Session R16: Superconducting Qubits: Gates, Couplers and Crosstalk I

Two-qubit gate with a parity-violated superconducting qubit

The second-order nonlinearity has been intensively studied over the decades in nonlinear optics for applications such as squeezed light sources and optical frequency combs. Three-wave mixing, the elementary process induced by the second-order nonlinearity, is also a key concept in the latest development of quantum transducers and has also been utilized for high-fidelity quantum manipulations of trapped ions with a parametric coupling. As is well known, a parity violation is required for the second-order nonlinearity. Here we propose a parity-violated superconducting qubit, which enables us to achieve a strong parametric coupling with a neighboring qubit based on the second-order nonlinearity. The qubit consists of a capacitively-shunted SNAIL [1] circuit under a finite flux bias. We demonstrate fast two-qubit gates (CZ, SWAP, and iSWAP) combined with echo pulses to suppress the effect of a residual longitudinal coupling and evaluate the average gate fidelities using an interleaved randomized benchmarking technique.

Ref. [1] N. E. Frattini et al., Appl. Phys. Lett. 110, 222603 (2017).   

Characterization and Tuneup of High-Fidelity Two-Qubit Operations on a Parametrically Driven Gate

Maintaining the fidelity of single- and two-qubit gates is a necessary requirement for building a quantum computer, especially in the NISQ era where every gate is critical. At the ideal limit without coherent or systematic errors, the gate fidelity fluctuates as T₁ fluctuates. It is important to be able to characterize the timescales over which system parameters may fall out of calibration, and the worst-performance cases should be reported. Here we present our work towards characterizing these effects, and we compare schemes for automating the recalibration process on a device consisting of fixed-frequency transmon qubits with a tunable coupler. The coupler is parametrically driven to selectively perform an iSWAP or CZ gate. Our results indicate that we are primarily limited by incoherent errors with single-qubit gates performing with >99.9% fidelity and two-qubit gates with fidelities >99.0%.   

Efficient cavity control with SNAP gates

Microwave cavities coupled to superconducting qubits have been demonstrated to be a promising platform for quantum information processing. A major challenge in this setup is to realize universal control over the cavity. A promising approach are selective number-dependent arbitrary phase (SNAP) gates combined with cavity displacements. Whereas it could be proven that this is a universal gate set, a central question remained open: how can a given target operation be realized efficiently with a sequence of these operations.

In this talk, we present a practical scheme to address this problem. We will compare our results to previously known techniques: for many experimentally relevant applications, we find that the sequence length can be reduced by a factor of around 10 or higher. We will also sketch the working principle of our method.   

Demonstrating a Continuous Set of Two-qubit Gates for Near-term Quantum Algorithms

Quantum algorithms offer a dramatic speedup for computational problems in machine learning, material science, and chemistry. However, any near-term realizations of these algorithms will need to be heavily optimized to fit within the finite resources offered by existing noisy quantum hardware. Here, taking advantage of the strong adjustable coupling of gmon qubits, we demonstrate a continuous two-qubit gate set that can provide a 3x reduction in circuit depth as compared to a standard gate decomposition. We implement two gate families: an iSWAP-like gate to attain an arbitrary swap angle, θ, and a CPHASE gate that generates an arbitrary conditional phase, Φ. Using one of each of these gates, we can perform an arbitrary two-qubit gate within the excitation-preserving subspace allowing for a complete implementation of the so-called Fermionic Simulation, or fSim, gate set. We benchmark the fidelity of the iSWAP-like and CPHASE gate families as well as 525 other fSim gates spread evenly across the entire fSim(θ, Φ) parameter space achieving purity-limited average two-qubit Pauli error of 3.8 x 10^-3 per fSim gate.   

