We report a superconducting artificial atom with an observed quantum coherence time of T * 2 =95µs and energy relaxation time T1=70µs. The system consists of a single Josephson junction transmon qubit embedded in an otherwise empty copper waveguide cavity whose lowest eigenmode is dispersively coupled to the qubit transition. We attribute the factor of four increase in the coherence quality factor relative to previous reports to device modifications aimed at reducing qubit dephasing from residual cavity photons. This simple device holds great promise as a robust and easily produced artificial quantum system whose intrinsic coherence properties are sufficient to allow tests of quantum error correction. PACS numbers: 03.67.Ac, 42.50.Pq, 85.25.-j Superconducting quantum circuits are a leading candidate technology for large scale quantum computing. They have been used to show a violation of a Bell-type inequality [1]; implement a simple two-qubit gate favorable for scaling [2]; generate three-qubit entanglement [3]; perform a routine relevant to error correction [4];and very recently to demonstrate a universal set of quantum gates with fidelities greater than 95% [5]. Most of these devices employ small angle-evaporated Josephson junctions as their critical non-linear circuit components. Devices designs appear to be consistent with the basic requirements for quantum error correction (QEC) and fault tolerance [6]. However, the construction and operation of much larger systems capable of meaningful tests of such procedures will require individual qubits and junctions with a very high degree of coherence. Current estimates for threshold error rates -and the cumulative nature of errors originating from control, measurement, and decoherence -make likely the need for quantum lifetimes at least 10 3 times longer than gate and measurement times [7], corresponding to 20 to 200µs for typical systems.To this end, improvements in qubit lifetimes have continued for the past decade, spurred largely by clever methods of decoupling noise and loss mechanisms from the qubit transition and thus realizing Hamiltonians more closely resembling their idealized versions. Recently, Paik, et al. made a breakthrough advance [8] by embedding a transmon qubit [9, 10] in a superconducting waveguide cavity. Dubbed three-dimensional circuit QED (3D cQED), this system produced significantly enhanced qubit lifetimes of T 1 =25-60µs and T * 2 =10-20µs, corresponding to quality factors for dissipation and decoherence of Q 1 ≈1.8×10 6 and Q 2 ≈7×10 5 , respectively.These results lead to two important questions. First, are similar coherence properties observable using other fabrication processes, facilities, and measurement setups? Second, what is the origin of the dephasing process suppressing T * 2 well below the no-pure-dephasing limit of 2T 1 ? Is it intrinsic to the junctions or to this qubit ar-chitecture? The weight and urgency of these questions are increased by implications on scaling potential: if the results are reproducible and decoherence tim...

We describe a scalable experimental protocol for estimating the average error of individual quantum computational gates. This protocol consists of interleaving random Clifford gates between the gate of interest and provides an estimate as well as theoretical bounds for the average error of the gate under test, so long as the average noise variation over all Clifford gates is small. This technique takes into account both state preparation and measurement errors and is scalable in the number of qubits. We apply this protocol to a superconducting qubit system and find a bounded average error of 0.003 [0,0.016] for the single-qubit gates X(π/2) and Y(π/2). These bounded values provide better estimates of the average error than those extracted via quantum process tomography.

Quantum process tomography is a necessary tool for verifying quantum gates and diagnosing faults in architectures and gate design. We show that the standard approach of process tomography is grossly inaccurate in the case where the states and measurement operators used to interrogate the system are generated by gates that have some systematic error, a situation all but unavoidable in any practical setting. These errors in tomography can not be fully corrected through oversampling or by performing a larger set of experiments. We present an alternative method for tomography to reconstruct an entire library of gates in a self-consistent manner. The essential ingredient is to define a likelihood function that assumes nothing about the gates used for preparation and measurement. In order to make the resulting optimization tractable we linearize about the target, a reasonable approximation when benchmarking a quantum computer as opposed to probing a black-box function.

The control and handling of errors arising from cross talk and unwanted interactions in multiqubit systems is an important issue in quantum information processing architectures. We introduce a benchmarking protocol that provides information about the amount of addressability present in the system and implement it on coupled superconducting qubits. The protocol consists of randomized benchmarking experiments run both individually and simultaneously on pairs of qubits. A relevant figure of merit for the addressability is then related to the differences in the measured average gate fidelities in the two experiments. We present results from two similar samples with differing cross talk and unwanted qubit-qubit interactions. The results agree with predictions based on simple models of the classical cross talk and Stark shifts.

We demonstrate improved operation of exchange-coupled semiconductor quantum dots by substantially reducing the sensitivity of exchange operations to charge noise. The method involves biasing a double dot symmetrically between the charge-state anticrossings, where the derivative of the exchange energy with respect to gate voltages is minimized. Exchange remains highly tunable by adjusting the tunnel coupling. We find that this method reduces the dephasing effect of charge noise by more than a factor of 5 in comparison to operation near a charge-state anticrossing, increasing the number of observable exchange oscillations in our qubit by a similar factor. Performance also improves with exchange rate, favoring fast quantum operations. DOI: 10.1103/PhysRevLett.116.110402 Gated semiconductor quantum dots are a leading candidate for quantum information processing due to their high speed, density, and compatibility with mature fabrication technologies [1,2]. Quantum dots are formed by spatially confining individual electrons using a combination of material interfaces and nanoscale metallic gates. Although several quantized degrees of freedom are available [3][4][5], the electron spin is often employed as a qubit due to its long coherence time [6,7]. Spin-spin coupling may be controlled via the kinetic exchange interaction, which has the benefit of short range and electrical controllability. Numerous qubit proposals use exchange, including as a two-qubit gate between ESR-addressed spins [8], a single axis of control in a two dot system also employing gradient magnetic fields [9] or spin-orbit couplings [10], or as a means of full qubit control on a restricted subspace of at least three coupled spins [11][12][13]. However, since exchange relies on electron motion, it is susceptible to electric field fluctuations, or charge noise. Limiting the consequence of this noise is critical to attaining performance of exchange-based qubits adequate for quantum information processing.Charge noise in semiconductor quantum dots may originate from a variety of sources including electric defects at interfaces and in dielectrics [14]. These defects typically result in electric fields that exhibit an approximate 1=f noise spectral density. Conventional routes for reducing charge noise include improving materials and interfaces [15] and dynamical decoupling [16][17][18][19]. In this Letter, rather than addressing the microscopic origins or detailed spectrum of charge noise, we introduce a "symmetric" mode of operation where the exchange interaction is less susceptible to that noise. This is done by biasing the device to a regime where the strength of the exchange interaction is first-order insensitive to dot chemical potential fluctuations but is still controllable by modulating the interdot tunnel barrier. This dramatically reduces the effects of charge noise.The principle of symmetric operation can be understood by treating charge noise as equivalent to voltage fluctuations on confinement gates. This approximation is valid when materi...