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arXiv:1210.4311

Modulated pulses compensating classical noise

Christopher Stihl, Benedikt Fauseweh, Stefano Pasini, Götz Silvester Uhrig

Published Oct 16, 2012• 1 citation
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Abstract

We consider pulses of finite duration for coherent control in the presence of classical noise. We derive the corrections to ideal, instantaneous pulses for the case of general decoherence (spin-spin relaxation and spin-lattice relaxation) up to and including the third order in the duration τ_p of the pulses. For pure dephasing (spin-spin relaxation only), we design πand π/2 pulses with amplitude and/or frequency modulation which resemble the ideal ones up to and including the second order in τ_p. For completely general decoherence including spin-lattice relaxation the corrections are computed up to and including the second order in τ_p as well. Frequency modulated pulses are determined which resemble the ideal ones. They are used to design a low-amplitude replacement for XY8 cycles. In comparison with pulses designed to compensate quantum noise less conditions have to be fulfilled. Consequently, we find that the classical pulses can be weaker and simpler than the corresponding pulses in the quantum case.

DOI
10.48550/arxiv.1210.4311

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We consider pulses of finite duration for coherent control in the presence of classical noise. We derive the corrections to ideal, instantaneous pulses for the case of general decoherence (spin-spin relaxation and spin-lattice relaxation) up to and including the third order in the duration τ_p of the pulses. For pure dephasing (spin-spin relaxation only), we design πand π/2 pulses with amplitude and/or frequency modulation which resemble the ideal ones up to and including the second order in τ_p. For completely general decoherence including spin-lattice relaxation the corrections are computed up to and including the second order in τ_p as well. Frequency modulated pulses are determined which resemble the ideal ones. They are used to design a low-amplitude replacement for XY8 cycles. In comparison with pulses designed to compensate quantum noise less conditions have to be fulfilled. Consequently, we find that the classical pulses can be weaker and simpler than the corresponding pulses in the quantum case.
Reliable long-time storage of arbitrary quantum states is a key element of quantum information processing. In order to dynamically decouple a spin or quantum bit from a dephasing environment by non-instantaneous pulses, we introduce an optimized sequence of N control π pulses that are realistic in the sense that they have a finite duration and a finite amplitude. We show that optimized dynamical decoupling is still applicable and that higher-order decoupling can be reached if shaped pulses are implemented. The sequence suppresses decoherence up to the order , with T being the total duration of the sequence and τ mx the maximum length of the pulses. The exponent depends on the shape of the pulse. Based on existing experiments, a concrete setup for the verification of the properties of the advocated sequence is proposed.
In the framework of nuclear magnetic resonance, we consider the general problem of the coherent control of a spin coupled to a bath by means of composite or continuous pulses of duration ${\ensuremath{\tau}}_{\text{p}}$. We show explicitly that it is possible to design the pulse in order to achieve a decoupling of the spin from the bath up to the third order in ${\ensuremath{\tau}}_{\text{p}}$. The evolution of the system is separated in the evolution of the spin under the action of the pulse and of the bath times correction terms. We derive the correction terms for a general time dependent axis of rotation and for a general coupling between the spin and the environment. The resulting corrections can be made to vanish by an appropriate design of the pulse. For $\ensuremath{\pi}$ and $\ensuremath{\pi}/2$ pulses, we demonstrate explicitly that pulses exist which annihilate the first- and the second-order corrections even if the bath is fully quantum mechanical, i.e., it displays internal dynamics. Such pulses will also be useful for quantum information processing.
