In this Letter we propose a fully scalable randomized benchmarking protocol for quantum information processors. We prove that the protocol provides an efficient and reliable estimate of the average error-rate for a set operations (gates) under a very general noise model that allows for both time and gate-dependent errors. In particular we obtain a sequence of fitting models for the observable fidelity decay as a function of a (convergent) perturbative expansion of the gate errors about the mean error. We illustrate the protocol through numerical examples.

We describe and expand upon the scalable randomized benchmarking protocol proposed in Phys. Rev. Lett. 106, 180504 (2011) which provides a method for benchmarking quantum gates and estimating the gate-dependence of the noise. The protocol allows the noise to have weak time and gate-dependence, and we provide a sufficient condition for the applicability of the protocol in terms of the average variation of the noise. We discuss how state preparation and measurement errors are taken into account and provide a complete proof of the scalability of the protocol. We establish a connection in special cases between the error rate provided by this protocol and the error strength measured using the diamond norm distance.

We present improvements in both theoretical understanding and experimental implementation of the cross resonance (CR) gate that have led to shorter two-qubit gate times and interleaved randomized benchmarking fidelities exceeding 99%. The CR gate is an all-microwave two-qubit gate offers that does not require tunability and is therefore well suited to quantum computing architectures based on 2D superconducting qubits. The performance of the gate has previously been hindered by long gate times and fidelities averaging 94-96%. We have developed a calibration procedure that accurately measures the full CR Hamiltonian. The resulting measurements agree with theoretical analysis of the gate and also elucidate the error terms that have previously limited the gate fidelity. The increase in fidelity that we have achieved was accomplished by introducing a second microwave drive tone on the target qubit to cancel unwanted components of the CR Hamiltonian.The cross resonance (CR) gate is an entangling gate for superconducting qubits that uses only microwave control [1,2] and has been the standard for multiqubit experiments in superconducting architectures using fixed-frequency transmon qubits [3,4]. Superconducting qubits arranged with shared quantum buses [5] allow qubit networks to be designed with any desired connectivity. This flexiblity of design also translates into a flexibility of control and many choices in entangling gate implementations. The CR gate is one choice of two-qubit gate that uses only microwave control, as opposed to using magnetic flux drives to tune two qubits into a specific resonance condition to entangle, as in the controlledPhase gate [6,7], or to tune a coupler directly [8][9][10][11]. The CR gate requires a small static coupling of the qubit pair that slightly hybridizes the combined system and one additional microwave drive. The relatively low overhead of the CR scheme (the additional control line is combined with a single-qubit drive at room temperature) makes it an attractive gate for use in quantum computing architectures based on planar superconducting qubits. Additionally, the CR gate is well-suited to transmon qubits [12], which have become the superconducting of choice due to promising long coherence and lifetimes [13,14], limited charge noise [15], and high single-qubit gate fidelities [16]. The microwave-only control allows the use of fixed-frequency transmons, further reducing the sources of possible noise [17]. Given all of these qualities, the CR gate has been a useful tool for application in multiqubit experiments, including demonstrations of parity measurements required for the surface code [3].Despite the appeal of the CR gate, its implementation has been hindered by slow gate times. The CR gate relies on an always-on qubit-qubit coupling, but large couplings can lead to crosstalk between qubits. This leads to a trade-off between fast, high-fidelity two-qubit gates and high-fidelity simultaneous single-qubit gates. As a result, typical CR gates between transmon devices have resu...

The ability to detect and deal with errors when manipulating quantum systems is a fundamental requirement for fault-tolerant quantum computing. Unlike classical bits that are subject to only digital bit-flip errors, quantum bits are susceptible to a much larger spectrum of errors, for which any complete quantum error-correcting code must account. Whilst classical bit-flip detection can be realized via a linear array of qubits, a general fault-tolerant quantum error-correcting code requires extending into a higher-dimensional lattice. Here we present a quantum error detection protocol on a two-by-two planar lattice of superconducting qubits. The protocol detects an arbitrary quantum error on an encoded two-qubit entangled state via quantum non-demolition parity measurements on another pair of error syndrome qubits. This result represents a building block towards larger lattices amenable to fault-tolerant quantum error correction architectures such as the surface code.

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.