Taming Entanglement

Using a spontaneous parametric-downconversion source of photon pairs, we are working towards the creation of arbitrary 2-qubit quantum states with high fidelity. Currently, all physically allowable combinations of polarization entanglement and mixture can be produced, including maximally-entangled mixed states. The states are experimentally measured and refined via computer-automated quantum-state tomography, and this system has also been used to perform single-qubit and ancilla-assisted quantum process tomography. Central to the long-term future of quantum information processing is the capability of performing extremely accurate and reproducible gate operations. The restrictions for fault-tolerant quantum computation are extremely demanding: the tolerable error-per-gate operation should be less than 10 −4 to 10 −6. Implementing such precise gate operations and preparing the requisite input states is therefore one of the key milestones for quantum information processing. Using optical realizations of qubits, e.g., polarization states of photons, we have the potential to meet these demanding tolerances. Therefore, although large-scale quantum computers will perhaps never be constructed solely using optical qubits, these systems nevertheless form a unique and convenient testbed with which to experimentally investigate the issues surrounding state creation, manipulation, and characterization, and also ways of dealing with decoherence. Figure 1. Spontaneous parametric downconversion.