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arXiv:0905.3254

Universal protection of unitary evolution from slow noise: dynamical control pushed to the extreme

Guy Bensky, E. Brion, F. Carlier, V. M. Akulin, Gershon Kurizki

Published May 20, 2009• 0 citations
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Abstract

We propose a technique that allows to simultaneously perform universal control of the evolution operator and compensate for the first order contribution of an arbitrary Hermitian constant noise. We show that, at least, a three-valued Hamiltonian is needed in order to protect the system against any such noise. This technique is illystrated by an explicit algorithm for a control sequence that is applied to numerically design a safe two-qubit gate.

DOI
10.48550/arxiv.0905.3254

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We propose a technique that allows to simultaneously perform universal control of the evolution operator and compensate for the first order contribution of an arbitrary Hermitian constant noise. We show that, at least, a three-valued Hamiltonian is needed in order to protect the system against any such noise. This technique is illystrated by an explicit algorithm for a control sequence that is applied to numerically design a safe two-qubit gate.
We propose a technique that allows to simultaneously perform universal control of the evolution operator of a system and compensate for the first-order contribution of any Hermitian constant noise and the action of the environment. We show that, at least, a three-valued Hamiltonian is needed in order to protect the system against any such noise and propose an explicit algorithm for finding an appropriate control sequence. This algorithm is applied to numerically design a safe gate in an atomic qutrit.
Realistic quantum computing is subject to noise. Therefore, an important frontier in quantum computing is to implement noise-resilient quantum control over qubits. At the same time, dynamical decoupling can protect the coherence of qubits. Here we demonstrate non-trivial quantum evolution steered by dynamical decoupling control, which simultaneously suppresses noise effects. We design and implement a self-protected controlled-NOT gate on the electron spin of a nitrogen-vacancy centre and a nearby carbon-13 nuclear spin in diamond at room temperature, by employing an engineered dynamical decoupling control on the electron spin. Final state fidelity of 0.91(1) is observed in preparation of a Bell state using the gate. At the same time, the qubit coherence time is elongated at least 30 fold. The design scheme does not require the dynamical decoupling control to commute with the qubit interaction and therefore works for general qubit systems. This work marks a step towards implementing realistic quantum computing systems. Realistic quantum computers require a high degree of qubit control and must also be resilient to noise. Using dynamical decoupling control techniques, Liuet al.implement a self-protected controlled-NOT gate for electron and nuclear spins that retains a high final state fidelity.
Abstract A protocol for implementing a universal set of quantum gates, which are safeguarded from noise through geometric, dephasing‐free, and dynamical‐decoupling approaches is proposed. With properly designed Hamiltonians, the desired interactions are realized directly within the Schrödinger picture rather than relying on the interaction picture. This design confers two advantages. One is that the system's evolution remains continuously protected by the noise‐mitigation strategies at every stage, eliminating the need to revert to the Schrödinger picture. The other one is that the dynamical decoupling technique effectively averages out small residual free terms in the system's Hamiltonian, thus reducing the stringent requirements for precise control of system parameters to avoid errors. Furthermore, numerical results are presented to support our protocol by finding gate fidelities in the absence (presence) of noises or errors, without or with dynamical decoupling technique.
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Parity-time (PT\mathcal{PT}PT)-symmetric non-Hermitian Hamiltonians bring about many novel features and interesting applications such as quantum gates faster than those in Hermitian systems and topological state transfer. The performance of evolutions under PT\mathcal{PT}PT-symmetric Hamiltonians is degraded by the inevitable noise and errors due to system-environment interaction and experimental imperfections. In contrast to Hermitian Hamiltonians, the fluctuations in dissipative beams that are utilized to generate non-Hermitian contributions in the PT\mathcal{PT}PT-symmetric Hamiltonians cause additional errors. Here we achieve the protection of PT\mathcal{PT}PT-symmetric Hamiltonians against noise acting along the qubit's quantization axis by combining quantum evolutions with dynamical decoupling sequences. We demonstrate the performance of our method by numerical simulations. Realistic noise sources and parameters are chosen including constant detuning error, time-varying detuning noise, and dissipative-beam noise. The fidelity of states after protected evolutions is well above those after unprotected evolutions under all the above situations. Our work paves the way for further studies and applications of non-Hermitian PT\mathcal{PT}PT-symmetric physics in noisy quantum systems.
We study the robustness of the evolution of a quantum system against small uncontrolled variations in parameters in the Hamiltonian. We show that the fidelity susceptibility, which quantifies the perturbative error to leading order, can be expressed in superoperator form and use this to derive control pulses that are robust to any class of systematic unknown errors. The proposed optimal control protocol is equivalent to searching for a sequence of unitaries that mimics the first-order moments of the Haar distribution, i.e., it constitutes a 1-design. We highlight the power of our results for error-resistant single- and two-qubit gates.