Testing the quantum-ness of qubits: Temporal Bell Inequalities

Interest in qubits has brought new attention to quantum measurement theory. Key results include the Heisenberg efficiency limit and the Korotkov-Averin bound on the signal to noise ratio of the qubit spectral density peak. We test these bounds by using two independent detectors measuring the same qubit [1]. While the Heisenberg efficiency limit cannot be surmounted, the Korotkov-Averin bound can be overcome. Continuous measurement theory also permits the simultaneous measurement of non-commuting observables. In the second part of the talk we treat a quantum-non-demoltion measurement of a charge qubit which acts like a quantum pump [2]. We test the quantum-ness of the qubit by violating a temporal Bell inequality (Leggett-Garg inequality).

[1] Andrew N. Jordan, Markus Buttiker, Phys. Rev. Lett. 95, 220401 (2005)
[2] Andrew N. Jordan, Alexander N. Korotkov, Markus Buttiker, cond-mat/0510782