Superconducting circuits can behave like atoms making transitions between two levels. Such circuits can test quantum mechanics at macroscopic scales and be used to conduct atomic-physics experiments on a silicon chip.Quantum bits, or qubits, form the heart of quantuminformation processing schemes. Because of the quantum parallelism and entanglement that arise from the superposition of states in two-level qubit systems, researchers expect eventual quantum computers to tackle tasks, such as factoring large numbers and simulating large quantum systems, that no ordinary computers can do in a practical time frame.Quantum computing involves preparing, manipulating, and reading out the quantum states of a many-qubit system. So it is desirable to have qubits that can be individually controlled. Moreover, they should be scalable; that is, simply adding more qubits should create a larger circuit capable of more complex calculations. Solid-state qubits satisfy these requirements.Fortunately, very small solid-state devices can behave quantum mechanically. As the size of a bulk conductor becomes increasingly smaller, its quasi-continuous electron conduction band turns into discrete energy levels. An example is a quantum dot, in which electrons are confined to a small semiconducting or metallic box or island composed of millions of atoms. The problem is that the electron states of that island quickly decohere as the microscopic degrees of freedom strongly interact with the environment. A bulk superconductor, in contrast, is composed of many paired electrons that condense into a single-level state. This superconducting state involves macroscopic degrees of freedom and thus exhibits better quantum coherence. By reducing the size of the superconductor, one can reduce the coupling of the superconducting state to the environment and thereby further improve the quantum coherence.Various experiments on superconducting circuits have demonstrated as much [1,2,3,4,5] and those schemes are regarded as promising candidates of qubits that can process quantum information (see Physics Today, June 2002, page 14). Not surprisingly, there is a deep analogy between natural atoms and the artificial atoms composed of electrons confined in small superconducting islands. Both have discrete energy levels and exhibit coher- * J.Q. You (jqyou@fudan.edu.cn) is a professor of physics at Fudan University in Shanghai, China. Franco Nori (nori@umich.edu) is a professor of physics and applied physics at the University of Michigan in Ann Arbor and the director of the Digital Materials Laboratory at Japan's Institute of Physical and Chemical Research (RIKEN). ent quantum oscillations between those levels-so-called Rabi oscillations. But whereas natural atoms are driven using visible or microwave photons that excite electrons from one state to another, the artificial atoms in the circuits are driven by currents, voltages, and microwave photons. The resulting electric and magnetic fields control the tunneling of electrons between the superconducting island and ...
We investigate the quantum dynamics of a Cooper-pair box with a superconducting loop in the presence of a nonclassical microwave field. We demonstrate the existence of Rabi oscillations for both single-and multi-photon processes and, moreover, we propose a new quantum computing scheme (including one-bit and conditional two-bit gates) based on Josephson qubits coupled through microwaves.
We report the first observation of the magnon-polariton bistability in a cavity magnonics system consisting of cavity photons strongly interacting with the magnons in a small yttrium iron garnet (YIG) sphere. The bistable behaviors are emerged as sharp frequency switchings of the cavity magnon-polaritons (CMPs) and related to the transition between states with large and small number of polaritons. In our experiment, we align, respectively, the [100] and [110] crystallographic axes of the YIG sphere parallel to the static magnetic field and find very different bistable behaviors (e.g., clockwise and counter-clockwise hysteresis loops) in these two cases. The experimental results are well fitted and explained as being due to the Kerr nonlinearity with either positive or negative coefficient. Moreover, when the magnetic field is tuned away from the anticrossing point of CMPs, we observe simultaneous bistability of both magnons and cavity photons by applying a drive field on the lower branch.
A goal of quantum information technology is to control the quantum state of a system, including its preparation, manipulation, and measurement. However, scalability to many qubits and controlled con-nectivity between any selected qubits are two of the major stumbling blocks to achieve quantum com-puting (QC). Here we propose an experimental method, using Josephson charge qubits, to efficiently solve these two central problems. The proposed QC architecture is scalable since any two charge qubits can be effectively coupled by an experimentally accessible inductance. More importantly, we formulate an efficient and realizable QC scheme that requires only one (instead of two or more) two-bit operation to implement conditional gates.
Magnon–polaritons are hybrid light–matter quasiparticles originating from the strong coupling between magnons and photons. They have emerged as a potential candidate for implementing quantum transducers and memories. Owing to the dampings of both photons and magnons, the polaritons have limited lifetimes. However, stationary magnon–polariton states can be reached by a dynamical balance between pumping and losses, so the intrinsically nonequilibrium system may be described by a non-Hermitian Hamiltonian. Here we design a tunable cavity quantum electrodynamics system with a small ferromagnetic sphere in a microwave cavity and engineer the dissipations of photons and magnons to create cavity magnon–polaritons which have non-Hermitian spectral degeneracies. By tuning the magnon–photon coupling strength, we observe the polaritonic coherent perfect absorption and demonstrate the phase transition at the exceptional point. Our experiment offers a novel macroscopic quantum platform to explore the non-Hermitian physics of the cavity magnon–polaritons.
