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AIP conference proceedings

Coherent Control of a Flux Qubit with Two-Frequency Microwave Pulses

Shiro Saito, T. Kutsuzawa, Takayoshi Meno, Hirotaka Tanaka, Masahito Ueda, H. Nakano, Kouichi Semba, Hideaki Takayanagi

Published Jan 1, 2006• 0 citations
Publisher

Abstract

Coherent control of a superconducting flux qubit has been achieved by using two‐frequency microwave pulses. We have observed Rabi oscillation resulting from parametric transitions between the qubit states when the qubit Larmor frequency matches the sum of the two microwave (MW) frequencies. The Rabi frequency is described by the product of two Bessel functions of the MW amplitudes. The parametric transition observed in our experiment offers useful flexibility for controlling flux qubits.

DOI
10.1063/1.2355012

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Coherent control of a superconducting flux qubit has been achieved by using two‐frequency microwave pulses. We have observed Rabi oscillation resulting from parametric transitions between the qubit states when the qubit Larmor frequency matches the sum of the two microwave (MW) frequencies. The Rabi frequency is described by the product of two Bessel functions of the MW amplitudes. The parametric transition observed in our experiment offers useful flexibility for controlling flux qubits.
Parametric control of a superconducting flux qubit has been achieved by using two-frequency microwave pulses. We have observed Rabi oscillations stemming from parametric transitions between the qubit states when the sum of the two microwave frequencies or the difference between them matches the qubit Larmor frequency. We have also observed multiphoton Rabi oscillations corresponding to one- to four-photon resonances by applying single-frequency microwave pulses. The parametric control demonstrated in this work widens the frequency range of microwaves for controlling the qubit and offers a high quality testing ground for exploring nonlinear quantum phenomena of macroscopically distinct states.
The quantum state of a flux qubit was successfully pulse controlled by using a resonant microwave. We observed Ramsey fringes by applying a pair of phase-shifted π∕2 microwave pulses without introducing detuning. With this method, the qubit state can be rotated on an arbitrary axis in the x-y plane of the Bloch sphere in a rotating frame. We successfully observed qubit Larmor precession stroboscopically with an angular velocity of 2π×11.4Grad∕s. In combination with Rabi pulses, this method enables us to achieve full control of single qubit rotation with shorter operation time.
We experimentally demonstrate coherent oscillations of a tunable superconducting flux qubit by manipulating its energy potential with a nanosecond-long pulse of magnetic flux. The occupation probabilities of two persistent current states oscillate at a frequency ranging from 6 GHz to 21 GHz, tunable by changing the amplitude of the flux pulse. The demonstrated operation mode could allow quantum gates to be realized in less than 100 ps, which is much shorter than gate times attainable in other superconducting qubits. Another advantage of this type of qubit is its immunity to both thermal and magnetic field fluctuations.
We report on two different manipulation procedures of a tunable rf SQUID. First, we operate this system as a flux qubit, where the coherent evolution between the two flux states is induced by a rapid change of the energy potential, turning it from a double well into a single well. The measured coherent Larmor-like oscillation of the retrapping probability in one of the wells has a frequency ranging from 6 to 20 GHz, with a theoretically expected upper limit of 40 GHz. Furthermore, here we also report a manipulation of the same device as a phase qubit. In the phase regime, the manipulation of the energy states is realized by applying a resonant microwave drive. In spite of the conceptual difference between these two manipulation procedures, the measured decay times of Larmor oscillation and microwave-driven Rabi oscillation are rather similar. Due to the higher frequency of the Larmor oscillations, the microwave-free qubit manipulation allows for much faster coherent operations.
We demonstrate coherent control and measurement of a superconducting qubit coupled to a superconducting coplanar waveguide resonator with a dynamically tunable qubit-cavity coupling strength. Rabi oscillations are measured for several coupling strengths showing that the qubit transition can be turned off by a factor of more than 1500. We show how the qubit can still be accessed in the off state via fast flux pulses. We perform pulse delay measurements with synchronized fast flux pulses on the device and observe T1{T}_{1}T1​ and T2{T}_{2}T2​ times of 1.6 and 1.9 $\ensuremath{\mu}$s, respectively. This work demonstrates how this qubit can be incorporated into quantum architectures.
We report measurements on two superconducting flux qubits coupled to a readout superconducting quantum interference device (SQUID). Two on-chip flux bias lines allow independent flux control of any two of the three elements, as illustrated by a two-dimensional qubit flux map. The application of microwaves yields a frequency-flux dispersion curve for 1- and 2-photon driving of the single-qubit excited state, and coherent manipulation of the single-qubit state results in Rabi oscillations and Ramsey fringes. This architecture should be scalable to many qubits and SQUIDs on a single chip.