A pair of optical pulses traveling through two dispersive media will become broadened and, as a result, the degree
of coincidence between the optical pulses will be reduced. For a pair of entangled photons, however, nonlocal
dispersion cancellation in which the dispersion experienced by one photon cancels the dispersion experienced
by the other photon is possible. In this paper, we report an experimental demonstration of nonlocal dispersion
cancellation using entangled photons. The degree of two-photon coincidence is shown to increase beyond
the limit attainable without entanglement. Our results have important applications in fiber-based quantum
communication and quantum metrology.
We demonstrate a linear optical device which performs optimal quantum measurement or minimum disturbance
measurement on a single-photon polarization qubit with the help of an ancillary path qubit introduced to the
same photon. We show theoretically and experimentally that this device satisfies the minimum disturbance
measurement condition by investigating the relation between the information gain (estimation fidelity) and
the state disturbance due to measurement (operation fidelity). Our implementation of minimal disturbance
measurement is postselection-free in the sense that all detection events are counted toward evaluation of the
estimation fidelity and the operation fidelity, i.e., there is no need for coincidence postselection of the detection
Recently, quantum systems with D-dimensional bases states (D > 2) or quDits have attracted much attention in the context of fundamental tests of quantum theory and potential applications in quantum information. In this paper, we discuss several schemes for generating entangled states of two ququarts (four-dimensional quantum systems). The ququart in our scheme is based on frequency-nondegenerate biphoton states of spontaneous parametric down-conversion and we show how the entangled states between two ququarts can be generated with simple linear optical elements, such as an ordinary 50/50 beamsplitter, a polarizing beamsplitter, or a dichroic beamsplitter. We also show that our scheme is capable of generating postselection-free two-ququart entangled states efficiently.