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...
We introduce a single-number metric, quantum volume, that can be measured using a concrete protocol on near-term quantum computers of modest size (n < ∼ 50), and measure it on several stateof-the-art transmon devices, finding values as high as 16. The quantum volume is linked to system error rates, and is empirically reduced by uncontrolled interactions within the system. It quantifies the largest random circuit of equal width and depth that the computer successfully implements. Quantum computing systems with high-fidelity operations, high connectivity, large calibrated gate sets, and circuit rewriting toolchains are expected to have higher quantum volumes. The quantum volume is a pragmatic way to measure and compare progress toward improved system-wide gate error rates for near-term quantum computation and error-correction experiments.
For superconducting qubits, microwave pulses drive rotations around the Bloch sphere. The phase of these drives can be used to generate zero-duration arbitrary "virtual" Z-gates which, combined with two X π/2 gates, can generate any SU(2) gate. Here we show how to best utilize these virtual Z-gates to both improve algorithms and correct pulse errors. We perform randomized benchmarking using a Clifford set of Hadamard and Z-gates and show that the error per Clifford is reduced versus a set consisting of standard finite-duration X and Y gates. Z-gates can correct unitary rotation errors for weakly anharmonic qubits as an alternative to pulse shaping techniques such as DRAG. We investigate leakage and show that a combination of DRAG pulse shaping to minimize leakage and Z-gates to correct rotation errors (DRAGZ) realizes a 13.3 ns X π/2 gate characterized by low error (1.95[3] × 10 −4 ) and low leakage (3.1[6] × 10 −6 ). Ultimately leakage is limited by the finite temperature of the qubit, but this limit is two orders-of-magnitude smaller than pulse errors due to decoherence.Computers based on quantum bits (qubits) are predicted to outperform classical computers for certain critical problems, e.g., factoring. Unlike a classical bit, which is discretely in the state 0 or 1, a qubit can be in a superposition state |Ψ = cos(θ/2)|0 + e iφ sin(θ/2)|1 where |0 and |1 are the quantum versions of the classical 0 and 1 states. This single-qubit superposition state can be geometrically represented as a point on the surface of a unit-sphere known as the Bloch sphere. Critical to implementing a quantum computer is the ability to control the state of the qubit, i.e., transform the qubit state arbitrarily between two points on the Bloch sphere. This is accomplished by unitary transformations (gates), which correspond to rotations of the state around different axes in the Bloch sphere representation. Physically, X and Y gates (rotations around the X and Y axes) are generated by modulating the coupling between the states |0 and |1 at the frequency difference between these states ω 01 = (E |1 −E |0 )/h. This modulation drive has the general form Ω(t) cos(ω D t − γ) where Ω(t) is the drive strength of the rotation, ω D is the drive frequency (ω D = ω 01 on resonance) and γ is the drive phase. The duration of the gate is set by the desired rotation angle and the drive strength. On-resonance, when γ = 0, the qubit state rotates around the X axis and when γ = π 2 the rotation is around the Y axis. Therefore, the geometric X and Y axes in the Bloch sphere correspond to a real π 2 phase difference between drive fields. Rotations around the remaining axis (Z axis), i.e., Zgates, correspond to a change in the relative phase between the |0 and |1 states. A Z-gate can be implemented by either detuning the frequency of the qubit with respect to the drive field for some finite amount of time (e.g. see Ref.[1]) or by composite X and Y gates. The result is that the qubit state rotates with respect to the X and Y axes. However, it is equivalent t...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.