Experimental implementation of universal nonadiabatic geometric quantum gates with a superconducting circuit

Using geometric phase to realize noise-resilient quantum computing is an important method to enhance control fidelity. In this work, we experimentally realize a universal nonadiabatic geometric quantum gate set in a superconducting qubit chain. We characterize the realized single- and two-qubit geometric gates with both quantum process tomography and randomized benchmarking methods. The measured average fidelities for single-qubit rotation gates and two-qubit controlled-Z gate are 0.9977 and 0.977, respectively. Besides, we also experimentally demonstrate the noise-resilient feature of the realized single-qubit geometric gates by comparing their performance with the conventional dynamic gates with different types of errors in the control field. Thus, our experiment proves a way to achieve high-fidelity geometric quantum gates for robust quantum computation.   

Bounds on cross-resonance gate fidelity in an extended parameter regime

The cross-resonance (CR) interaction is the primary method for enabling two-qubit gates in systems with fixed-frequency transmons using only microwave control. Eliminating the need for tunable circuit elements reduces the system's susceptibility to low-frequency noise, but the cost is more complicated coherent dynamics during the gate [1,2,3,4], both between the targeted qubit pair and to qubits that are nominally idle (spectators). We bound the two-qubit CR gate fidelity as set by unitary dynamics, showing its sensitivity to the parameters governing the two-qubit interaction (coupling strength, relative energies, anharmonicities, and total gate time) and to parameters required to build larger devices (frequency allocation of neighboring qubits). As we reach the limit of fidelities achieved by control of our current devices, consideration of this larger parameter space will let us design devices with higher performance.

[1] Magesan, Gambetta, arXiv:1804.04073 (2018)
[2] Sheldon, Magesan, Chow, Gambetta, Phys. Rev. A 93, 060302 (2016)
[3] Tripathi, Khezri, Korotkov, Phys. Rev. A 100, 012301 (2019)
[4] Kirchhoff, Keßler, Liebermann, Assémat, Machnes, Motzoi, Wilhelm, Phys. Rev. A 97, 042348 (2018)

  

Superconducting qubit gates based on accelerated adiabatic evolution

Quantum gates based on adiabatic evolution are in principle desirable because of their intrinsic robustness against small errors or imperfections in the control pulses. In practice however, the requirement of extremely long evolution times makes them very susceptible to dissipation and noise, resulting in poor fidelities. Recently, protocols based on shortcuts to adiabaticity have been used to design accelerated version of adiabatic quantum gates [1,2]. These gates can be fast while still possessing some of the robustness properties of purely adiabatic gates. We perform detailed theoretical studies and simulations exploring the performance of such gates in realistic superconducting qubit platforms. This includes single qubit gates in protected qubit architectures such as fluxonium, as well as two-qubit gates in cavity-coupled transmon architectures [3].
[1] H. Ribeiro and A. A. Clerk, Phys. Rev. A 100, 032323 (2019).
[2] T. Wang et al., Phys. Rev. Applied 11, 034030 (2019).
[3] D. J. Egger et al. Phys. Rev. Applied 11, 014017 (2019).   

Microwave-activated entangling gates in high coherence superconducting qubits

We report experimental progress on microwave-activated entangling gates with capacitively coupled fluxonium qubits. When biased at the flux sweet-spot, individual qubit transition has long coherence (the best device has T₂ > 400 us) [1]. A control-Z gate can be implemented by sending a short 2π-pulse at the frequency near the 1→2 transition of the target qubit [2]. The gate transition has higher frequency and larger matrix element than the qubit transition, resulting in fast gate and minimal spurious phase errors. Another microwave entangling gate, similar to the cross-resonance gate in transmon [3], can be applied to the computational subspace. We discuss qubits' design and fabrication, initialization, readout, and benchmarking of the gates.

[1] Long B. Nguyen, et. al., Phys. Rev. X (2019).
[2] Konstantin N. Nesterov, et. al., Phys. Rev. A 98, 030301 (2018).
[3] Jerry M. Chow, et. al., Phys. Rev. Lett. 107, 080502 (2011).   

