Interfacing photonic and solid-state qubits within a hybrid quantum architecture offers a promising route towards large scale distributed quantum computing. Ideal candidates for coherent qubit interconversion are optically active spins, magnetically coupled to a superconducting resonator. We report on an on-chip cavity QED experiment with magnetically anisotropic Er(3+)∶Y2SiO5 crystals and demonstrate collective strong coupling of rare-earth spins to a lumped element resonator. Moreover, the electron spin resonance and relaxation dynamics of the erbium spins are detected via direct microwave absorption, without the aid of a cavity.

Based on a real-time measurement of the motion of a single ion in a Paul trap, we demonstrate its electro-mechanical cooling below the Doppler limit by homodyne feedback control (cold damping). The feedback cooling results are well described by a model based on a quantum mechanical Master Equation.PACS numbers: 3.65. Ta, 42.50.Lc 32.80.Pj, 42.50.Ct, 42.50.Vk, 32.80.Lg Quantum optics, and more recently mesoscopic condensed matter physics, have taken a leading role in realizing individual quantum systems, which can be monitored continuously in quantum limited measurements, and at the same time can be controlled by external fields on time scales fast in comparison with the system evolution. Examples include cold trapped ions and atoms [1], cavity QED [2,3,4,5] and nanomechanical systems [6]. This setting opens the possibility of manipulating individual quantum systems by feedback, a problem which is not only of a fundamental interest in quantum mechanics, but also promises a new route to generating interesting quantum states in the laboratory. First experimental efforts to realize quantum feedback have been reported only recently. While not all of them may qualify as quantum feedback in a strict sense, feedback has been applied to various quantum systems [5,7,8,9,10,11]. On the theory side, this has motivated during the last decade the development of a quantum feedback theory [12,13], where the basic ingredients are the interplay between quantum dynamics and the back-action of the measurement on the system evolution. In this letter we report a first experiment to demonstrate quantum feedback control, i.e. quantum feedback cooling, of a single trapped ion by monitoring the fluorescence of the laser driven ion in front of a mirror. We establish a continuous measurement of the position of the ion which allows us to act back in a feedback loop demonstrating "cold damping" [14,15]. We will show that quantum control theory based on a quantum optical modelling of the system dynamics and continuous measurement theory of photodetection provides a quantitative understanding of the experimental results.We study a single 138 Ba + ion in a miniature Paul trap which is continuously laser-excited and laser-cooled to the Doppler limit on its S 1/2 to P 1/2 transition at 493 nm, as outlined in Fig. 1. The ion is driven by a laser near the atomic resonance, and the scattered light is emitted both into the radiation modes reflected by the mirror, as well as the other (background) modes of the quantized light field [16]. Light scattered into the mirror modes can either reach the photodetector directly, or after reflection from the mirror. From the resulting interference the motion of the ion (its projection onto the ion-mirror axis) is detected as a vibrational sideband in the fluctuation spectrum of the photon counting signal [17]. Of the three sidebands at about (1,1.2,2.3) MHz, corresponding to the three axes of vibration, we observe the one at ν = 1 MHz. It has a width Γ ≈ 400 Hz and is superimposed on the background shot noise...

Superconducting microwave resonators are reliable circuits widely used for detection and as test devices for material research. A reliable determination of their external and internal quality factors is crucial for many modern applications, which either require fast measurements or operate in the single photon regime with small signal to noise ratios. Here, we use the circle fit technique with diameter correction and provide a step by step guide for implementing an algorithm for robust fitting and calibration of complex resonator scattering data in the presence of noise. The speedup and robustness of the analysis are achieved by employing an algebraic rather than an iterative fit technique for the resonance circle.