We consider the coherent control of a quantum bit by the use of short pulses with finite duration ${\ensuremath{\tau}}_{\mathrm{p}}$. By shaping the pulse, we perturbatively decouple the dynamics of the bath from the dynamics of the quantum bit during the pulse. Such shaped pulses provide single quantum bit gates robust against decoherence which are useful for quantum-information processing. We extend previous results in two ways: (i) we treat frequency-modulated pulses and (ii) we pass from time-independent baths to analytically time-dependent baths. First- and second-order solutions for $\ensuremath{\pi}$ and $\ensuremath{\pi}/2$ pulses are presented. They are useful in experiments where amplitude modulation is difficult to realize.
Numerically we simulate the effect of optimized coherent control pulses with a finite duration on a qubit in a bath of spins. The pulses of finite duration are compared with ideal instantaneous pulses. In particular, we show that properly designed short pulses can approximate ideal instantaneous pulses up to a certain order in the shortness of the pulse. We provide examples of such pulses, quantify the discrepancy from the ideal case, and compare their effect for various ranges of the coupling constants.
In order to achieve the high-fidelity quantum control needed for a broad range of quantum information technologies, reducing the effects of noise and system inhomogeneities is an essential task. It is well known that a system can be decoupled from noise or made insensitive to inhomogeneous dephasing dynamically by using carefully designed pulse sequences based on square or delta-function waveforms such as Hahn spin echo or CPMG. However, such ideal pulses are often challenging to implement experimentally with high fidelity. Here, we uncover a new geometrical framework for visualizing all possible driving fields, which enables one to generate an unlimited number of smooth, experimentally feasible pulses that perform dynamical decoupling or dynamically corrected gates to arbitrarily high order. We demonstrate that this scheme can significantly enhance the fidelity of single-qubit operations in the presence of noise and when realistic limitations on pulse rise times and amplitudes are taken into account.
Dynamical decoupling and shaped control are standard tools for mitigating dephasing noise in solid-state qubits. Most analyses either assume commuting pure dephasing with longitudinal control, where filter-function theory applies exactly, or move to fully numerical non-commuting dynamics with limited interpretability. Here I introduce Q-Loop, a scalar modulation protocol designed to suppress low-frequency 1/f γ dephasing more efficiently than standard Carr–Purcell (CP) sequences, and I connect its performance across both regimes. First, in the commuting pure-dephasing limit I compute the filter functions F(ω, τ ) and decoherence functionals χ(τ ) for free evolution, CP, and Q-Loop under 1/f γ noise with γ ∈ {0.5, 0.9, 1.2}. For γ = 0.9 I find that Q-Loop reduces the integrated noise susceptibility to χQ-Loop/χCP ≈ 0.36, with even stronger suppression for steeper spectra (χQ-Loop/χCP ≈ 0.14 at γ = 1.2). Monte Carlo simulations of classical noise trajectories verify that the resulting coherence W(τ ) = ⟨e −iϕ⟩ matches the analytic prediction e −χ(τ) . Second, I move beyond the commuting approximation and study an n-qubit GHZ state evolving under a time-dependent Hamiltonian with transverse control and longitudinal dephasing noise, H(t) = β 2 y(t) Xn i=1 Xi + 1 2 Xn i=1 Bi(t)Zi, where Bi(t) are mixed local+collective classical 1/f 0.9 fields and y(t) is either Q-Loop or CP. Using QuTiP to simulate unitary dynamics conditioned on classical noise, I determine the minimum control amplitude β ∗ (n) required to maintain fidelity F ≥ 0.9 with the GHZ state at a fixed interrogation time. For n = 2, 3, 4 qubits I obtain β ∗ Q-Loop ≃ (2.03, 2.28, 2.53) and β ∗ CP ≃ (15.53, 15.53, 16.00), corresponding to an average efficiency gain ⟨β ∗ CP/β∗ Q-Loop⟩ ≈ 6.9. Thus the spectral advantage of Q-Loop in reducing χ(τ ) is reflected in a substantial reduction of transverse control power required to stabilize entanglement under realistic non-commuting dynamics. I also discuss the limitations of the present analysis—classical noise, finite system size, and the restricted range n = 2–4—and outline how the framework can be extended to larger systems and more microscopic noise models.