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Superconductor Science and Technology

Sub-nanosecond operations on superconducting quantum register based on Ramsey patterns

M. V. Bastrakova, N. V. Klenov, Vsevolod I. Ruzhickiy, Igor I. Soloviev, Сатанин Аркадий Михайлович

Published Feb 14, 2022• 21 citations
Publisher

Abstract

Abstract An ultrafast qubit control concept is proposed and analyzed theoretically to reduce the duration of operations with single and multiple superconducting qubits. It is based on the generation of Ramsey fringes due to unipolar sub-nanosecond control pulses. The interference of waves of qubit states population propagating forward and backward in time play a key role in the concept. The influence of the shape and duration of control pulses on the contrast of the interference pattern is revealed in the frame of Ramsey’s paradigm. Protocols for the observation of Ramsey oscillations and an implementation of various gate operations are developed for flux qubits. We also suggest a notional engineering solution for creating the required sub-nanosecond control pulses with the desired shape and amplitude. It is demonstrated that this makes it possible to control the quantum states of the system with a fidelity of more than 99%.

DOI
10.1088/1361-6668/ac5505

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In this paper we discuss methods for controlling the states of interacting superconducting flux qubits using energy-efficient devices of rapid single flux quantum logic (the resonators with Josephson nonlinearity). A comparative analysis for one - and two-qubit quantum logical operations performed both within the traditional control technique using Rabi pulses and using picosecond single unipolar magnetic field pulses is carried out. It is shown that by optimizing the shape and parameters of unipolar control pulses (associated, for example, with the propagation of fluxons in the transmission lines) it is possible to implement all the basic operations with the fidelity of more than 97 %. The efficiency of the developed technique was demonstrated for programming a two-bit quantum processor that implements the simplest Deutsch-Joza algorithm.

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Abstract An ultrafast qubit control concept is proposed and analyzed theoretically to reduce the duration of operations with single and multiple superconducting qubits. It is based on the generation of Ramsey fringes due to unipolar sub-nanosecond control pulses. The interference of waves of qubit states population propagating forward and backward in time play a key role in the concept. The influence of the shape and duration of control pulses on the contrast of the interference pattern is revealed in the frame of Ramsey’s paradigm. Protocols for the observation of Ramsey oscillations and an implementation of various gate operations are developed for flux qubits. We also suggest a notional engineering solution for creating the required sub-nanosecond control pulses with the desired shape and amplitude. It is demonstrated that this makes it possible to control the quantum states of the system with a fidelity of more than 99%.
An ultrafast qubit control concept is proposed to reduce the duration of operations with a single and multiple superconducting qubits. It is based on the generation of Ramsey fringes due to unipolar picosecond control pulses. The key role in the concept is played by the interference of waves of qubit states population propagating forward and backward in time. The influence of the shape and duration of control pulses on the contrast of the interference pattern is revealed in the frame of Ramsey's paradigm. Protocols for observation of Ramsey oscillations and implementation of various gate operations are developed. We also suggest a notional engineering solution for creating the required picosecond control pulses with desired shape and amplitude. It is demonstrated that this makes it possible to control the quantum states of the system with the fidelity of more than 99%.
Superconducting devices based on the Josephson effect are effectively used for the implementation of qubits and quantum gates. The manipulation of superconducting qubits is generally performed by using microwave pulses with frequencies from 5 to 15 GHz, obtaining a typical operating frequency from 100 MHz to 1 GHz. A manipulation based on simple pulses in the absence of microwaves is also possible. In our system, a magnetic flux pulse modifies the potential of a double SQUID qubit from a symmetric double well to a single deep-well condition. By using this scheme with a Nb/AlO x /Nb system, we obtained coherent oscillations with sub-nanosecond period (tunable from 50 to 200 ps), very fast with respect to other manipulating procedures, and with a coherence time up to 10 ns, of the order of that obtained with similar devices and technologies but using microwave manipulation. We introduce ultrafast manipulation, presenting experimental results, new issues related to this approach (such as the use of a compensation procedure for canceling the effect of 'slow' fluctuations) and open perspectives, such as the possible use of RSFQ logic for qubit control.
Abstract The conventional technique for controlling the states of individual qubits and quantum registers is based on the use of high-frequency modulated pulses whose carrier frequency is close to the transition frequency between the ground states of “artificial atoms” and “molecules” (the Rabi technique). In this paper, an alternative concept for controlling superconducting qubits is developed that is based on the idea of fast excitation of a quantum system by solitary unmodulated pulses. The formation of entangled Bell states in a two-qubit superconducting register under the action of field pulses obtained by rapid single flux quantum (RSFQ) devices is demonstrated by numerical simulation. The tomography of quantum states is performed by solving the density matrix equation. It is shown that the technique of “unipolar pulses” allows one to speed up the formation of entangled states (at picosecond times) with a fidelity of 95–98%. The execution of the basic quantum algorithms is demonstrated: the Deutsch–Jozsa algorithm with an accuracy of more than 98% and Grover’s algorithm with an accuracy of 93%.
Abstract Methods for controlling states of interacting superconducting flux qubits using power-efficient devices of fast single-quantum logic (Josephson nonlinearity cavities) are studied. One- and two-qubit quantum logical operations performed within the conventional control technique using Rabi pulses and using picosecond single unipolar magnetic field pulses are comparatively analyzed. It is shown that all main operations can be implemented with an accuracy of better than 97% due to optimization of the shape and parameters of unipolar control pulses (associated with, e.g., propagation of fluxons in transmission lines). The efficiency of the developed technique for programming a two-qubit quantum processor implementing the simplest Deutsch–Jozsa algorithm is demonstrated.
We have observed coherent time evolution between two quantum states of a superconducting flux qubit comprising three Josephson junctions in a loop. The superposition of the two states carrying opposite macroscopic persistent currents is manipulated by resonant microwave pulses. Readout by means of switching-event measurement with an attached superconducting quantum interference device revealed quantum-state oscillations with high fidelity. Under strong microwave driving, it was possible to induce hundreds of coherent oscillations. Pulsed operations on this first sample yielded a relaxation time of 900 nanoseconds and a free-induction dephasing time of 20 nanoseconds. These results are promising for future solid-state quantum computing.
In this paper we discuss methods for controlling the states of interacting superconducting flux qubits using energy-efficient devices of rapid single flux quantum logic (the resonators with Josephson nonlinearity). A comparative analysis for one - and two-qubit quantum logical operations performed both within the traditional control technique using Rabi pulses and using picosecond single unipolar magnetic field pulses is carried out. It is shown that by optimizing the shape and parameters of unipolar control pulses (associated, for example, with the propagation of fluxons in the transmission lines) it is possible to implement all the basic operations with the fidelity of more than 97 %. The efficiency of the developed technique was demonstrated for programming a two-bit quantum processor that implements the simplest Deutsch-Joza algorithm.