Demonstration of entangling gate for all-to-all connected superconducting qubits

Exploring highly connected networks of qubits is invaluable for implementing various quantum codes and simulations. All-to-all connectivity allows for entangling qubits with reduced gate depth. In the ion community, the Mølmer-Sørensen gate is routinely used to entangle over a dozen qubits with high fidelity. We report on the observed fidelity for a Mølmer-Sørensen-like interaction through the use of shared coplanar waveguide (CPW) resonators to couple multiple superconducting qubits. This gate allows us to selectively entangle any subset of the qubits coupled to the shared resonator.   

Multi-qubit gate mediated by a shared microwave resonator: Error analysis

Using microwave dressing of superconducting qubits, it is possible to implement a multi-qubit gate via a shared resonator. The gate investigated here is analogous to the Mølmer-Sørensen gate commonly used for trapped-ion qubits, but using the photon mode of the resonator instead of the phonon mode of the ions. The gate requires the use of two-photon transitions induced by bi-chromatic fields. We discuss in this talk the different sources of errors currently limiting the fidelity of the gate, relative to the coherence of the driving fields and the lifetimes of the dressed qubits.   

Fidelity Optimization of the Cross-resonance Gate on a Multi-qubit Quantum Processor

In this work, we benchmark the performance of the cross-resonance gate in a multi-qubit setting, and evaluate gate fidelity as a function of circuit and control parameters, such as qubit detuning, effective coupling rate, and control pulse shape. We use numerical techniques to efficiently optimize gate parameter configurations. Lastly, we monitor the error syndromes of calibrated gates to investigate their sensitivity to environmental drift. This work outlines systematic calibration and operation of cross-resonance gates with model-guided optimization for general algorithmic utilization in near-term quantum computers.   

A continuously tunable coupler for switching off adjacent qubit coupling in a superconducting circuit

Controllable interaction between superconducting qubits is desirable for large-scale quantum computation and simulation. We experimentally realize a simply designed and flux-tunable coupler with continuous tunability that can turn off the adjacent qubit coupling in a superconducting circuit. Based on this coupler, we demonstrate a new scheme for a two-qubit controlled-Z gate: two Xmon qubits, originally far detuned, are first gradually brought into resonance while adjusting the coupler to keep the qubit-qubit coupling off; then the qubit-qubit coupling is turned on by the tunable coupler for the necessary interaction; finally the two qubits are brought out of resonance to the idle points while keeping the coupling off. This scheme not only efficiently suppresses the leakage out of the computational subspace but also allows for a large qubit-qubit interaction strength. We achieve an average controlled-Z gate fidelity of 98.3%, characterized via quantum process tomography and dominantly limited by the system decoherence.   

Cancellation of unwanted ZZ interactions by superconducting qubit engineering

We present a method to cancel the unwanted always-on ZZ interaction in transmon based qubit architectures by qubit engineering. We employ a new engineered qubit, which operates as a weakly anharmonic oscillator and has anharmonicity comparable to the transmon qubit, but with a positive sign. We test this concept on the cross-resonance-gate architecture [1] and outline various strategies for optimal chip design. Based on that, we design a new architecture for a superconducting quantum processor that has an inherent suppression of the unwanted ZZ interactions. Our work addresses on one of the major challenges in a commonly used architecture for superconducting quantum processors, and it enables higher fidelity multiqubit gates.

[1] Chow, J. M. et al. Phys. Rev. Lett. 107, 080502 (2011).   

Implementation of a conditional-phase gate by using in-situ tunable ZZ-interactions

High fidelity two-qubit gates exhibiting low crosstalk are essential building blocks for gate-based quantum information processing. In superconducting circuits two-qubit gates are typically based either on RF-controlled interactions or on the in-situ tunability of qubit frequencies. Here, we present an alternative approach using a tunable ZZ-interaction between two-qubits, mediated by a flux-tunable coupler element. By adding a direct capacitive coupling path between the qubits, we are able to control the ZZ-coupling rate over three orders of magnitude. Using this coupling mechanism we implement a conditional-phase gate without relying on resonant exchange of excitations and characterize its performance in terms of gate fidelity, leakage and residual coupling in the idle configuration.