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AIP Advances

Tunable superconducting qudit mediated by microwave photons

Sung Un Cho, Myung‐Ho Bae, Kicheon Kang, Nam Kim

Published Aug 1, 2015• 1 citation
Publisher

Abstract

We have investigated the time-domain characteristics of the Autler-Townes doublet in a superconducting circuit. The transition probabilities between the ground state and the Autler-Townes doublet states are shown to be controlled in a phase-coherent manner using a well-known microwave pulse pattern technique. The experimental results are well explained by a numerical simulation based on the Markovian master equation. Our result indicates that the Autler-Townes doublet states might be useful as a tunable qudit for implementation of quantum information processing, in particular as a multivalued quantum logic element.

DOI
10.1063/1.4930103

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We have investigated the time-domain characteristics of the Autler-Townes doublet in a superconducting circuit. The transition probabilities between the ground state and the Autler-Townes doublet states are shown to be controlled in a phase-coherent manner using a well-known microwave pulse pattern technique. The experimental results are well explained by a numerical simulation based on the Markovian master equation. Our result indicates that the Autler-Townes doublet states might be useful as a tunable qudit for implementation of quantum information processing, in particular as a multivalued quantum logic element.
Routers, switches, and repeaters are essential components of modern information-processing systems. Similar devices will be needed in future superconducting quantum computers. In this work we investigate experimentally the time evolution of Autler-Townes splitting in a superconducting phase qubit under the application of a control tone resonantly coupled to the second transition. A three-level model that includes independently determined parameters for relaxation and dephasing gives excellent agreement with the experiment. The results demonstrate that the qubit can be used as a ON/OFF switch with 100 ns operating time-scale for the reflection/transmission of photons coming from an applied probe microwave tone. The ON state is realized when the control tone is sufficiently strong to generate an Autler-Townes doublet, suppressing the absorption of the probe tone photons and resulting in a maximum of transmission.
Microwave photons in superconducting circuits is a promising approach for quantum computing due to long coherence times and the easy controllability of superconducting devices. However, as the search of the ultimate design of a quantum computer is still ongoing, various suggestions for the realization of a quantum bit, qubit, exist even in the area of superconducting circuits. In this thesis, we study theoretically quantum computing using itinerant microwave photons as qubits. Basic tools for treating the reflection of wave packets and the noise generated by resistive elements in a circuit are derived starting from a widely used model for superconducting transmission lines. We also present simple quantum devices that can act as quantum gates for microwave qubits, including a novel suggestion for a tunable phase shifter. A tunable phase shifter is a useful component in the study of microwave photons in general as well as in quantum computing. The tunable phase shifter consists of three superconducting quantum interference devices (SQUIDs) that can be treated as tunable inductors in the low-power regime. The results of this thesis can be used both in applications related to quantum computing and in the general study of quantum mechanics in circuits. In particular, we conclude that there exist feasible system parameters, for which it should be possible to demonstrate the tunable phase shifter utilizing available experimental techniques. We also show the principle how the possible nonlinearity of the phase shifter can be used in a two-qubit gate providing interactions between microwave photons.
We address recent advances in quantum optics with propagating microwaves\nin superconducting circuits. This research field exploits on the fact that the coupling between a superconducting artificial atom and propagating microwave photons in a one-dimensional (1D) open transmission line can be made strong enough to observe quantum effects, without using any cavity to confine the microwave photons.\nWe embed an artificial atom, a superconducting transmon qubit, in a 1D open\ntransmission line and investigate the scattering properties of coherent microwaves. When an input coherent state, with an average photon number much less than 1, is on resonance with the artificial atom, we observe extinction of up to 99% in the forward propagating field. We observe the strong nonlinearity of the artificial atom and under strong driving we observe the Mollow triplet. We also study the statistics of the reflected and transmitted beams, which are predicted to be nonclassical states. In particular, we demonstrate photon antibunching in the reflected beam by measuring the second-order correlation function. By applying a second control tone, we observe the Autler-Townes splitting and a giant cross-Kerr effect.\nFurthermore, we demonstrate fast operation of a single-photon router using\nthe Autler-Townes splitting. This device provides important steps towards the realization of a quantum network. This thesis describes the motivation, theoretical background, design, implementation and measurement results.
We propose a tunable nonlinear interaction for the implementation of quantum logic operations on pairs of superconducting resonators, where the two-resonator interaction is mediated by a transmon quantum bit (qubit). This interaction is characterized by a high on-to-off coupling ratio and allows for fast qubit-type and ddd-level system (qudit)-type operations for quantum information processing with multiphoton cavity states. We present analytical and numerical calculations showing that these operations can be performed with practically unit fidelity in absence of any dissipative phenomena, whereas physical two-photon two-resonator operations can be realized with a fidelity of 99.9% in presence of qubit and resonator decoherence. The resonator-qubit-resonator system proposed in this Letter can be implemented using available planar or three-dimensional microwave technology.
We propose a scheme to control the shape of the Autler-Townes (AT) doublet in the photoelectron spectrum from atomic resonance-enhanced multiphoton ionization (REMPI). The scheme is based on the interference of two AT doublets created by ionization of the strongly driven atom from the ground and the resonantly excited state using tailored bichromatic femtosecond laser pulses. In this scheme, the quantum phase of the photoelectrons is crucial for the manipulation of the AT doublet. The laser polarization state and the relative optical phase between the two colors are used to manipulate the interference pattern. We develop an analytical model to describe the bichromatic REMPI process and provide a physical picture of the control mechanism. To validate the model, the results are compared to an ab initio calculation based on the solution of the two-dimensional time-dependent Schr\"odinger equation for the nonperturbative interaction of an atom with intense polarization-shaped bichromatic femtosecond-laser pulses. Our results indicate that the control mechanism is robust with respect to the laser intensity, facilitating its experimental observation.
To do large-scale quantum information processing, it is necessary to control the interactions between individual qubits while retaining quantum coherence. To this end, superconducting circuits allow for a high degree of flexibility. We report on the time-domain tunable coupling of optimally biased superconducting flux qubits. By modulating the nonlinear inductance of an additional coupling element, we parametrically induced a two-qubit transition that was otherwise forbidden. We observed an on/off coupling ratio of 19 and were able to demonstrate a simple quantum protocol.