@article { WOS:000661896800004,
title = {Efficient Stabilized Two-Qubit Gates on a Trapped-Ion Quantum Computer},
journal = {Phys. Rev. Lett.},
volume = {126},
number = {22},
year = {2021},
month = {JUN 4},
publisher = {AMER PHYSICAL SOC},
type = {Article},
abstract = {In order to scale up quantum processors and achieve a quantum advantage, it is crucial to economize on the power requirement of two-qubit gates, make them robust to drift in experimental parameters, and shorten the gate times. Applicable to all quantum computer architectures whose two-qubit gates rely on phase-space closure, we present here a new gate-optimizing principle according to which negligible amounts of gate fidelity are traded for substantial savings in power, which, in turn, can be traded for substantial increases in gate speed and/or qubit connectivity. As a concrete example, we illustrate the method by constructing optimal pulses for entangling gates on a pair of ions within a trapped-ion chain, one of the leading quantum computing architectures. Our method is direct, noniterative, and linear, and, in some parameter regimes, constructs gate-steering pulses requiring up to an order of magnitude less power than the standard method. Additionally, our method provides increased robustness to mode drift. We verify the new trade-off principle experimentally on our trapped-ion quantum computer.},
issn = {0031-9007},
doi = {10.1103/PhysRevLett.126.220503},
author = {Blumel, Reinhold and Grzesiak, Nikodem and Nguyen, Nhung H. and Green, Alaina M. and Li, Ming and Maksymov, Andrii and Linke, Norbert M. and Nam, Yunseong}
}
@article { WOS:000665681100001,
title = {Optimal calibration of gates in trapped-ion quantum computers},
journal = {Quantum Sci. Technol.},
volume = {6},
number = {3},
year = {2021},
month = {JUL},
publisher = {IOP Publishing Ltd},
type = {Article},
abstract = {To harness the power of quantum computing, it is essential that a quantum computer provide maximal possible fidelity for a quantum circuit. To this end, much work has been done in the context of qubit routing or embedding, i.e., mapping circuit qubits to physical qubits based on gate performance metrics to optimize the fidelity of execution. Here, we take an alternative approach that leverages a unique capability of a trapped-ion quantum computer, i.e., the all-to-all qubit connectivity. We develop a method to determine a fixed number (budget) of quantum gates that, when calibrated, will maximize the fidelity of a batch of input quantum programs. This dynamic allocation of calibration resources on randomly accessible gates, determined using our heuristics, increases, for a wide range of calibration budget, the average fidelity from 70\% or lower to 90\% or higher for a typical batch of jobs on an 11-qubit device, in which the fidelity of calibrated and uncalibrated gates are taken to be 99\% and 90\%, respectively. Our heuristics are scalable, more than 2.5 orders of magnitude faster than a randomized method for synthetic benchmark circuits generated based on real-world use cases.},
keywords = {connectivity, fidelity optimization, gate calibrations},
issn = {2058-9565},
doi = {10.1088/2058-9565/abf718},
author = {Maksymov, Andrii and Niroula, Pradeep and Nam, Yunseong}
}
@article {niroula_quantum_2021,
title = {A quantum algorithm for string matching},
journal = {npj Quantum Inform.},
volume = {7},
number = {1},
year = {2021},
note = {Place: HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY Publisher: NATURE RESEARCH Type: Article},
month = {feb},
abstract = {Algorithms that search for a pattern within a larger data-set appear ubiquitously in text and image processing. Here, we present an explicit, circuit-level implementation of a quantum pattern-matching algorithm that matches a search string (pattern) of length M inside a longer text of length N. Our algorithm has a time complexity of (O) over tilde root N, while the space complexity remains modest at O(N+ M). We report the quantum gate counts relevant for both pre-fault-tolerant and fault-tolerant regimes.},
doi = {10.1038/s41534-021-00369-3},
author = {Niroula, Pradeep and Nam, Yunseong}
}
@article { WOS:000677582200001,
title = {Resource-Optimized Fermionic Local-Hamiltonian Simulation on a Quantum Computer for Quantum Chemistry},
