Decoherence-free measurements for quantum information processors

The measurement of discrete quantum systems is a vital ingredient for quantum information processors. These systems, being fragile in nature, are characterized by a perturbation or collapse of their states whenever they are measured due to measurement-induced decoherence. We propose a quantum measurement model that allows for the extraction of information from any superposed quantum system without inducing a total collapse. The methodology used involves the coupling of a quantum probe through suitable interactions that create a partial entanglement between probe and quantum system. This entanglement in turn transfers information about the quantum system to the probe. Therefore, by making measurements on the probe rather than the superposed quantum system itself, we avert a total collapse to the quantum system. Our measurement model is applied to quantum computers for the measurement of their superposed states. The use of the measurement scheme for solving computational problems on the quantum computer is presented. In addition, the measurement scheme is invoked as an eavesdropping scheme on quantum cryptography systems; we show the degree to which their security is comprised. Lastly, the system performance of multi-user quantum cryptography networks is simulated, culminating in the experimental demonstration of a six-user quantum cryptographic network system.