Universal Gate for Fixed-Frequency Qubits via a Tunable Bus

Abstract

A challenge for constructing large circuits of superconducting qubits is to balance addressability, coherence and coupling strength. High coherence can be attained by building circuits from fixedfrequency qubits, however, leading techniques cannot couple qubits that are far detuned. Here we introduce a method based on a tunable bus which allows for the coupling of two fixed-frequency qubits even at large detunings. By parametrically oscillating the bus at the qubit-qubit detuning we enable a resonant exchange (XX+YY) interaction. We use this interaction to implement a 183 ns two-qubit iSWAP gate between qubits separated in frequency by 854 MHz with a measured average fidelity of 0.9823(4) from interleaved randomized benchmarking. This gate may be an enabling technology for surface code circuits and for analog quantum simulation.

Analysis of a parametrically driven exchange-type gate and a two-photon excitation gate between superconducting qubits

“…Here, we demonstrate significantly faster gate times when the bSWAP interaction is mediated parametrically by a flux-modulation of the tunable coupler using a similar device as in Ref. [26].On the theory level, fast modulations at elevated frequencies pose a challenge for methodology as commonly employed adiabatic approximations break down. In order to compare experimental results with theory, we develop a generalized, time-dependent Schrieffer-Wolff transformation [34,35], which explicitely incorporates the time-dependence of the ancilla transmon frequency as well as counter-rotating terms in the coupling between ancilla and computational transmons [36].…”

“…The effective Hamiltonian in this case is H iSWAP ∝ XX + YY. With this method gate fidelities of 97% have been shown for gates lasting less than 200 ns [26]. Unfortunately, for shorter pulses it becomes hard to avoid unwanted transitions to higher-excited qubit levels as well as qubit-coupler transitions occurring at similar frequencies.…”

“…[26] between two transmon qubits mediated by a third, ancilla transmon device (bus), which couples dispersively to both qubits and whose frequency is modulated by an external magnetic flux. Such a flux-modulation scheme provides frequency-selectivity and allows to use fixed-frequency computational qubits, thereby minimizing the sensitvity of the device with respect to magnetic flux noise and disorder effects.…”

“…Moreover, the quest for useful quantum computations before full quantum error correction becomes available may be assisted by efficient, short-depth gate sequences based on two-or multi-qubit gates [14,15] with versatile types of interactions. In particular, parametric schemes based on tunable couplers have been proposed and recently realized as a means to achieve fast gates with high fidelities [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31].…”

Quantum arithmetics via computation with minimized external control: The half-adder

“…It is worth noting that the majority of the network is made up of ZZ interactions, which is obtainable in superconducting circuits, among others, as shown by Geller et al [47]. Additionally, McKay et al showed that it was possible to obtain (XX + Y Y ) interactions in a superconducting circuit [48], lending itself nicely for the interactions between the second and third qubits of this network. This makes the quantum half-adder with this network configuration almost fully implementable in a superconducting circuit.…”