Experiments with Generalized Quantum Measurements and Entangled Photon Pairs

[摘要] This thesis describes a linear-optical device for performing generalized quantum measurementson quantum bits (qubits) encoded in photon polarization, the implementationof said device, and its use in two diff erent but related experiments. The device works bycoupling the polarization degree of freedom of a single photon to a `mode;; or `path;; degreeof freedom, and performing a projective measurement in this enlarged state space in orderto implement a tunable four-outcome positive operator-valued measure (POVM) on theinitial quantum bit. In both experiments, this POVM is performed on one photon from atwo-photon entangled state created through spontaneous parametric down-conversion.In the fi rst experiment, this entangled state is viewed as a two-qubit photonic clusterstate, and the POVM as a means of increasing the computational power of a given resourcestate in the cluster-state model of quantum computing. This model traditionallyachieves deterministic outputs to quantum computations via successive projective measurements,along with classical feedforward to choose measurement bases, on qubits in a highly entangledresource called a cluster state; we show that `virtual qubits;; can be appended to agiven cluster by replacing some projective measurements with POVMs. Our experimentaldemonstration fully realizes an arbitrary three-qubit cluster computation by implementingthe POVM, as well as fast active feed-forward, on our two-qubit photonic cluster state.Over 206 diff erent computations, the average outputdelity is 0.9832+/- 0.0002; furthermorethe error contribution from our POVM device and feedforward is only of order 10^-3, lessthan some recent thresholds for fault-tolerant cluster computing.In the second experiment, the POVM device is used to implement a deterministicprotocol for remote state preparation (RSP) of arbitrary photon polarization qubits. RSPis the act of preparing a quantum state at a remote location without actually transmittingthe state itself. We are able to remotely prepare 178 diff erent pure and mixed qubitstates with an averagedelity of 0.995. Furthermore, we study the thefidelity achievableby RSP protocols permitting only classical communication, without shared entanglement,and compare the resulting benchmarks for averagefidelity against our experimental results.Our experimentally-achieved average fi delities surpass the classical thresholds wheneverclassical communication alone does not trivially allow for perfect RSP.

[发布机构] University of Waterloo

[效力级别] quantum optics

[关键词] quantum information;quantum optics;quantum computing;quantum measurement;Physics