We have measured the plasma resonances of an array of Josephson junctions in the regime E(J)>>E(C), up to the ninth harmonic by incorporating it as part of a resonator capacitively coupled to a coplanar waveguide. From the characteristics of the resonances, we infer the successful implementation of a superinductance, an electrical element with a nondissipative impedance greater than the resistance quantum [R(Q)=h/(2e)(2) is approximately equal to 6.5 kΩ] at microwave frequencies. Such an element is crucial for preserving the quantum coherence in circuits exploiting large fluctuations of the superconducting phase. Our results show internal losses less than 20 ppm, self-resonant frequencies greater than 10 GHz, and phase-slip rates less than 1 mHz, enabling direct application of such arrays for quantum information and metrology. Arrays with a loop geometry also demonstrate a new manifestation of flux quantization in a dispersive analog of the Little-Parks effect.
Quantum physics was invented to account for two fundamental features of measurement resultstheir discreetness and randomness. Emblematic of these features is Bohr's idea of quantum jumps between two discrete energy levels of an atom 1 . Experimentally, quantum jumps were first observed in an atomic ion driven by a weak deterministic force while under strong continuous energy measurement 2-4 . The times at which the discontinuous jump transitions occur are reputed to be fundamentally unpredictable. Can there be, despite the indeterminism of quantum physics, a possibility to know if a quantum jump is about to occur or not? Here, we answer this question affirmatively by experimentally demonstrating that the jump from the ground to an excited state of a superconducting artificial three-level atom can be tracked as it follows a predictable "flight," by monitoring the population of an auxiliary energy level coupled to the ground state. The experimental results demonstrate that the jump evolution when completed is continuous, coherent, and deterministic. Furthermore, exploiting these features and using real-time monitoring and feedback, we catch and reverse a quantum jump mid-flight, thus deterministically preventing its completion. Our results, which agree with theoretical predictions essentially without adjustable parameters, support the modern quantum trajectory theory 5-9 and provide new ground for the exploration of real-time intervention techniques in the control of quantum systems, such as early detection of error syndromes.Bohr conceived of quantum jumps 1 in 1913, and while Einstein elevated their hypothesis to the level of a quantitative rule with his AB coefficient theory 10,11 , Schrödinger strongly objected to their existence 12 . The nature and existence of quantum jumps remained a subject of controversy for seven decades until they were directly observed in a single system 2-4 . Since then, quantum jumps have been observed in a variety of atomic [13][14][15][16] and solid-state 17-21 systems. Recently, quantum jumps have been recognized as an essential phenomenon in quantum feedback control 22,23 , and in particular, for detecting and correcting decoherence-induced errors in quantum information systems 24,25 .
Dephasing induced by residual thermal photons in the readout resonator is a leading factor limiting the coherence times of qubits in the circuit QED architecture. This residual thermal population, of the order of 10 −1 -10 −3 , is suspected to arise from noise impinging on the resonator from its input and output ports. To address this problem, we designed and tested a new type of band-pass microwave attenuator that consists of a dissipative cavity well thermalized to the mixing chamber stage of a dilution refrigerator. By adding such a cavity attenuator inline with a 3D superconducting cavity housing a transmon qubit, we have reproducibly measured increased qubit coherence times. At base temperature, through Hahn echo experiment, we measured T2e/2T1 = 1.0 (+0.0/−0.1) for two qubits over multiple cooldowns. Through noise-induced dephasing measurement, we obtained an upper bound 2 × 10 −4 on the residual photon population in the fundamental mode of the readout cavity, which to our knowledge is the lowest value reported so far. These results validate an effective method for protecting qubits against photon noise, which can be developed into a standard technology for quantum circuit experiments.
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