Besides the conventional transverse couplings between superconducting qubits (SQs) and electromagnetic fields, there are additional longitudinal couplings when the inversion symmetry of the potential energies of the SQs is broken. We study nonclassical-state generation in a SQ which is driven by a classical field and coupled to a single-mode microwave field. We find that the classical field can induce transitions between two energy levels of the SQs, which either generate or annihilate, in a controllable way, different photon numbers of the cavity field. The effective Hamiltonians of these classical-field-assisted multiphoton processes of the singlemode cavity field are very similar to those for cold ions, confined to a coaxial RF-ion trap and driven by a classical field. We show that arbitrary superpositions of Fock states can be more efficiently generated using these controllable multiphoton transitions, in contrast to the single-photon resonant transition when there is only a SQ-field transverse coupling. The experimental feasibility for different SQs is also discussed.
The engineering of quantum devices has reached the stage where we now have small scale quantum processors containing multiple interacting qubits within them. Simple quantum circuits have been demonstrated and scaling up to larger numbers is underway [1,2]. However as the number of qubits in these processors increases, it becomes challenging to implement switchable or tunable coherent coupling among them. The typical approach has been to detune each qubit from others or the quantum bus it connected to [1, 2], but as the number of qubits increases this becomes problematic to achieve in practice due to frequency crowding issues. Here, we demonstrate that by applying a fast longitudinal control field to the target qubit, we can turn off its couplings to other qubits or buses (in principle on/off ratio higher than 100 dB). This has important implementations in superconducting circuits as it means we can keep the qubits at their optimal points, where the coherence properties are greatest, during coupling/decoupling processing. Our approach suggests a new way to control coupling among qubits and data buses that can be naturally scaled up to large quantum processors without the need for auxiliary circuits and yet be free of the frequency crowding problems.Superconducting quantum circuits [3,4] are promising candidates to realize quantum processors and simulators. They have been used to demonstrate various quantum algorithms and implement thousands of quantum operations within their coherence time [5], in which controllable couplings are inevitable. The typical way to couple/decouple two superconducting quantum elements with always-on coupling is to tune their frequencies in or out of resonance [6][7][8][9][10][11][12]. This method is widely adopted even for the most recent universal gate implementations [5,[13][14][15]. However, it suffers from several defects, namely, it is technically difficult to avoid frequency crowding problem in large scale circuits; the qubits cannot always work at the coherent optimal point and the fast tuning of the qubit frequency results in non-adiabatic information leakage. To overcome the above problems, significant * Electronic address: yuxiliu@mail.tsinghua.edu.cn † Electronic address: xbzhu16@ustc.edu.cn effort has been devoted both theoretically [16,17] and experimentally [18-23] to develop couplers for parametrically tuning the coupling strength between two elements. Recently high coherence and fast tunable coupling has been demonstrated [24], however, these quantum or classical couplers increase the complexity of the circuits and introduce new decoherence sources. Therefore, the implementation of quantum switch for coherent coupling between quantum elements is still a big challenge in scalable quantum circuits.In this letter, we demonstrate a simple yet reliable method to switch on/off the coupling between a quantum resonator and a superconducting flux qubit [25] via a control field, longitudinally applied to the qubit [26,27]. Our system is a gap tunable flux qubit [28,29], coupled to...
We propose a method to generate entangled states of the vibrational modes of N membranes which are coupled to a cavity mode via the radiation pressure. Using sideband excitations, we show that arbitrary entangled states of vibrational modes of different membranes can be produced in principle by sequentially applying a series of classical pulses with desired frequencies, phases and durations. As examples, we show how to synthesize several typical entangled states, for example, Bell states, NOON states, GHZ states and W states. The environmental effect, information leakage, and experimental feasibility are briefly discussed. Our proposal can also be applied to other experimental setups of optomechanical systems, in which many mechanical resonators are coupled to a common sing-mode cavity field via the radiation pressure.
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