Experimental Implementation of the Optimal Linear-Optical Controlled Phase Gate

Quantum information processing offers potential for further progress of information technologies. Theoretical research in this field has already predicted a whole range of significant improvements which could be achieved by application of quantum physics instead of classic computing methods. It becomes apparent that the basic building blocks of future computers will be the so-called c-phase (controlled phase) gates. These gates process 2 bites of quantum information (qubits) and their function is similar to CNOT operation known from classic ICT. Present-day experimental research is however faced with various technological barriers which prevents routine implementation of quantum technologies into everyday practice. Expecially the low efficiency (low success probability) is limiting factor.

In our experiment we have dealt with an important question: course of success probabilityof the c-phase gate in dependency to its settings. Theoretical study of authors from Cambridge predicted unusual course of this dependency. Linear optical components as polarization phase plates or light polarizators were used. Photon pairs carrying quantum information (qubits) processed by our gate were generated in spontaneous frequency conversion. With quantum tomography and estimation were discovered the detailed parameters of transformation which the gate executes on both qubits.

The most important output of the experiment has been a verification of a theoretical prediction for success probability as a function of gate parameter settings. Comparison of experimentally acquired values with theoretical curve is depicted on image below. Adjustability of our gate parameters in dependecy on requirements of upplied quantum protocol extends the options of its effective implementation for instance in the generation of non-classical light and other fields.

Success probability of the gate.

¹Joint Laboratory of Optics of Palacký University and Institute of Physics of Academy of Sciences of the Czech Republic, Olomouc, Czech Republic
²Institute of Physics and Astronomy, University of Potsdam, Potsdam, Germany
³Institute for Advanced Study Berlin, Berlin, Germany
⁴Department of Optics, Faculty of Science, Palacký University, Olomouc, Czech Republic