interaction with the cavity mode. The disappearance of a peak on time 0 is observed by second-order correlation function measurement using two single photon detectors. It means radiated photons are single photons. From S. Kang et al., Opt. Express 19, 2440 (2010).
A single-atom-cavity system can be used to realize a single-qubit quantum memory. Instead of trapping an atom in the cavity, if we trap some atoms in an one-dimensional optical lattice by one atom per one lattice site and drag the optical lattice, we can selectively couple the cavity with each atom. Using this method, the multi-qubit quantum memory can be realized. In this case, the atoms should be moved as fast as possible, while keeping the atoms in their motional ground state of the lattice potential. In this regard, we study the change of atomic dynamics in moving optical lattice. The resonance fluorescence spectrum of cold atoms provides the information on temperature and quantum mechanical motional states of the atoms. The resonance fluorescence spectrum of a few atoms is measured by photon-counting second-order-correlation spectroscopy(PCSOCS) method, developed by our laboratory[H. Hong et al., Opt. Lett. 31, 3182 (2006)]. By using PCSOCS, we can measure the spectrum of extremely small signals, about femto-Watt level. In the previous study with PCSOCS method, we measured the resonance fluorescence of single Rubidium atoms in optical lattice micro-potential formed at the center of magneto-optical trap. When the atoms are localized in the optical lattice minima, the spectrum exhibits the three narrow peaks, which are affected by Lamb-Dicke effect. As increasing the speed of the moving optical lattice, broadened Doppler peaks start to grow while the narrow peaks get smaller. These Doppler peaks reflect non-localized atoms. Comparison of the ratio between two atom groups(localized, non-localized) is currently under investigation.
Single rubidium atom trapped in an optical lattice and its fluorescence spectrum. Besides strong Rayleigh peak, the spectrum exhibits weak Stokes and anti-Stokes Raman sidebands. We can obtain the tunneling rate between adjacent potential minima by analyzing the linewidth of the central Rayleigh peak(~kHz). From W. Kim et al., Nano Letters 11, 729 (2011).
[Click here to see the real-time spectrum measurement]
Resonance fluorescence spectrum of atoms in optical lattice. (a) The spectrum of atoms in stationary optical lattice. (b) The spectrum of atoms in moving optical lattice. Broadened peaks, generated by non-localized atoms, appear besides Rayleigh peak.
[Click here to see the spectrum change as the optical lattice speed increases]
"B-Team" Members: From the left, Kyungwon An, Jung-Ryul Kim, Myunggyu Hwang and Kyeongock Chong. Collaborators Dr. Sungsam Kang (Korea U.), Dr. Youngwoon Choi (MIT) and Dr. Seokchan Youn (Univ. Bonn)
Last updated: February 25, 2014