When eavesdropping on a quantum channel, two distinct properties of quantum physics arise. First, if the quantum system is prepared in one of several non-orthogonal states, then no measurement can discriminate these states with certainty. Second, any measurement that acquires information about a system must necessarily disturb it. We investigate a simple and optimal eavesdropping scheme that minimizes the disturbance caused by a given amount of information gain, and show that an in principle demonstration of such a scheme could be performed using existing experimental techniques in single-photon quantum technologies.
Two-qubit entangling gates allow the realization of new types of generalized quantum measurement. We discuss, and use photonic systems to demonstrate, two instances of this: two-qubit entangling measurements realizing superior discrimination of locally prepared two-qubit quantum states relative to what is achievable with local measurements and classical communication; and nondestructive weak measurements with postselection, leading to quantum weak values.
The performance of nondeterministic nonlinear gates in linear optics relies on the photon counting scheme being employed and the efficiencies of the detectors in such schemes. We assess the performance of the nonlinear sign gate, which is a critical component of linear optical quantum computing, for two standard photon counting methods: the double detector array and the visible light photon counter. Our analysis shows that the double detector array is insufficient to provide the photon counting capability for effective nondeterministic nonlinear transformations, and we determine the gate fidelity for both photon counting methods as a function of detector efficiencies.