Nondestructive detector for exchange symmetry of photonic qubits

We experimentally realize a quantum Fredkin gate and use it for constructing a nondestructive detector of exchange symmetry for qubits. The detector, which discriminates between symmetric and anti-symmetric quantum states of two qubits, allows us to directly measure a purity and overlap of quantum states and implement a quantum state programmable measurement. Furthermore, the nondestructive nature of the detector can be used for analysis of the back-action of the measurements, as well as for realization of nonlinear quantum operations, such as quantum purification and quantum cloning. As a whole, the experiment demonstrates the utility of exchange symmetry measurements and their potential for multi-step characterization and processing of quantum states. Quantum devices: detecting and putting exchange symmetry at use A new detector can identify the exchange symmetry of pairs of qubits in a non-destructive way. Michal Mičuda and colleagues from Palacký University, Czech Republic, have experimentally realized a quantum Fredkin gate in a linear quantum optics platform. The gate exchanges the state of two input qubits, here encoded in the polarization of two distinguishable photons, depending on the state of a third ancillary photon. By placing the ancillary qubit in a superposition state, the authors achieve a non-destructive projection of the qubit pair onto their symmetric or anti-symmetric subspace—namely the ensemble of states that are respectively invariant or acquire a global negative sign upon qubit exchange. As the authors show, discriminating the exchange symmetry of a qubit pair can be used to determine purity of quantum states, and to generate non-linear operations.