journal = {Quantum},
volume = {5},
year = {2021},
month = {JUL 26},
pages = {1-36},
publisher = {VEREIN FORDERUNG OPEN ACCESS PUBLIZIERENS QUANTENWISSENSCHAF},
type = {Article},
abstract = {The ability to simulate a fermionic system on a quantum computer is expected to revolutionize chemical engineering, materials design, nuclear physics, to name a few. Thus, optimizing the simulation circuits is of significance in harnessing the power of quantum computers. Here, we address this problem in two aspects. In the fault-tolerant regime, we optimize the R-z and T gate counts along with the ancilla qubit counts required, assuming the use of a product-formula algorithm for implementation. We obtain a savings ratio of two in the gate counts and a savings ratio of eleven in the number of ancilla qubits required over the state of the art. In the pre-fault tolerant regime, we optimize the two-qubit gate counts, assuming the use of the variational quantum eigensolver (VQE) approach. Specific to the latter, we present a framework that enables bootstrapping the VQE progression towards the convergence of the ground-state energy of the fermionic system. This framework, based on perturbation theory, is capable of improving the energy estimate at each cycle of the VQE progression, by about a factor of three closer to the known ground-state energy compared to the standard VQE approach in the test-bed, classically-accessible system of the water molecule. The improved energy estimate in turn results in a commensurate level of savings of quantum resources, such as the number of qubits and quantum gates, required to be within a pre-specified tolerance from the known ground-state energy. We also explore a suite of generalized transformations of fermion to qubit operators and show that resource-requirement savings of up to more than 20\%, in small instances, is possible.},
issn = {2521-327X},
author = {Wang, Qingfeng and Li, Ming and Monroe, Christopher and Nam, Yunseong}
}
@article { ISI:000524530000001,
title = {Ground-state energy estimation of the water molecule on a trapped-ion quantum computer},
journal = {npj Quantum Inform.},
volume = {6},
number = {1},
year = {2020},
month = {APR 3},
pages = {33},
publisher = {NATURE PUBLISHING GROUP},
type = {Article},
abstract = {Quantum computing leverages the quantum resources of superposition and entanglement to efficiently solve computational problems considered intractable for classical computers. Examples include calculating molecular and nuclear structure, simulating strongly interacting electron systems, and modeling aspects of material function. While substantial theoretical advances have been made in mapping these problems to quantum algorithms, there remains a large gap between the resource requirements for solving such problems and the capabilities of currently available quantum hardware. Bridging this gap will require a co-design approach, where the expression of algorithms is developed in conjunction with the hardware itself to optimize execution. Here we describe an extensible co-design framework for solving chemistry problems on a trapped-ion quantum computer and apply it to estimating the ground-state energy of the water molecule using the variational quantum eigensolver (VQE) method. The controllability of the trapped-ion quantum computer enables robust energy estimates using the prepared VQE ansatz states. The systematic and statistical errors are comparable to the chemical accuracy, which is the target threshold necessary for predicting the rates of chemical reaction dynamics, without resorting to any error mitigation techniques based on Richardson extrapolation.},
doi = {10.1038/s41534-020-0259-3},
author = {Nam, Yunseong and Chen, Jwo-Sy and Pisenti, Neal C. and Wright, Kenneth and Delaney, Conor and Maslov, Dmitri and Brown, Kenneth R. and Allen, Stewart and Amini, Jason M. and Apisdorf, Joel and Beck, Kristin M. and Blinov, Aleksey and Chaplin, Vandiver and Chmielewski, Mika and Collins, Coleman and Debnath, Shantanu and Hudek, Kai M. and Ducore, Andrew M. and Keesan, Matthew and Kreikemeier, Sarah M. and Mizrahi, Jonathan and Solomon, Phil and Williams, Mike and Wong-Campos, Jaime David and Moehring, David and Monroe, Christopher and Kim, Jungsang}
}