We analyze the optical selection rules of the microwave-assisted transitions in a flux qubit superconducting quantum circuit (SQC). We show that the parities of the states relevant to the superconducting phase in the SQC are well defined when the external magnetic flux phi(e) = phi(0)/2; then the selection rules are the same as the ones for the electric-dipole transitions in usual atoms. When phi(e) does not = phi(0)/2, the symmetry of the potential of the artificial "atom" is broken, a so-called delta-type "cyclic" three-level atom is formed, where one- and two-photon processes can coexist. We study how the population of these three states can be selectively transferred by adiabatically controlling the electromagnetic field pulses. Different from lambda-type atoms, the adiabatic population transfer in our three-level delta atom can be controlled not only by the amplitudes but also by the phases of the pluses.

We propose an experimentally realizable method to control the coupling between two flux qubits. In our proposal, the bias fluxes are always fixed for these two inductively coupled qubits. The detuning of these two qubits can be initially chosen to be sufficiently large, so that their initial interbit coupling is almost negligible. When a variable frequency or time-dependent magnetic flux (TDMF) is applied to one of the qubits, a well-chosen frequency of the TDMF can be used to compensate the initial detuning and to couple two qubits. This proposed method avoids fast changes of either qubit frequencies or the amplitudes of the bias magnetic fluxes through the qubit loops, and also offers a remarkable way to implement any logic gate, as well as tomographically measure flux qubit states.

Based on the interaction between the radiation field and a superconductor, we propose a way to engineer quantum states using a SQUID charge qubit inside a microcavity. This device can act as a deterministic single photon source as well as generate any Fock states and an arbitrary superposition of Fock states for the cavity field. The controllable interaction between the cavity field and the qubit can be realized by the tunable gate voltage and classical magnetic field applied to the SQUID. PACS numbers: 42.50.Dv, 74.50.+r, 42.50.Ct The generation of quantum states of the radiation field has been a topic of growing interest in recent years. This is because of possible applications in quantum communication and information processing, such as quantum networks, secure quantum communications, and quantum cryptography [1]. Based on the interaction between the radiation field and atoms, many theoretical schemes have been proposed for the generation of Fock states [2,3] and their arbitrary superpositions [4,5]. Experiments have generated single-photon states in quantum dots [6], atoms inside a microcavity [7], and other systems [8]. A superposition of the vacuum and one-photon states can also be experimentally created by truncating an input coherent state or using cavity quantum electrodynamics [9]. However, how to generate an arbitrary photon state by virtue of the interaction between the radiation field and solid state quantum devices seems to be unknown both theoretically and experimentally. Recent progress in superconducting quantum devices (e.g., [10,11]) makes it possible to do quantum state engineering experiments in these systems, and also there have been proposals on superconducting qubits interacting with the nonclassical electromagnetic field [12,13,14,15,16,17].Here, we present an experimentally feasible scheme to generate quantum states of a single-mode cavity field in the microwave regime by using the photon transition between the ground and first excited states of a macroscopic two-level system formed by a superconducting quantum interference device (SQUID). This artificial two-level "atom" can be easily controlled by an applied gate voltage V g and the flux Φ c generated by the classical magnetic field through the SQUID (e.g., [14,18]). The process of generating photon states in this device includes three main steps: (i) The artificial atom operates at the degeneracy point by choosing appropriate values for V g and Φ c . There is no interaction between the quantized cavity field and "atom" at this stage. (ii) Afterwards new V g and Φ c are selected such that the cavity field interacts resonantly with the "atom" and evolves during a designated time. (iii) The above two steps can be repeated until a desired state is obtained. Finally, the flux Φ c can be adjusted to a special value, then the interaction is switched off, and the desired photon state appears in the cavity. This process is similar to that of a micromaser [2] and it is described below.Model.-The macroscopic two-level system studied here is ...

We propose an approach to reconstruct any superconducting charge qubit state by using quantum state tomography. This procedure requires a series of measurements on a large enough number of identically prepared copies of the quantum system. The experimental feasibility of this procedure is explained and the time scales for different quantum operations are estimated according to experimentally accessible parameters. Based on the state tomography, we also investigate the possibility of the process tomography.

