1508 ISSN 1054-660X, Laser Physics, 2006, Vol. 16, No. 11, pp. 1508–1511. © MAIK “Nauka / Interperiodica” (Russia), 2006. Original Text © Astro, Ltd., 2006. Quantum computation employs the principle of coherent superposition and quantum entanglement to solve certain problems, such as factoring large integers and searching data in an array, much faster than a clas- sical computer [1]. The basic building blocks of a quan- tum computer are quantum logic gates. It was shown that any quantum computation can be reduced to a sequence of two classes of quantum gates, namely, uni- versal two-qubit logic gates and one-qubit local opera- tions [2]. Such gates have been discussed and imple- mented in many physical systems including trapped ions [3], liquid-state nuclear magnetic resonance (NMR) [4], quantum dots [5], optical lattices [6], linear optics [7], and cavity QED [8–15]. Because of some demonstrated advantages, cavity QED with high-Q cavities and long-lived Rydberg atoms or trapped ions is often given more attention [16]. Broadly speaking, there are many different ways to perform quantum computation in cavity QED, for example, quantum information (a qubit) can be encoded in photonic states and/or atomic internal states. In most early cavity-QED-based proposals, the light field in the cavity was always used as a qubit [8– 10, 12–14] or as a “bus” to couple the different sub- systems [11]. Thus, how to prevent the photonic decay is one of the main tasks for quantum information pro- cessing in cavity QED. To this end, several schemes have been proposed very recently by encoding the quantum information in the atomic internal states and virtually exciting a cavity mode during the logic gate operations [15]. On the other hand, the requirements for constructing quantum logic operations demand strong coupling and precise control for the interactions between atoms and light fields [16, 17], but this is not an easy task, in par- ticular, at a single-photon level. One way to realize such strong coupling is to adopt small cavities; however, this will restrict the power to increase the scale of the quan- tum computation, because it is difficult to individually address each atom during quantum logic operations when atoms are in a small cavity. In this paper, we present a feasible quantum compu- tation scheme based on quantum switches (QS) [18] in a high-Q cavity in which the quantum information is encoded in atomic states. The distinct characteristic of our scheme is that the implementation of quantum logic operations completely adopts the interactions between atoms and classical light fields without the need for the quantum cavity modes as qubits or as a “bus” all the time. Our scheme thus possesses the following main advantages. (i) The strict requirements for the cavity, such as very low photonic decay, are greatly relaxed. (ii) The proposed scheme can be easily extended to the case of multiple target qubits, because the classical light field can be applied to many atoms and lets them undergo the same rotations. (iii) In the previous propos- als, such as in [8–10], a technique that transfers the quantum states of the cavity mode from and to the atomic states [19] is always used to encode the quantum information in the atomic states, which certainly con- sumes operating time, while in our scheme the quantum logic operations between atoms are realized in one step even in the case of multiple target qubits; thus, our scheme is simple and time-saving. (iv) In our scheme, the atoms are sufficiently separated and can be individ- ually addressed. The atomic states can be detected in a selective and sensitive way by field ionization [20]. Quantum Computation Based on a Quantum Switch in Cavity QED X.-L. Feng a, b , L. C. Kwek a, c , C. H. Lai a , and C. H. Oh a a Department of Physics, Blk S12, National University of Singapore 2, Science Drive 3, 117542 Singapore b Key Laboratory for High Intensity Optics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai, 201800, China c National Institute of Education, Nanyang Technological University, 1 Nanyang Walk, 639798 Singapore e-mail: phyfxl@nus.edu.sg Received May 6, 2006 Abstract—A feasible scheme for constructing quantum logic gates is proposed on the basis of quantum switches in cavity QED. It is shown that the light field which is fed into the cavity due to the passage of an atom in a certain state can be used to manipulate the conditioned quantum logical gate. In our scheme, the quantum information is encoded in the states of Rydberg atoms and the cavity mode is not used as logical qubits or as a communicating “bus”; thus, the effect of atomic spontaneous emission can be neglected and the strict require- ments for the cavity can be relaxed. PACS numbers: 03.67.Lx, 42.50.-p, 32.80.-t DOI: 10.1134/S1054660X0611003X QUANTUM INFORMATION AND QUANTUM COMPUTATION