Date: 2012-02-08

**The "Achilles and the Tortoise" Paradox and its Applications in Quantum Optics**

**Dr. Yu-Ping Huang (Northwestern University)**

The "Achilles and the Tortoise" Paradox, raised by Zeno of Elea more than 2000 years ago, has led to the discovery of quantum Zeno effect (QZE) by which the motion of a system can be frozen through frequent observation of the system's quantum state. Demonstrated first in a system of two-level atoms in 1989, QZE has since attracted intensive attention in interdisciplinary fields due to the prospect of achieving strong interaction between objects that physically never couple. In this talk, I will first discuss the fundamental physics underlying the QZE and some counter-intuitive phenomena it leads to. Then, I will present some potent applications of QZE in the field of quantum optics. These include a) implementation of deterministic quantum logic with single atoms/single photons; b) realization of decoherence-free phase gate with laser-excited semiconductor quantum dots; c) construction of "interaction-free" all-optical switches capable of ultralow energy dissipation (down to 100 aJ or less per operation). I will also discuss a proof-of-principle experiment being carried out in the Center for Photonic Communication & Computing, whose initial result highlights an avenue to novel all-optical devices for quantum applications with single photons.

**References:**

Y. P. Huang and M. G. Moore, "Interaction- and measurement-free Quan-tum Zeno gates asymptotically on-demand for single-atom and single-photonqubits," Phys. Rev. A **77**, 062332 (2008).

K. J. Xu, Y. P. Huang, M. G. Moore, and C. Piermarocchi, "Two-Qubit Con-ditional Phase Gate in Laser-Excited Semiconductor Quantum Dots Usingthe Quantum Zeno Effect," Phys. Rev. Lett. **103**, 037401 (2009),

Y.-P. Huang and P. Kumar, "Interaction-free all-optical switching in χ(2)mi-crodisks for quantum applications," Opt. Lett., **35**, 2376 (2010).

Y.-P. Huang, J. B. Altepeter, and P. Kumar, "Interaction-free all-opticalswitching via the quantum Zeno effect," Phys. Rev. A 82, 063826 (2010).

Y.-P. Huang and P. Kumar, "Anti-bunched Emission of Photon Pairs via Quantum Zeno Blockade," Phys. Rev. Lett. **108**, 030502 (2012)