Qubit Architecture with High Coherence and Fast Tunable Coupling

Abstract

We introduce a superconducting qubit architecture that combines high-coherence qubits and tunable qubit-qubit coupling. With the ability to set the coupling to zero, we demonstrate that this architecture is protected from the frequency crowding problems that arise from fixed coupling. More importantly, the coupling can be tuned dynamically with nanosecond resolution, making this architecture a versatile platform with applications ranging from quantum logic gates to quantum simulation. We illustrate the advantages of dynamic coupling by implementing a novel adiabatic controlled-Z gate, at a speed approaching that of single-qubit gates. Integrating coherence and scalable control, our "gmon" architecture is a promising path towards large-scale quantum computation and simulation.The fundamental challenge for quantum computation and simulation is to construct a large-scale network of highly connected coherent qubits [1, 2]. Superconducting qubits use macroscopic circuits to process quantum information and are a promising candidate towards this end [3]. Over the last several years, materials research and circuit optimization have led to significant progress in qubit coherence [4][5][6]. Superconducting qubits can now perform hundreds of operations within their coherence times, allowing for research into complex algorithms such as error correction [7,8].It is desirable to combine these high-coherence qubits with tunable inter-qubit coupling; the resulting architecture would allow for both coherent local operations and dynamically varying qubit interactions. For quantum simulation, this would provide a unique opportunity to investigate dynamic processes in non-equilibrium condensed matter phenomena [9][10][11][12][13]. For quantum computation, such an architecture would provide isolation for single-qubit gates while at the same time enabling fast two-qubit gates that minimize errors from decoherence. Despite previous successful demonstrations of tunable coupling [14][15][16][17][18][19][20][21][22][23], these applications have yet to be realized due to the challenge of incorporating tunable coupling with high coherence devices.Here, we introduce a planar qubit architecture that combines high coherence with tunable inter-qubit coupling g. This "gmon" device is based on the Xmon transmon design [5], but now gives nanosecond control of the coupling strength with a measured on/off coupling ratio exceeding 1000. We find that our device retains the high coherence inherent in the Xmon design, with the coupler providing unique advantages in constructing single-and two-qubit quantum logic gates. With the coupling turned off, we demonstrate that our architecture is protected from the frequency crowding problems that arise from fixed coupling. Our single-qubit gate fidelity is nearly independent of the qubit-qubit detuning, even when operating the qubits on resonance. By dynamically tuning the coupling, we implement a novel adiabatic controlled-Z gate at a speed approaching that of single-qubit gates.A two-qubit unit cell with tun...

Full two-photon down-conversion of a single photon

“…In addition to this, the superconducting quantum interference device (SQUID) that controls the transmon frequency is small and shielded away from any coupling with the transmission line. Inductive couplings between transmons have been demonstrated, however [29,30]. We make use of similar ideas to envision a different coupling architecture that allows one to break the parity symmetry in the transmon setup.…”

Tunable coupler for superconducting Xmon qubits: Perturbative nonlinear model

“…Most importantly, the design introduces tunability without compromising high coherence. Tunably coupled Xmon's based on this design, which are called gmon qubits, have been demonstrated recently [24].…”

“…In the weakly coupled limit the effective coupling strength-half the splitting between the symmetric and antisymmetric eigenstates-is approximately [24] …”

Overlap junctions for high coherence superconducting qubits

“…This non-linearity separates the two lowest energy levels from higher excitations, forming a two-level system as the physical qubit. Coherence times of superconducting qubits have been increased significantly in both 2D and 3D geometries (∼10-100µs) [1][2][3][4][5]. These relatively long coherence times, combined with fast, high-fidelity gate schemes, have enabled the demonstration of quantum error detection with superconducting devices [6][7][8].…”