Adiabatic Two-Qubit Gates In Superconducting Circuits

Superconducting circuit has been proved a promising candidate for quantum com-puting.High fidelity quantum gates,especially two-qubit gates,are keys to the imple-mentation of quantum algorithms.During past decades,extensive efforts have been made to achieve speedy and precise two-qubit gates.In general,there are three sources of gate errors:decoherence,non-ideal inter-actions,and imperfect control.?1?Decoherence is the dominant source in most gate protocols.Since improving coherence is a long-term effort that requires upgrades in fabrication processes and extensive studies in materials,reducing gate time is the most direct way to enhance fidelity.?2?Parasitic coupling is a thorny problem in scalable quantum processors.Adding an extra coupler between qubits can suppress many non-ideal interactions,which has attracted much attention recently.?3?For a perfect con-trol,massive calibrations are needed to compensate both pulse distortion and control crosstalk.Therefore,it is desirable to design a more robust gate scheme which is in-sensitive to the errors of control parameters.Since the adiabatic evolution is immune to the local fluctuations,people naturally involve the adiabatic concept in gate opera-tions.However,the simple adiabatic evolution subject to the constraint of the adiabatic condition.The lengthening gate time degrades the fidelity by introducing more de-coherence.To mitigate this issue,people propose various protocols to accelerate the adiabatic gates,but a few are suitable for the two-qubit system.In this thesis,I will first introduce the basic concepts in superconducting quantum computing and summarize the most popular two-qubit gate protocols.Then we will propose two adiabatic protocols to implement fast,robust,and precise two-qubit gates.The first one is a two-qubit-superadiabatic?TQSA?gate protocol based on para-metric modulation.In the scheme,a parametric modulating field provides tunable effective coupling between two qubits,enabling us to construct a target superadia-batic Hamiltonian.We experimentally demonstrate TQSA gates in superconducting circuits consisting of multiple qubits.Using superadiabatic evolution,we implement both SWAP gate and CZ gate.We track the state evolution in the{|01?,|10?}subspace,and find no nonadiabatic error during the SWAP operation.Then we investigate the robustness of TQSA gates against the variations of control parameters.A superadia-batic CZ gate is finally demonstrated with a fidelity of 94.2%,which is mainly limited by decoherence.Using numerical simulation,we prove that gate fidelity can theoreti-cally reach 99.9%with higher coherence,which is promising for quantum information processing.The second one is a fast-adiabatic c-phase gate protocol for fixed frequency qubits coupled via tunable couplers.The gate is realized by adiabatically modulating the fre-quency of the coupler,which turns ZZ coupling on and off to accumulate a conditional phase.We show that high contrast ZZ coupling can be achieved in a full transmon system.In our proposed scheme with typical circuit parameters,the coupling strength can vary from?10 MHz?“on”state?to?1 k Hz?“off”state?.Another key advantage is the strong coupling??100 MHz?between the coupler and computation space,which elevates the speed limit of an adiabatic quantum gate.Our simulation shows that the error rate of a CZ gate using our protocol can be less than 10^-6,with a short gate time less than 60 ns.The high fidelity and fast gate speed suggest that our protocol provides a promising tool for quantum computing.