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ReportsQuantum Coherence in an Optical Modulator 
Semiconductor quantum well electroabsorption modulators are widely used to modulate near-infrared (NIR) radiation at frequencies below 0.1 terahertz (THz). Here, the NIR absorption of undoped quantum wells was modulated by strong electric fields with frequencies between 1.5 and 3.9 THz. The THz field coupled two excited states (excitons) of the quantum wells, as manifested by a new THz frequency- and power-dependent NIR absorption line. Nonperturbative theory and experiment indicate that the THz field generated a coherent quantum superposition of an absorbing and a nonabsorbing exciton. This quantum coherence may yield new applications for quantum well modulators in optical communications.
1 Physics Department and Institute for Quantum and Complex Dynamics (iQCD), Broida Hall Building 572, Room 3410, University of California, Santa Barbara, CA 93106, USA.
2 Electrical and Computer Engineering Department, University of California, Santa Barbara, CA 93106, USA. 3 Center for Nanotechnology, NASA Ames Research Center, MS 229-1, Moffett Field, CA 94035, USA. 4 Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA. 5 Georgia Tech Lorraine, Metz Technopole, 2-3 rue Marconi, 57070 Metz, France. * Present address: JILA, University of Colorado, 440 UCB, Boulder, CO 80309, USA.
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née Ciulin. Present address: Department of Chemistry, University of Aarhus, Langelandsgade 140, DK-8000 Århus C, Denmark. 
To whom correspondence should be addressed. E-mail: Science. ISSN 0036-8075 (print), 1095-9203 (online)
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