Single-photon detection is a requisite technique in quantum-optics experiments in both the optical and the microwave domains. However, the energy of microwave quanta are four to five orders of magnitude less than their optical counterpart, making the efficient detection of single microwave photons extremely challenging. Here we demonstrate the detection of a single microwave photon propagating through a waveguide. The detector is implemented with an impedance-matched artificial Λ system comprising the dressed states of a driven superconducting qubit coupled to a microwave resonator. Each signal photon deterministically induces a Raman transition in the Λ system and excites the qubit. The subsequent dispersive readout of the qubit produces a discrete ‘click'. We attain a high single-photon-detection efficiency of 0.66±0.06 with a low dark-count probability of 0.014±0.001 and a reset time of ∼400 ns. This detector can be exploited for various applications in quantum sensing, quantum communication and quantum information processing.
The parametric phase-locked oscillator (PPLO) is a class of frequency-conversion device, originally based on a nonlinear element such as a ferrite ring, that served as a fundamental logic element for digital computers more than 50 years ago. Although it has long since been overtaken by the transistor, there have been numerous efforts more recently to realize PPLOs in different physical systems such as optical photons, trapped atoms, and electromechanical resonators. This renewed interest is based not only on the fundamental physics of nonlinear systems, but also on the realization of new, high-performance computing devices with unprecedented capabilities. Here we realize a PPLO with Josephson-junction circuitry and operate it as a sensitive phase detector. Using a PPLO, we demonstrate the demodulation of a weak binary phase-shift keying microwave signal of the order of a femtowatt. We apply PPLO to dispersive readout of a superconducting qubit, and achieved high-fidelity, single-shot and non-destructive readout with Rabi-oscillation contrast exceeding 90%.
We report single-shot readout of a superconducting flux qubit by using a flux-driven Josephson parametric amplifier (JPA). After optimizing the readout power, gain of the JPA and timing of the data acquisition, we observe the Rabi oscillations with a contrast of 74% which is mainly limited by the bandwidth of the JPA and the energy relaxation of the qubit. The observation of quantum jumps between the qubit eigenstates under continuous monitoring indicates the nondestructiveness of the readout scheme. arXiv:1309.6706v1 [cond-mat.supr-con]
By driving a dispersively coupled qubit-resonator system, we realize an "impedance-matched" Λ system that has two identical radiative decay rates from the top level and interacts with a semi-infinite waveguide. It has been predicted that a photon input from the waveguide deterministically induces a Raman transition in the system and switches its electronic state. We confirm this through microwave response to a continuous probe field, observing near-perfect (99.7%) extinction of the reflection and highly efficient (74%) frequency down-conversion. These proof-of-principle results lead to deterministic quantum gates between material qubits and microwave photons and open the possibility for scalable quantum networks interconnected with waveguide photons. DOI: 10.1103/PhysRevLett.113.063604 PACS numbers: 42.50.Pq, 03.67.Lx, 85.25.Cp In one-dimensional optical systems, interference between an incident photon field and radiation from a quantum emitter (natural or artificial atom) is drastically enhanced due to the low dimensionality [1,2]. This may be contrasted with the three-dimensional case, where the spatial mode mismatch between the incident and scattered fields prevents perfect interference [3]. In particular, when the quantum emitter is coupled to the end of a semi-infinite waveguide and when its excited state has two radiative decay paths (i.e., a so-called Λ or Δ-type three-level system) with equal decay rates, a resonant incident photon into the emitter deterministically induces a Raman transition, and is never reflected due to destructive interference with the reemitted photon [4]. This phenomenon is called "impedance matching," in analogy with the suppression of wave reflection in an electric circuit terminated by its characteristic impedance [5].Artificial atoms in superconducting circuits have proven to be versatile quantum mechanical systems for realizing a variety of intriguing quantum optical phenomena. In circuit quantum electrodynamics (QED) [6,7], strong coupling of a superconducting qubit with a resonator photon is readily achieved. Moreover, an artificial atom coupled directly with a microwave transmission line demonstrates near-perfect reflection of the incident field [8,9]. Recently, we have theoretically shown that an impedance-matched Λ system can be implemented by using the dressed states of a driven circuit-QED system [10]. Although Λ and Δ systems have been proposed theoretically [11][12][13][14] and implemented experimentally with a flux qubit by using the lowest three levels of its asymmetric double-well potential [15][16][17], realizing an impedance-matched Λ system has remained elusive. Here, we experimentally demonstrate impedance matching in the driven circuit-QED system. Using this system, we demonstrate near-perfect absorption of the incident microwave and its frequency down-conversion with a conversion efficiency of 74%. These results and their associated agreement with our model calculations indicate that each incident microwave photon deterministically induces a Raman transit...
We propose a scalable scheme to implement quantum computation in graphene nanoribbon. It is shown that electron or hole can be naturally localized in each zigzag region for a graphene nanoribbon with a sequence of Z-shaped structure without exploiting any confined gate. An onedimensional graphene quantum dots chain is formed in such graphene nanoribbon, where electron or hole spin can be encoded as qubits. The coupling interaction between neighboring graphene quantum dots is found to be always-on Heisenberg type. Applying the bang-bang control strategy and decoherence free subspaces encoding method, universal quantum computation is argued to be realizable with the present techniques.
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