Weak continuous measurements of multiqubits systems

Abstract

In this review we summarize our recent experiments on the investigation on superconducting qubits. Instead of strong projective measurement used by other groups in their first pioneering experiments we have proposed and realized a weak continuous readout which belongs to the class of quantum non-demolition measurements. Moreover, our scheme enables to measure a superconducting qubit at the so called sweet (or magic) point where a qubit is in a superposition of two classical states and its sensitivity to external noise is minimized. In this scheme, which is widely used nowadays, the superconducting oscillator coupled to superconducting qubit is used as a detector of the qubit's state. Such system is analogue to a system of a single atom interacting with photons in a cavity, which allows to study quantum electrodynamics in artificial macroscopic systems. Pushing this analogy we demonstrate Sisyphus cooling and amplification caused by energy exchange between an oscillator and a flux qubit. Using the Sisyphus effect we show consistency between the adiabatic weak continuous measurement in the ground state and the spectroscopic measurement. This allows us to characterize the more complicated system of coupled qubits by making use of the same method. We have realized and studied fixed ferromagnetic, antiferromagnetic as well as tunable qubit-qubit coupling. We argue that ground state measurements can be used for characterization of entangled states in coupled flux qubits. E. Il'ichev (B) · 123 134 E. Il'ichev et al.

Multiphoton transitions in Josephson-junction qubits (Review Article)

“…A phase (amplitude) shift provides information about the dynamics of a quantum system. In particular, for probing the qubit's state, several types of oscillators have been already used: an LC tank circuit [73,74], a nanomechanical one [43,75], and a transmission line resonator [76,77]. If the resonator quantization energy ω p is smaller than the thermal excitation energy k B T , the resonator can be considered as a classical oscillator.…”

Circuit QED: superconducting qubits coupled to microwave photons

“…In recent years there were many theoretical proposals for coupling qubits to either three-dimensional cavities or lumped element resonators and for protocols for analyzing microwave photon states (Shnirman et al, 1997;Makhlin et al, 2001;Buisson and Hekking, 2001;Marquardt and Bruder, 2001;Al-Saidi and Stroud, 2001;Plastina and Falci, 2003;Blais et al, 2003;Yang et al, 2003;You and Nori, 2003;Menzel et al, 2010;Koch and Le Hur, 2009;Koch et al, 2010;Nunnenkamp et al, 2011;Viehmann et al, 2011) (and others too numerous to list) and there has been a flurry of experiments on circuit QED (Wallraff et al, 2004;Chiorescu et al, 2004;Wallraff et al, 2005;Schuster et al, 2007b;Johansson et al, 2006;Siddiqi et al, 2006;Boulant et al, 2007;Schuster et al, 2007a;Wallraff et al, 2007;Sillanpää et al, 2007;Houck et al, 2007;Leek et al, 2007;Majer et al, 2007;Astafiev et al, 2007;Metcalfe et al, 2007;Deppe et al, 2008;Fink et al, 2008;Hofheinz et al, 2008;Schreier et al, 2008;Houck et al, 2008;Fragner et al, 2008;Grajcar et al, 2008;Sandberg et al, 2008;Il'ichev et al, 2009;…”