%0 Journal Article %D 2020 %T Fault-Tolerant Operation of a Quantum Error-Correction Code %A Laird Egan %A Dripto M. Debroy %A Crystal Noel %A Andrew Risinger %A Daiwei Zhu %A Debopriyo Biswas %A Michael Newman %A Muyuan Li %A Kenneth R. Brown %A Marko Cetina %A Christopher Monroe %X

Quantum error correction protects fragile quantum information by encoding it in a larger quantum system whose extra degrees of freedom enable the detection and correction of errors. An encoded logical qubit thus carries increased complexity compared to a bare physical qubit. Fault-tolerant protocols contain the spread of errors and are essential for realizing error suppression with an error-corrected logical qubit. Here we experimentally demonstrate fault-tolerant preparation, rotation, error syndrome extraction, and measurement on a logical qubit encoded in the 9-qubit Bacon-Shor code. For the logical qubit, we measure an average fault-tolerant preparation and measurement error of 0.6% and a transversal Clifford gate with an error of 0.3% after error correction. The result is an encoded logical qubit whose logical fidelity exceeds the fidelity of the entangling operations used to create it. We compare these operations with non-fault-tolerant protocols capable of generating arbitrary logical states, and observe the expected increase in error. We directly measure the four Bacon-Shor stabilizer generators and are able to detect single qubit Pauli errors. These results show that fault-tolerant quantum systems are currently capable of logical primitives with error rates lower than their constituent parts. With the future addition of intermediate measurements, the full power of scalable quantum error-correction can be achieved. 

%8 9/24/2020 %G eng %U https://arxiv.org/abs/2009.11482 %0 Journal Article %D 2019 %T Ground-state energy estimation of the water molecule on a trapped ion quantum computer %A Yunseong Nam %A Jwo-Sy Chen %A Neal C. Pisenti %A Kenneth Wright %A Conor Delaney %A Dmitri Maslov %A Kenneth R. Brown %A Stewart Allen %A Jason M. Amini %A Joel Apisdorf %A Kristin M. Beck %A Aleksey Blinov %A Vandiver Chaplin %A Mika Chmielewski %A Coleman Collins %A Shantanu Debnath %A Andrew M. Ducore %A Kai M. Hudek %A Matthew Keesan %A Sarah M. Kreikemeier %A Jonathan Mizrahi %A Phil Solomon %A Mike Williams %A Jaime David Wong-Campos %A Christopher Monroe %A Jungsang Kim %X

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 a scalable co-design framework for solving chemistry problems on a trapped ion quantum computer, and apply it to compute the ground-state energy of the water molecule. The robust operation of the trapped ion quantum computer yields energy estimates with errors approaching the chemical accuracy, which is the target threshold necessary for predicting the rates of chemical reaction dynamics.

%8 03/07/2019 %G eng %U https://arxiv.org/abs/1902.10171 %0 Journal Article %D 2019 %T Quantum Computer Systems for Scientific Discovery %A Yuri Alexeev %A Dave Bacon %A Kenneth R. Brown %A Robert Calderbank %A Lincoln D. Carr %A Frederic T. Chong %A Brian DeMarco %A Dirk Englund %A Edward Farhi %A Bill Fefferman %A Alexey V. Gorshkov %A Andrew Houck %A Jungsang Kim %A Shelby Kimmel %A Michael Lange %A Seth Lloyd %A Mikhail D. Lukin %A Dmitri Maslov %A Peter Maunz %A Christopher Monroe %A John Preskill %A Martin Roetteler %A Martin Savage %A Jeff Thompson %A Umesh Vazirani %X

The great promise of quantum computers comes with the dual challenges of building them and finding their useful applications. We argue that these two challenges should be considered together, by co-designing full stack quantum computer systems along with their applications in order to hasten their development and potential for scientific discovery. In this context, we identify scientific and community needs, opportunities, and significant challenges for the development of quantum computers for science over the next 2-10 years. This document is written by a community of university, national laboratory, and industrial researchers in the field of Quantum Information Science and Technology, and is based on a summary from a U.S. National Science Foundation workshop on Quantum Computing held on October 21-22, 2019 in Alexandria, VA.

