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arXiv:1105.4303 · Physical Review A

Quadratic dynamical decoupling with nonuniform error suppression

Gregory Quiroz, Daniel A. Lidar

Published Oct 17, 2011• 18 citations
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Abstract

We analyze numerically the performance of the near-optimal quadratic dynamical decoupling (QDD) single-qubit decoherence errors suppression method [J. West et al., Phys. Rev. Lett. 104, 130501 (2010)]. The QDD sequence is formed by nesting two optimal Uhrig dynamical decoupling sequences for two orthogonal axes, comprising ${N}_{1}$ and ${N}_{2}$ pulses, respectively. Varying these numbers, we study the decoherence suppression properties of QDD directly by isolating the errors associated with each system basis operator present in the system-bath interaction Hamiltonian. Each individual error scales with the lowest order of the Dyson series, therefore immediately yielding the order of decoherence suppression. We show that the error suppression properties of QDD are dependent upon the parities of ${N}_{1}$ and ${N}_{2}$, and near-optimal performance is achieved for general single-qubit interactions when ${N}_{1}={N}_{2}$.

DOI
10.1103/physreva.84.042328

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We prove the universality of the generalized QDDN1N2{}_{{N}_{1}{N}_{2}}N1​N2​​ (quadratic dynamical decoupling) pulse sequence for near-optimal suppression of general single-qubit decoherence. Earlier work showed numerically that this dynamical decoupling sequence, which consists of an inner Uhrig DD (UDD) and outer UDD sequence using N1{N}_{1}N1​ and N2{N}_{2}N2​ pulses, respectively, can eliminate decoherence to O(TN)O({T}^{N})O(TN) using O(N2)O({N}^{2})O(N2) unequally spaced ``ideal'' (zero-width) pulses, where TTT is the total evolution time and N=N1=N2N={N}_{1}={N}_{2}N=N1​=N2​. A proof of the universality of QDD has been given for even N1{N}_{1}N1​. Here we give a general universality proof of QDD for arbitrary N1{N}_{1}N1​ and N2{N}_{2}N2​. As in earlier proofs, our result holds for arbitrary bounded environments. Furthermore, we explore the single-axis (polarization) error suppression abilities of the inner and outer UDD sequences. We analyze both the single-axis QDD performance and how the overall performance of QDD depends on the single-axis errors. We identify various performance effects related to the parities and relative magnitudes of N1{N}_{1}N1​ and N2{N}_{2}N2​. We prove that using QDDN1N2{}_{{N}_{1}{N}_{2}}N1​N2​​ decoherence can always be eliminated to O(TminN1,N2)O({T}^{\mathrm{min}{{N}_{1},{N}_{2}}})O(TminN1​,N2​).
Dynamical decoupling (DD) is perhaps the simplest and least resource-intensive error-suppression strategy for improving quantum computer performance. Here we report on a large-scale survey of the performance of 60 different DD sequences from ten families, including basic as well as advanced sequences with high-order error cancelation properties and built-in robustness. The survey is performed using three different superconducting-qubit IBMQ devices, with the goal of assessing the relative performance of the different sequences in the setting of arbitrary quantum state preservation. We find that the high-order universally robust (UR) and quadratic DD (QDD) sequences generally outperform all other sequences across devices and pulse-interval settings. Surprisingly, we find that DD performance for basic sequences such as the Carr-Purcell-Meiboom-Gill and XY4 sequences can be made to nearly match that of UR and QDD sequences by optimizing the pulse interval, with the optimal interval being substantially larger than the minimum interval possible on each device.
We consider a system with general decoherence and a quadratic dynamical decoupling sequence (QDD) for the coherence control of a qubit coupled to a bath of spins. We investigate the influence of the geometry and of the initial conditions of the bath on the performance of the sequence. The overall performance is quantified by a distance norm ddd. It is expected that ddd scales with $\ensuremath{\tau}$, the total duration of the sequence, as ${\ensuremath{\tau}}^{\mathrm{min}{{N}_{x},{N}_{z}}+1}$, where Nx{N}_{x}Nx​ and Nz{N}_{z}Nz​ are the number of pulses of the outer and of the inner sequence, respectively. We show both numerically and analytically that the state of the bath can boost the performance of QDD under certain conditions: The scaling of QDD for a given number of pulses can be enhanced by a factor of 2 if the bath is prepared in a highly symmetric state and if the system Hamiltonian is SU(2) invariant.
Nested Uhrig dynamical decoupling (NUDD) is a highly efficient quantum error suppression scheme that builds on optimized single axis UDD sequences. We prove the universality of NUDD and analyze its suppression of different error types in the setting of generalized control pulses. We present an explicit lower bound for the decoupling order of each error type, which we relate to the sequence orders of the nested UDD layers. We find that the error suppression capabilities of NUDD are strongly dependent on the parities and relative magnitudes of all nested UDD sequence orders. This allows us to predict the optimal arrangement of sequence orders. We test and confirm our analysis using numerical simulations.
We introduce a high-order dynamical decoupling (DD) scheme for arbitrary bounded system-bath interactions in the weak-coupling regime. Given any decoupling group G\mathcal GG that averages the interaction to zero, our construction {guarantees the existence of pulse sequences with at most (∣G∣−1)K(|\mathcal G|-1)K(∣G∣−1)K pulses}, while canceling all error terms linear in the system-bath coupling strength up to order KKK in the total evolution time. As a corollary, for an nnn-qubit system with kkk-local system-bath interactions, we obtain an O(nk−1K)\mathcal{O}(n^{k-1}K)O(nk−1K)-pulse sequence, a significant improvement over existing schemes with O(exp⁡(n))\mathcal{O}(\exp(n))O(exp(n)) pulses (for k=O(1)k=\mathcal{O}(1)k=O(1)). The construction is obtained via a mapping to the continuous necklace-splitting problem, which asks how to cut a multi-colored interval into pieces that give each party the same share of every color. We provide explicit pulse sequences for suppressing general single-qubit decoherence, prove that the pulse count is asymptotically optimal, and verify the predicted error scaling in numerical simulations. For the same number of pulses, we observe that our sequences outperform the state-of-the-art Quadratic DD in the weak-coupling regime. We also construct explicit high-order sequences for suppressing representative 2-local noise models. Finally, the same construction extends to suppress slow, time-dependent classical noise and to filter-function design.