We reveal the cooperative effect of coherent and dissipative magnon-photon couplings in an open cavity magnonic system, which leads to nonreciprocity with a considerably large isolation ratio and flexible controllability. Furthermore, we discover unidirectional invisibility for microwave propagation, which appears at the zero-damping condition for hybrid magnon-photon modes. A simple model is developed to capture the generic physics of the interference between coherent and dissipative couplings, which accurately reproduces the observations over a broad range of parameters. This general scheme could inspire methods to achieve nonreciprocity in other systems. Γ ZDC ω m ω c J Γe iπ ω c ω m J Γ ω m ω c Γe iπ ZDC ω m ω c J J ω m = ω c + 2JΓ α
We propose an experimentally feasible method for enhancing the atom-field coupling as well as the ratio between this coupling and dissipation (i.e., cooperativity) in an optical cavity. It exploits optical parametric amplification to exponentially enhance the atom-cavity interaction and, hence, the cooperativity of the system, with the squeezing-induced noise being completely eliminated. Consequently, the atom-cavity system can be driven from the weak-coupling regime to the strong-coupling regime for modest squeezing parameters, and even can achieve an effective cooperativity much larger than 100. Based on this, we further demonstrate the generation of steady-state nearly maximal quantum entanglement. The resulting entanglement infidelity (which quantifies the deviation of the actual state from a maximally entangled state) is exponentially smaller than the lower bound on the infidelities obtained in other dissipative entanglement preparations without applying squeezing. In principle, we can make an arbitrarily small infidelity. Our generic method for enhancing atom-cavity interaction and cooperativities can be implemented in a wide range of physical systems, and it can provide diverse applications for quantum information processing.
Oxalic acid secretion from roots is considered to be an important mechanism for aluminum (Al) resistance in buckwheat (Fygopyrum esculentum Moench). Nonetheless, only a single Al-resistant buckwheat cultivar was used to investigate the significance of oxalic acid in detoxifying Al. In this study, we investigated two buckwheat cultivars, Jiangxi (Al resistant) and Shanxi (Al sensitive), which showed significant variation in their resistance to Al stress. In the presence of 0 to 100 mM Al, the inhibition of root elongation was greater in Shanxi than that in Jiangxi, and the Al content of root apices (0-10 mm) was much lower in Jiangxi. However, the dependence of oxalic acid secretion on external Al concentration and the time course for secretion were similar in both cultivars. Furthermore, the variation in Al-induced oxalic acid efflux along the root was similar, showing a 10-fold greater efflux from the apical 0-to 5-mm region than from the 5-to 10-mm region. These results suggest that both Shanxi and Jiangxi possess an equal capacity for Al-dependent oxalic acid secretion. Another two potential Al resistance mechanisms, i.e. Al-induced alkalinization of rhizosphere pH and root inorganic phosphate release, were also not involved in their differential Al resistance. However, after longer treatments in Al (10 d), the concentrations of phosphorus and Al in the roots of the Al-resistant cultivar Jiangxi were significantly higher than those in Shanxi. Furthermore, more Al was localized in the cell walls of the resistant cultivar. All these results suggest that while Al-dependent oxalic acid secretion might contribute to the overall high resistance to Al stress of buckwheat, this response cannot explain the variation in tolerance between these two cultivars. We present evidence suggesting the greater Al resistance in buckwheat is further related to the immobilization and detoxification of Al by phosphorus in the root tissues.Ionic aluminum (Al) is highly toxic to plant growth and appears to interfere with a number of physiological and biochemical processes (Rengel, 1992;Kochian, 1995). However, species vary widely in their ability to resist the harmful effect of Al, and significant differences in Al resistance have even been reported between genotypes of the same species (Yang et al., 2005). Over the past few decades, concerted efforts have been made to understand the genetic and physiological basis of Al resistance in many different species. As proposed by Taylor (1991), Al resistance mechanisms can be grouped into two categories. One is based on excluding Al from the root cells, and the other relies on improving the resistance of plants to the Al ions once they enter the cytosol. Among the likely exclusion mechanisms, a role for organic acid efflux has been well documented in several species (Ma, 2000;Ryan et al., 2001;Kochian et al., 2004). Other potential exclusion mechanisms include increases in rhizospheric pH (Degenhardt et al., 1998), phosphate efflux (Pellet et al., 1996), the secretion of proteins to bind Al ...
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