Dielectric loss from two-level states is shown to be a dominant decoherence source in superconducting quantum bits. Depending on the qubit design, dielectric loss from insulating materials or the tunnel junction can lead to short coherence times. We show that a variety of microwave and qubit measurements are well modeled by loss from resonant absorption of two-level defects. Our results demonstrate that this loss can be significantly reduced by using better dielectrics and fabricating junctions of small area . With a redesigned phase qubit employing low-loss dielectrics, the energy relaxation rate has been improved by a factor of 20, opening up the possibility of multiqubit gates and algorithms.

Although Josephson junction qubits show great promise for quantum computing, the origin of dominant decoherence mechanisms remains unknown. Improving the operation of a Josephson junction based phase qubit has revealed microscopic two-level systems or resonators within the tunnel barrier that cause decoherence. We report spectroscopic data that show a level splitting characteristic of coupling between a two-state qubit and a two-level system. Furthermore, we show Rabi oscillations whose "coherence amplitude" is significantly degraded by the presence of these spurious microwave resonators. The discovery of these resonators impacts the future of Josephson qubits as well as existing Josephson technologies.

One of the many challenges of building a scalable quantum computer is single-shot measurement of all the quantum bits (qubits). We have used simultaneous single-shot measurement of coupled Josephson phase qubits to directly probe interaction of the qubits in the time domain. The concept of measurement crosstalk is introduced, and we show that its effects are minimized by careful adjustment of the timing of the measurements. We observe the antiphase oscillation of the two-qubit 01 and 10 states, consistent with quantum mechanical entanglement of these states, thereby opening the possibility for full characterization of multiqubit gates and elementary quantum algorithms.

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We have detected coherent quantum oscillations between Josephson phase qubits and microscopic critical-current fluctuators by implementing a new state readout technique that is an order of magnitude faster than previous methods. The period of the oscillations is consistent with the spectroscopic splittings observed in the qubit's resonant frequency. The results point to a possible mechanism for decoherence and reduced measurement fidelity in superconducting qubits and demonstrate the means to measure two-qubit interactions in the time domain.Superconducting circuits based on Josephson tunnel junctions have attracted renewed attention because of their potential use as quantum bits (qubits) in a quantum computer [1]. Rapid progress toward this goal has led to the observation of Rabi oscillations in charge, flux, phase, and hybrid charge/flux based Josephson qubits [2,3,4,5]. Another milestone toward building a scalable quantum computer is the coherent coupling of two qubits. While coupled-qubit interactions have been inferred spectroscopically [6,7] and a two-qubit quantum gate has been implemented[8], the direct detection of correlations of qubit states in the time domain remains to be demonstrated. One obstacle to observing two-qubit dynamics is that the single-shot state readout time must be much shorter than the qubit coherence time (∼ 10 − 100 ns) and the timescale of the coupled-qubit interaction. Fast readout techniques are also needed for error correction algorithms [9].Here we report a state measurement of the phase qubit that has high fidelity and a duration of only 2 − 4 ns. Using this new readout technique, we directly detect time-domain quantum oscillations between the qubit and the recently discovered spurious resonators associated with critical-current fluctuators in Josephson tunnel junctions [10]. These results explicitly illustrate the mechanism by which critical-current fluctuators decohere phase qubits. We also present a model that attributes a reduction in measurement fidelity to the spurious resonators, and we speculate that qubit-fluctuator coupling contributes to decoherence and loss of fidelity in the flux and charge/flux qubits as well. In addition to revealing these new aspects of qubit physics, the few-nanosecond measurement technique will be valuable for future experiments on coupled qubits.The design and fabrication of the Josephson phase qubits used in this experiment have been described previously [2,10], and Fig. 1a shows their principal circuitry. The qubit consists of a superconducting loop containing a single Josephson junction. The qubit is inductively coupled to a line carrying a flux bias current I φ = I dc + δI(t), where I dc varies slowly and a short pulse δI(t) is used for the fast readout scheme. The microwave current I µw induces Rabi oscillations, and it is capacitively coupled to the qubit after passing through low-temperature attenuators (not shown). The dashed box in Fig. 1a surrounds on-chip components kept near 25 mK. Figure 1b shows the potential energy landscape ...