Optimal Qubit Control Using Single-Flux Quantum Pulses

Liebermann, Per J.; Wilhelm, Frank K.

doi:10.1103/physrevapplied.6.024022

Cited by 64 publications

(55 citation statements)

References 26 publications

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“…As a first application with AlphaZero, we demonstrate optimal control using Single Flux Quantum (SFQ) pulses [45,54,55]. The aim is to control the quantum system by using a pulse train that consists of individual, very short pulses typically in the pico-second scale.…”

Section: B Digital Gate Sequencesmentioning

confidence: 99%

“…The optimization task is to find the input string that maximizes the fidelity functional (1). The current approach for this type optimization is to apply a genetic algorithm (GA) [45,57,58]. Besides GA and AlphaZero, we also compare two conventional algorithms, Q-learning and stochastic descent (SD) as in Ref.…”

Section: B Digital Gate Sequencesmentioning

confidence: 99%

“…Similar to Ref. [45], we used a population size of 70 and a mutation probability of 0.001. At each iteration we would select 2 × 30 parent solutions.…”

Section: Ga Implementationmentioning

confidence: 99%

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Global optimization of quantum dynamics with AlphaZero deep exploration

et al. 2020

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While a large number of algorithms for optimizing quantum dynamics for different objectives have been developed, a common limitation is the reliance on good initial guesses, being either random or based on heuristics and intuitions. Here we implement a tabula rasa deep quantum exploration version of the Deepmind AlphaZero algorithm for systematically averting this limitation. AlphaZero employs a deep neural network in conjunction with deep lookahead in a guided tree search, which allows for predictive hidden variable approximation of the quantum parameter landscape. To emphasize transferability, we apply and benchmark the algorithm on three classes of control problems using only a single common set of algorithmic hyperparameters. AlphaZero achieves substantial improvements in both the quality and quantity of good solution clusters compared to earlier methods. It is able to spontaneously learn unexpected hidden structure and global symmetry in the solutions, going beyond even human heuristics. arXiv:1907.05672v1 [quant-ph]

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Section: B Digital Gate Sequencesmentioning

confidence: 99%

Section: B Digital Gate Sequencesmentioning

confidence: 99%

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Global optimization of quantum dynamics with AlphaZero deep exploration

et al. 2020

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“…We define V and E in this way with the goal of separating control in the qubit subspace from leakage elimination: navigation through the subsequence graph G preserves rotation in the qubit subspace, but movement from vertex to vertex can change leakage out of the computational subspace substantially, as one can see from Eq. (18). A trivial example of the subsequence graph is shown in Fig.…”

Section: Leakage Suppressionmentioning

confidence: 99%

Hardware-Efficient Qubit Control with Single-Flux-Quantum Pulse Sequences

McDermott

Vavilov

2019

Phys. Rev. Applied

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The hardware overhead associated with microwave control is a major obstacle to scale-up of superconducting quantum computing. An alternative approach involves irradiation of the qubits with trains of Single Flux Quantum (SFQ) pulses, pulses of voltage whose time integral is precisely equal to the superconducting flux quantum. Here we describe the derivation and validation of compact SFQ pulse sequences in which classical bits are clocked to the qubit at a frequency that is roughly a factor 5 higher than the qubit oscillation frequency, allowing for variable pulse-to-pulse timing. The control sequences are constructed by repeated streaming of short subsequence registers that are designed to suppress leakage out of the computational manifold. With a single global clock, high-fidelity (> 99.99%) control of qubits resonating at over 20 distinct frequencies is possible. SFQ pulses can be stored locally and delivered to the qubits via a proximal classical Josephson digital circuit, offering the possibility of a streamlined, low-footprint classical coprocessor for monitoring errors and feeding back to the qubit array.

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“…For example, one approach to scalable qubit control involves manipulation of qubits by quantized voltage pulses derived from the classical Single Flux Quantum (SFQ) digital logic family [8,9]; here, local generation of QPs during each voltage pulse is inevitable. Due to the local nature of dissipation, the QP density may become large, x x * , and QP recombination accompanied by phonon emission to the substrate emerges as the leading mechanism of QP relaxation.…”

mentioning

confidence: 99%

Phonon-mediated quasiparticle poisoning of superconducting microwave resonators

et al. 2017

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Nonequilibrium quasiparticles represent a significant source of decoherence in superconducting quantum circuits. Here we investigate the mechanism of quasiparticle poisoning in devices subjected to local quasiparticle injection. We find that quasiparticle poisoning is dominated by the propagation of pair-breaking phonons across the chip. We characterize the energy dependence of the timescale for quasiparticle poisoning. Finally, we observe that incorporation of extensive normal metal quasiparticle traps leads to a more than order of magnitude reduction in quasiparticle loss for a given injected quasiparticle power.Gate and measurement fidelities of superconducting qubits have reached the threshold for fault-tolerant operations [1,2]; however, continued progress in the field will require improvements in coherence and the development of scalable approaches to multiqubit control. Recently it was shown that nonequilibium quasiparticles (QPs) represent a dominant source of qubit decoherence [3,4]. Quasiparticles are also a source of decoherence in topologically protected Majorana qubits [5]. Most commonly, superconducting quantum circuits are operated in such a way that there is no explicit dissipation of power on the quantum chip; nevertheless, stray infrared light from higher temperature stages leads to a dilute background of nonequilibrium QPs in the superconducting thin films. According to [6], the leading mechanism for QP relaxation at low densityis trapping by localized defects or vortices, where n QP is the QP density and n CP is the density of Cooper pairs (4 × 10 6 µm −3 in aluminum). In this regime, QPs propagate diffusively through the superconductor until they are trapped.For future multiqubit processors, however, it might be necessary to integrate proximal classical control or measurement elements tightly with the quantum circuit, leading to a nonnegligible level of local power dissipation. For example, one approach to scalable qubit control involves manipulation of qubits by quantized voltage pulses derived from the classical Single Flux Quantum (SFQ) digital logic family [8,9]; here, local generation of QPs during each voltage pulse is inevitable. Due to the local nature of dissipation, the QP density may become large, x x * , and QP recombination accompanied by phonon emission to the substrate emerges as the leading mechanism of QP relaxation. The emitted phonons can travel great distances through the substrate until they are absorbed by the superconducting film, leading to the generation of new QP pairs in remote regions of the film [10,11].In this Letter, we present experiments to characterize the dynamics of QP poisoning in superconducting thin films subjected to direct QP injection, so that recombination is important and a significant flux of pair-breaking phonons is emitted to the substrate. We show that cuts in the superconducting film, which eliminate direct diffusion of QPs, have little influence on QP poisoning far from the injection site; however, the incorporation of normal metal QP traps l...

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Optimal Qubit Control Using Single-Flux Quantum Pulses

Cited by 64 publications

References 26 publications

Global optimization of quantum dynamics with AlphaZero deep exploration

Global optimization of quantum dynamics with AlphaZero deep exploration

Hardware-Efficient Qubit Control with Single-Flux-Quantum Pulse Sequences

Phonon-mediated quasiparticle poisoning of superconducting microwave resonators

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