10.1117/2.1200904.1550 Bang-bang decoupling used in quantum optics for the first time Sajeev Damodarakurup, Marco Lucamarini, Giovanni Di Giuseppe, David Vitali, and Paolo Tombesi Experimental suppression of polarization decoherence in a ring cavity uses a simple open-loop control technique known as bang-bang decou- pling. In recent years new strategies for maintaining the coherence (i.e., constant relative phase) of quantum systems have appeared. 1, 2 Even if photons interact weakly with their surroundings, the re- sulting decoherence (i.e., randomization of the relative phase) may significantly affect their polarization state during propaga- tion within dispersive media. We show how the polarization de- coherence of a photon circling in a ring cavity can be suppressed by adapting to the field of quantum optics the open-loop pro- tection technique known as bang-bang (BB). This method is al- ready being employed for nuclear spins and nuclear-quadrupole qubits. It has potential practical application in the field of optical fibers. In BB, the system undergoes a sequence of suitably tailored unitary operations aimed at decoupling the system from the sur- rounding environment. For a polarization qubit, conveniently represented as a point on the surface of a Bloch sphere (see Figure 1), BB consists of alternating fast 180 ◦ flips of the po- larization state about the two axes, z and x. 3 The physical idea behind BB comes from nuclear magnetic resonance (NMR) spec- troscopy, where the controls are implemented in time as a se- quence of strong and rapid pulses. In the photon case BB is ap- plied in space: while circling in the cavity, the photon passes di- rectly through the optical elements that implement the BB trans- formations on it. Figure 2 depicts our experimental apparatus. A laser diode at 800nm with a bandwidth of about 15nm is pulsed at 100KHz. The laser is injected into a triangular ring cavity com- posed of a spherical mirror and two planar mirrors, and the beam is attenuated so that it provides an average of one photon per pulse. In the cavity the photon undergoes two kinds of polarization decoherence. One arises from the polarization-dependent reflec- tivity of the plane mirrors. We introduce the other manually: it Figure 1. Bloch-sphere representation of polarization states of light. The BB unitary operations, X and Z, correspond to π -rotations about the x- and z-axes. { H, V}, {D, A}, and {R, L} represent the eigen- states of the three polarization bases. comes from the birefringent crystals positioned in front of the plane mirrors, which are labeled B X in Figure 2. These crystals introduce a controlled birefringence along various axes of the Bloch sphere. We quantify the amount of decoherence by evalu- ating the purity of the experimentally reconstructed output den- sity matrices (see Figure 3). A full BB decoupling cycle is com- posed of the double application of the X and Z flips during the system evolution. Two waveplates, labeled X and Z in Figure 2, make these flips occur. In our first experiment we studied the mirror-reflectivity- dependent decoherence for different polarization input states. Figure 3 reports the corresponding output density Continued on next page