Interfacing superconducting quantum processors, working in the GHz frequency range, with optical quantum networks and atomic qubits is a challenging task for the implementation of distributed quantum information processing as well as for quantum communication. Using spin ensembles of rare-earth ions provides an excellent opportunity to bridge microwave and optical domains at the quantum level. In this Rapid Communication, we demonstrate the ultralow-power, on-chip, electron-spin-resonance spectroscopy of Er 3+ spins doped in a Y 2 SiO 5 crystal using a high-Q, coplanar, superconducting resonator.Quantum communication is a rapidly developing field of science and technology, which allows the transmission of information in an intrinsically secure way. 1 As well as its classical counterpart, a quantum communication network can combine various types of systems which transmit, receive, and process information using quantum algorithms. 2 For example, the nodes of such a network can be implemented by superconducting (SC) quantum circuits operated in the GHz frequency range, 3 whereas fiber optics operated at near infrared can be used to link them over long distances. For the reversible transfer of quantum states between systems operating at GHz and optical frequency ranges, one must use a hybrid system. 4 Spin ensembles coupled to a microwave resonator or to a SC qubit represent one of the possible implementations of such a system. 5-8 The collective coupling strength of a spin ensemble is increased with respect to a single spin by the square root of the number of spins. Transparent crystals doped with paramagnetic ions often possess long coherence times, 9,10 and the collective coupling has been recently demonstrated with nitrogen-vacancy centers in diamond, 11-13 organic molecules, 14 and (Cr 3+ ) ions in ruby. 12 In this Rapid Communication, we report on the ultralowpower electron-spin-resonance (ESR) spectroscopy of an erbium-ion spin ensemble at sub-Kelvin temperatures using a high-Q, coplanar, SC resonator. The Er 3+ ions are distinct from other spin ensembles due to their optical transition at the telecom C band, i.e., inside the so-called erbium window at 1.54 μm wavelength, and their long measured optical coherence time. 15 The energy-level diagram of erbium ions embedded inside a crystal is shown in Fig. 1(a). The electronic configuration of a free Er 3+ ion is 4f 11 , with a 4 I term. The spin-orbit coupling splits it into several fine structure levels. An optical transition at the telecom wavelength occurs between the ground state 2S+1 L J = 4 I 15/2 and the first excited state 4 I 13/2 , where S, L, and J are the respective spin, orbital, and total magnetic momenta of the ion. The weak crystal field splits the ground state into eight (J + 1/2) Kramers doublets. 16 At cryogenic temperature, only the lowest doublet Z 1 is populated, therefore the system can be described as an effective electronic spin with S = 1/2. However, erbium has five even isotopes, 162 Er, 164 Er, 166 Er, 168 Er, and 170 Er, and one odd ...

We demonstrate a new method to directly manipulate the state of individual two-level systems (TLSs) in phase qubits. It allows one to characterize the coherence properties of TLSs using standard microwave pulse sequences, while the qubit is used only for state readout. We apply this method to measure the temperature dependence of TLS coherence for the first time. The energy relaxation time T 1 is found to decrease quadratically with temperature for the two TLSs studied in this work, while their dephasing time measured in Ramsey and spin-echo experiments is found to be T 1 limited at all temperatures. DOI: 10.1103/PhysRevLett.105.230504 PACS numbers: 03.67.Lx, 03.65.Yz, 74.50.+r, 85.25.Am In the early 1970s, measurements of the thermal properties of amorphous materials [1] led to the development of a phenomenological model to explain their specific heat and thermal conductivity at low temperature. Anderson, Halperin, and Varma [2], as well as Phillips [3], suggested the presence of an ensemble of two-level systems in the amorphous material, originating from quantum tunneling of individual atoms or a small group of atoms between two metastable lattice positions.The tunneling model was intensively tested experimentally by ensemble measurements performed on samples having a large two-level system (TLS) density, such as glasses. As an example, the life time of thermally exited states could be measured from the heat release of a sample after a rapid cool-down. Also, quantum coherent measurements of the decoherence times of (near-) resonantly excited subsets of TLSs were performed by monitoring the response to acoustic [4] or electric echo pulses. Interpretation of these experiments inherently requires a statistical analysis of an inhomogeneous ensemble of TLSs, which is characterized by a distribution of dipole orientations and strengths as well as a spread of local strain fields. Since the microscopic nature of TLSs remains an actively debated topic, it is crucial to gain a deeper understanding by observing the properties of individual TLSs and hereby raise the veil imposed by averaging. Experiments on individual TLSs became possible with the advent of superconducting quantum bit (qubit) circuits [5]. Individual TLSs can couple strongly to the qubit via their electric dipole moment when they are located in the dielectric of the thin ( % 2 nm) tunnel barrier of the Josephson junctions forming the qubit. This coupling manifests itself as an avoided level crossing in the qubit spectrum at bias values for which a certain TLS energy splitting ÁE matches the energy difference between the two qubit states [6]. The coupling strength between a TLS and the qubit follows directly from the magnitude of the avoided level crossing S as indicated in Fig. 1(b).Time-resolved experiments on phase qubits have demonstrated that an individual TLS can be manipulated using the qubit as a tool to both fully control and read out its state, and their possible use as a quantum memory has been demonstrated [7]. Recently [8], we showed that...