Int. J. Emerg. Sci., 3(4), 359-369, December 2013 ISSN: 2222-4254 © IJES 359 Depth-Optimized Reversible 4:2 and 6:2 Compressors Mahmood Kalemati, Mariam Zomorodi Moghadam, Keivan Navi Faculty of Electrical and Computer Engineering, Shahid Beheshti University, G. C., Tehran, Iran m.kalemati@mail.sbu.ac.ir, m_zomorodi@sbu.ac.ir, navi@sbu.ac.ir Abstract. Reversible circuit design is an attractive research study that has applications in quantum computing, low power design and cryptography. In other side, two of the most important circuits in arithmetic units are Multipliers and multi-operand addition circuits. Conventional multiplier has three main stages which partial product reduction stage has an impressive impact in the total delay of the circuit. This stage is a multi-operand addition that reduces the number of partial products. Half adders and full adders are the main arithmetic components that are used in the reduction stage of multipliers. Compressors are other components that are used in the reduction stage of multipliers and multi-operand additions. In this paper, we propose new realization of reversible 4:2 and 6:2 compressors based on conventional irreversible compressor architectures where cascading our proposed reversible 4:2 and 6:2 compressors have depth less than existing works. Keywords: Reversible circuit, Reversible compressor, Quantum cost, Quantum gate, Constant inputs, Garbage outputs 1 INTRODUCTION In irreversible computation, information loss will lead to energy dissipation. Landauer showed that losing information in irreversible computation consumes power which the amount of energy dissipation is estimated at least KTLn2 Joules [1, 2]. In this equation, K=1.380×10 -23 JK -1 is Boltzmann constant and T is denoted as environment temperature. In high performance digital circuits that power consumption is a serious problem [3], this type of energy loss will be more important. In 1973 Bennett showed that reversible circuits can save this amount of energy loss [4]. A reversible circuit implements one to one function and the number of inputs and outputs in a reversible circuit is equal that cause to have no information loss and it is possible to determine inputs from outputs [1, 5]. Low power circuit design, quantum computing and cryptography are some applications of reversible circuits design. In low power circuit design, several works have been done in existing CMOS technology design. For example in [6], a carry look-ahead adder has been designed in reversible manner using control gates. In addition to quantum computing, quantum gates are inherently reversible [3] and quantum circuits are built based on reversible logic. Several metrics in reversible circuit design are considered. Quantum cost and gate number are the metrics that indicate the number of quantum