We propose a spectroscopic approach to probe tiny vibrations of a nanomechanical resonator (NAMR), which may reveal classical or quantum behavior depending on the decoherence-inducing environment. Our proposal is based on the detection of the voltage-fluctuation spectrum in a superconducting transmission line resonator (TLR), which is indirectly coupled to the NAMR via a controllable Josephson qubit acting as a quantum transducer. The classical (quantum mechanical) vibrations of the NAMR induce symmetric (asymmetric) Stark shifts of the qubit levels, which can be measured by the voltage fluctuations in the TLR. Thus, the motion of the NAMR, including if it is quantum mechanical or not, could be probed by detecting the voltage-fluctuation spectrum of the TLR. Introduction.-Since the beginning of quantum theory, many researchers have tried to monitor macroscopic quantum effects with mechanical resonators (see, e.g., [1]). This relates to the debate on the quantum-classical mechanics boundary for macroscopic objects and the mechanisms of quantum decoherence [2]. Besides superconductivity and Bose-Einstein condensates, quantum oscillations of nanomechanical resonators (NAMRs) could also provide an attractive platform for experimentally testing quantum phenomena at macroscopic scales. Furthermore, reaching the quantum limit of mechanical motions could open new avenues of technology [3], in, e.g., high precision measurement, quantum computation, and even gravitational wave detection.A mechanical resonator may reveal either quantum or classical behavior, depending on the decoherence-inducing environment [2]. Phenomenologically (see, e.g., Ref.[4]), if the energy (hν) of the vibration (with frequency ν) quanta is larger than the thermal energy k B T , then the mechanical oscillation could be regarded as quantum mechanical. NAMRs with low thermal occupation number have recently been experimentally studied [4,5]. These nanodevices, containing 10 10 -10 12 atoms, work at very low temperatures (in the mK-range) and sufficiently high frequencies (GHz-range), approaching the quantum limit. A formidable challenge (see, e.g., [4,5]) in this field is how to sensitively detect the quivering of the detected nanodevice, and quantitatively verify whether it is quantum mechanical or not. Indeed, it is difficult to directly detect [5,6] the tiny displacements of a NAMR, vibrating at GHz frequencies, using the available displacementdetection techniques. Also, the usual position-measurement method is ultimately limited by the always-present "zeropoint motion" fluctuations in the quantum regime [1].Here, we propose a promising indirect method to detect the mechanical oscillation of a NAMR approaching its quantum limit. Instead of attempting to further improve the sensitivity of the usual force/displacement detection [5] or to redesign the tested nanostructure [4], our proposal is based on the detection of the voltage-fluctuation spectrum in a superconducting transmission line resonator (TLR). A controllable Josephson

We propose an approach to coherently transfer populations between selected quantum states in one-and two-qubit systems by using controllable Stark-chirped rapid adiabatic passages (SCRAPs). These evolutiontime insensitive transfers, assisted by easily implementable single-qubit phase-shift operations, could serve as elementary logic gates for quantum computing. Specifically, this proposal could be conveniently demonstrated with existing Josephson phase qubits. Our proposal can find an immediate application in the readout of these qubits. Indeed, the broken parity symmetries of the bound states in these artificial "atoms" provide an efficient approach to design the required adiabatic pulses. PACS number(s): 42.50.Hz, 03.67.Lx, 85.25.Cp.Introduction.-The field of quantum computing is attracting considerable experimental and theoretical attention. Usually, elementary logic gates in quantum computing networks are implemented using precisely designed resonant pulses. The various fluctuations and operational imperfections that exist in practice (e.g., the intensities of the applied pulses and decoherence of the systems), however, limit these designs. For example, the usual π-pulse driving for performing a single-qubit NOT gate requires both a resonance condition and also a precise value of the pulse area. Also, the difficulty of switching on/off interbit couplings [1] strongly limits the precise design of the required pulses for two-qubit gates.

We propose a mechanism to interface a transmission line resonator (TLR) with a nano-mechanical resonator (NAMR) by commonly coupling them to a charge qubit, a Cooper pair box with a controllable gate voltage. Integrated in this quantum transducer or simple quantum network, the charge qubit plays the role of a controllable quantum node coherently exchanging quantum information between the TLR and NAMR. With such an interface, a quasi-classical state of the NAMR can be created by controlling a single-mode classical current in the TLR. Alternatively, a "Cooper pair" coherent output through the transmission line can be driven by a single-mode classical oscillation of the NAMR.

We demonstrate photon-noise limited performance at sub-millimeter wavelengths in feedhorn-coupled, microwave kinetic inductance detectors (MKIDs) made of a TiN/Ti/TiN trilayer superconducting film, tuned to have a transition temperature of 1.4 K. Micro-machining of the silicon-on-insulator wafer backside creates a quarter-wavelength backshort optimized for efficient coupling at 250 µm. Using frequency read out and when viewing a variable temperature blackbody source, we measure device noise consistent with photon noise when the incident optical power is > 0.5 pW, corresponding to noise equivalent powers > 3×10 −17 W/ √ Hz. This sensitivity makes these devices suitable for broadband photometric applications at these wavelengths.