Projective Measurement Scheme for Solid-State Qubits

We present an effective measurement scheme for the solid-state qubits that does not introduce extra decoherence to the qubits until the measurement is switched on by a resonant pulse. The resonant pulse then maximally entangles the qubit with the detector. The scheme has the feature of being projective, noiseless, and switchable. This method is illustrated on the superconducting persistent-current qubit, but can be applied to the measurement of a wide variety of solid-state qubits, the direct detection of the electromagnetic signals of which gives poor resolution of the qubit states. Quantum computation in solid-state systems is a growing field. [1–8] Among various physical realizations, solid-state qubits have the advantage of being scalable to large number of qubits and that the quantum states can be engineered by various techniques. Successful implementations of qubits have been achieved in several mesoscopic systems. [9–13] Effective measurement of quantum bits is a crucial step in quantum computing. An ideal measurement of the qubit is a projective measurement [14] that correlates each state of the quantum bit with a macroscopically resolvable state. In practice, it is often hard to design an experiment that can both projectively measure a solid-state qubit effectively and meanwhile does not couple environmental noise to the qubit. Often in solid-state systems, the detector is fabricated onto the same chip as the qubit and couples with qubit all the time. On the one hand, noise should not be introduced to the qubit via the coupling with the detector.