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arXiv:2110.06664 · Physica Scripta

Integrated programmable controlled phase gate design for quantum information processing

Yalın Başay, Serdar Kocaman

Published Sep 11, 2023• 2 citations
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Abstract

Abstract An integrated programmable controlled-phase (CPHASE) gate has been proposed for quantum information processing applications. This gate can introduce arbitrary phase difference to the target qubit in the case of the control qubit being in the state of |1〉. As the desired phase difference can be utilized after fabrication, unlike the conventional controlled-phase gates that provide hard-coded phase shift, such an integrated gate is expected to pave the way for more versatile operations of current integrated optical circuits as well as possible new applications.

DOI
10.1088/1402-4896/acf895

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Abstract An integrated programmable controlled-phase (CPHASE) gate has been proposed for quantum information processing applications. This gate can introduce arbitrary phase difference to the target qubit in the case of the control qubit being in the state of |1〉. As the desired phase difference can be utilized after fabrication, unlike the conventional controlled-phase gates that provide hard-coded phase shift, such an integrated gate is expected to pave the way for more versatile operations of current integrated optical circuits as well as possible new applications.
Photons are preferable carriers of quantum information having little interaction with environment. Most crucial elements of photonic quantum information processing are two qubit gates. Although usage of controlled-not gates for photonic two qubit operations is common, controlled-phase gates provide more versatile utilization by enabling desired phase difference to be generated between qubits. Ability to determine the phase after fabrication yields extra benefit. We demonstrate a design of integrated photonic controlled-phase gate with programmable phase difference. We used a unique and practical polarization rotator which is ring resonator with a single notch.
We present a linear-optical scheme for a controlled-phase gate with tunable phase shift programmed by a qubit state. In contrast to all previous tunable controlled-phase gates, the phase shift is not hard-coded into the optical setup, but can be tuned to any value from 0 to π by the state of a so-called program qubit . Our setup is feasible with current level of technology using only linear-optical components. We provide an experimental feasibility study to assess the gate’s implementability. We also discuss options for increasing the success probability up to 1/12; this approaches the success probability of an optimal non-programmable tunable controlled-phase gate.
Optical quantum computing (QC) increasingly uses integrated optics based experiments which permit circuit compactness and phase stability [1]. However, despite the rapid adaptations of integrated waveguide devices for quantum photonics, initial gate demonstrations operate in post-selection, thus not allowing scaling of a quantum circuit beyond the depth of a single gate. Recently, a number of quantum circuits have been demonstrated using the femtosecond laser direct write (FLDW) technique [2]. This technique induces refractive index change in glass substrates which can form three-dimensional waveguide devices. Here we demonstrate a potentially scalable waveguide gate for QC, a controlled-phase gate or Knill gate, produced using the FLDW technique. This gate produces a phase shift on a target qubit conditional on the state of a control qubit, as shown in Fig. 1(a). It requires four photons for operation, two of which act as the target and control path-encoded qubits and two ancillas which herald the successful operation of the circuit [3].
Abstract Limited by the decoherence of qubits as well as the errors of quantum gates, near-term superconducting quantum computers can only run low-depth quantum circuits to achieve acceptable fidelity. One possible way to overcome these limitations is to construct quantum circuits with additional high-fidelity expressive multi-qubit gates. Recently, a new three-qubit gate, denoted as Controlled-CPHASE-SWAP (CCZS), has been implemented through simultaneous Controlled-Z (CZ) gates. The CCZS gate takes less time than a single CZ gate and can be implemented at the coherence limit. However, how to use the CCZS gate in quantum circuit synthesis remains unexplored. In this paper, we construct the quantum fan-out/parity gates, the controlled-phase gate and the locally fully connected CZ gates with the CCZS gate, respectively. Furthermore, applications of the CCZS gate in quantum error correction, quantum Fourier transform and quantum approximate optimization algorithm are also proposed. We evaluate the performance of the CCZS gate in quantum circuit synthesis through simulation and explore its potential advantages over CZ gates.
Despite progress towards achieving low error rates with superconducting qubits, error-prone two-qubit gates remain a bottleneck for realizing large-scale quantum computers. Therefore, a systematic framework to design high-fidelity gates becomes imperative. One type of two-qubit gate in superconducting qubits is the controlled-phase (CPHASE) gate, which utilizes a conditional interaction between higher energy levels of the qubits controlled by a baseband flux pulse on one of the qubits or a tunable coupler. In this work, we study an adiabatic implementation of CPHASE gates and formulate the design of the control trajectory for the gate as a pulse-design problem. We show in simulation that the Chebyshev-based trajectory can, in certain cases, enable gates with gate infidelity lower by an average of 23.3% when compared to the widely used Slepian-based trajectory.