Flux qubit as a sensor for a magnetometer with quantum limited sensitivity

We propose to use the quantum properties of a superconducting flux qubit in the construction of a magnetometer with quantum limited sensitivity. The main advantage of a flux qubit is that its noise is rather low, and its transfer functions relative to the measured flux can be made to be about 10mV/$\Phi_0$, which is an order of magnitude more than the best value for a conventional SQUID magnetometer. We analyze here the voltage-to-flux, the phase-to-flux transfer functions and the main noise sources. We show that the experimental characteristics of a flux qubit, obtained in recent experiments, allow the use of a flux qubit as magnetometer with energy resolution close to the Planck constant.Comment: 3 pages, 6 figure

Low frequency Rabi spectroscopy for a dissipative two-level system

We have analyzed the interaction of a dissipative two level quantum system with high and low frequency excitation. The system is continuously and simultaneously irradiated by these two waves. If the frequency of the first signal is close to the level separation the response of the system exhibits undamped low frequency oscillations whose amplitude has a clear resonance at the Rabi frequency with the width being dependent on the damping rates of the system. The method can be useful for low frequency Rabi spectroscopy in various physical systems which are described by a two level Hamiltonian, such as nuclei spins in NMR, double well quantum dots, superconducting flux and charge qubits, etc. As the examples, the application of the method to a nuclear spin and to the readout of a flux qubit are briefly discussed.Comment: 4 pages, 3 figures, the figures are modifie

Is a single photon's wave front observable?

The ultimate goal and the theoretical limit of weak signal detection is the ability to detect a single photon against a noisy background. [...] In this paper we show, that a combination of a quantum metamaterial (QMM)-based sensor matrix and quantum non-demolition (QND) readout of its quantum state allows, in principle, to detect a single photon in several points, i.e., to observe its wave front. Actually, there are a few possible ways of doing this, with at least one within the reach of current experimental techniques for the microwave range. The ability to resolve the quantum-limited signal from a remote source against a much stronger local noise would bring significant advantages to such diverse fields of activity as, e.g., microwave astronomy and missile defence. The key components of the proposed method are 1) the entangling interaction of the incoming photon with the QMM sensor array, which produces the spatially correlated quantum state of the latter, and 2) the QND readout of the collective observable (e.g., total magnetic moment), which characterizes this quantum state. The effects of local noise (e.g., fluctuations affecting the elements of the matrix) will be suppressed relative to the signal from the spatially coherent field of (even) a single photon.Comment: 13 pages, 4 figure

Effect of Cherenkov radiation on the jitter of solitons in the driven underdamped Frenkel-Kontorova model

The effect of complex dynamics of solitons on the output noise of the system (thermal jitter) is studied in the frame of the driven underdamped Frenkel-Kontorova model. In contrast to the continuous case, we have observed a dramatic splash of the jitter. It is demonstrated that this jitter increase is related to the joining of an initial soliton with the one generated by large amplitude oscillations of the Cherenkov radiation tail, which results in the establishment of a unified soliton structure