Chris Macklin

A near–quantum-limited Josephson traveling-wave parametric amplifier

Stringing together a powerful amplifier Amplifying microwave signals with high gain and across a broad range of frequencies is crucial in solid-state quantum information processing (QIP). Achieving broadband operation is especially tricky. Macklin et al. engineered an amplifier that contains a long chain of so-called Josephson junctions (see the Perspective by Cleland). The amplifier exhibited high gain over a gigahertz-sized bandwidth and was able to perform high-fidelity qubit readout. Because the amplifier will be capable of reading out as many as 20 qubits simultaneously, it may help to scale up QIP protocols. Science, this issue p. 307; see also p. 280 A microwave amplifier containing a chain of Josephson junctions shows promising performance. [Also see Perspective by Cleland] Detecting single–photon level signals—carriers of both classical and quantum information—is particularly challenging for low-energy microwave frequency excitations. Here we introduce a superconducting amplifier based on a Josephson junction transmission line. Unlike current standing-wave parametric amplifiers, this traveling wave architecture robustly achieves high gain over a bandwidth of several gigahertz with sufficient dynamic range to read out 20 superconducting qubits. To achieve this performance, we introduce a subwavelength resonant phase-matching technique that enables the creation of nonlinear microwave devices with unique dispersion relations. We benchmark the amplifier with weak measurements, obtaining a high quantum efficiency of 75% (70% including noise added by amplifiers following the Josephson amplifier). With a flexible design based on compact lumped elements, this Josephson amplifier has broad applicability to microwave metrology and quantum optics.

Observation of Measurement-Induced Entanglement and Quantum Trajectories of Remote Superconducting Qubits

The creation of a quantum network requires the distribution of coherent information across macroscopic distances. We demonstrate the entanglement of two superconducting qubits, separated by more than a meter of coaxial cable, by designing a joint measurement that probabilistically projects onto an entangled state. By using a continuous measurement scheme, we are further able to observe single quantum trajectories of the joint two-qubit state, confirming the validity of the quantum Bayesian formalism for a cascaded system. Our results allow us to resolve the dynamics of continuous projection onto the entangled manifold, in quantitative agreement with theory.

Heralded State Preparation in a Superconducting Qubit

We demonstrate high-fidelity, quantum nondemolition, single-shot readout of a superconducting flux qubit in which the pointer state distributions can be resolved to below one part in 1000. In the weak excitation regime, continuous measurement permits the use of heralding to ensure initialization to a fiducial state, such as the ground state. This procedure boosts readout fidelity to 93.9% by suppressing errors due to spurious thermal population. Furthermore, heralding potentially enables a simple, fast qubit reset protocol without changing the system parameters to induce Purcell relaxation.

Dispersive readout of a flux qubit at the single-photon level

A superconducting flux qubit is inductively coupled to a Superconducting QUantum Interference Device (SQUID) magnetometer, capacitively shunted to form a 1.294-GHz resonator. The qubit-state-dependent resonator frequency is weakly probed with a microwave signal and detected with a Microstrip SQUID Amplifier. At a mean resonator occupation

Towards quantum-noise limited multiplexed microwave readout of qubits

\bar{n}

Resonant phase matching of Josephson junction traveling wave parametric amplifiers

= 1.5 photons, the readout visibility is increased by a factor of 4.5 over that using a cryogenic semiconductor amplifier. As

Erratum: Observation of Measurement-Induced Entanglement and Quantum Trajectories of Remote Superconducting Qubits [Phys. Rev. Lett.112, 170501 (2014)]

\bar{n}

Resonant phase matching of Josephson junction traveling wave parametric amplifiers.

is increased from 0.008 to 0.1, no reduction in

Parametric amplification in Josephson junction embedded transmission lines

T_1

Josephson traveling-wave parametric amplifier for superconducting qubit readout

is observed, potentially enabling continuous monitoring of the qubit state.