%8 12/16/2019 %G eng %U https://arxiv.org/abs/1912.07577 %0 Journal Article %D 2019 %T Quantum Simulators: Architectures and Opportunities %A Ehud Altman %A Kenneth R. Brown %A Giuseppe Carleo %A Lincoln D. Carr %A Eugene Demler %A Cheng Chin %A Brian DeMarco %A Sophia E. Economou %A Mark A. Eriksson %A Kai-Mei C. Fu %A Markus Greiner %A Kaden R. A. Hazzard %A Randall G. Hulet %A Alicia J. Kollár %A Benjamin L. Lev %A Mikhail D. Lukin %A Ruichao Ma %A Xiao Mi %A Shashank Misra %A Christopher Monroe %A Kater Murch %A Zaira Nazario %A Kang-Kuen Ni %A Andrew C. Potter %A Pedram Roushan %X

Quantum simulators are a promising technology on the spectrum of quantum devices from specialized quantum experiments to universal quantum computers. These quantum devices utilize entanglement and many-particle behaviors to explore and solve hard scientific, engineering, and computational problems. Rapid development over the last two decades has produced more than 300 quantum simulators in operation worldwide using a wide variety of experimental platforms. Recent advances in several physical architectures promise a golden age of quantum simulators ranging from highly optimized special purpose simulators to flexible programmable devices. These developments have enabled a convergence of ideas drawn from fundamental physics, computer science, and device engineering. They have strong potential to address problems of societal importance, ranging from understanding vital chemical processes, to enabling the design of new materials with enhanced performance, to solving complex computational problems. It is the position of the community, as represented by participants of the NSF workshop on "Programmable Quantum Simulators," that investment in a national quantum simulator program is a high priority in order to accelerate the progress in this field and to result in the first practical applications of quantum machines. Such a program should address two areas of emphasis: (1) support for creating quantum simulator prototypes usable by the broader scientific community, complementary to the present universal quantum computer effort in industry; and (2) support for fundamental research carried out by a blend of multi-investigator, multi-disciplinary collaborations with resources for quantum simulator software, hardware, and education. 

%8 12/14/2019 %G eng %U https://arxiv.org/abs/1912.06938 %0 Journal Article %D 2019 %T Two-qubit entangling gates within arbitrarily long chains of trapped ions %A Kevin A. Landsman %A Yukai Wu %A Pak Hong Leung %A Daiwei Zhu %A Norbert M. Linke %A Kenneth R. Brown %A Luming Duan %A Christopher R. Monroe %X

Ion trap systems are a leading platform for large scale quantum computers. Trapped ion qubit crystals are fully-connected and reconfigurable, owing to their long range Coulomb interaction that can be modulated with external optical forces. However, the spectral crowding of collective motional modes could pose a challenge to the control of such interactions for large numbers of qubits. Here, we show that high-fidelity quantum gate operations are still possible with very large trapped ion crystals, simplifying the scaling of ion trap quantum computers. To this end, we present analytical work that determines how parallel entangling gates produce a crosstalk error that falls off as the inverse cube of the distance between the pairs. We also show experimental work demonstrating entangling gates on a fully-connected chain of seventeen 171Yb+ ions with fidelities as high as 97(1)%.

%8 05/28/2019 %G eng %U https://arxiv.org/abs/1905.10421 %0 Journal Article %J Physical Review Letters %D 2018 %T Robust two-qubit gates in a linear ion crystal using a frequency-modulated driving force %A Pak Hong Leung %A Kevin A. Landsman %A Caroline Figgatt %A Norbert M. Linke %A Christopher Monroe %A Kenneth R. Brown %X

In an ion trap quantum computer, collective motional modes are used to entangle two or more qubits in order to execute multi-qubit logical gates. Any residual entanglement between the internal and motional states of the ions will result in decoherence errors, especially when there are many spectator ions in the crystal. We propose using a frequency-modulated (FM) driving force to minimize such errors and implement it experimentally. In simulation, we obtained an optimized FM gate that can suppress decoherence to less than 10−4 and is robust against a frequency drift of more than ±1 kHz. The two-qubit gate was tested in a five-qubit trapped ion crystal, with 98.3(4)% fidelity for a Mølmer-Sørensen entangling gate and 98.6(7)% for a controlled-not (CNOT) gate. We also show an optimized FM two-qubit gate for 17 ions, proving the scalability of our method.

%B Physical Review Letters %V 120 %P 020501 %8 2018/01/09 %G eng %U https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.120.020501 %N 2 %R 10.1103/PhysRevLett.120.020501 %0 Journal Article %D 2016 %T Co-Designing a Scalable Quantum Computer with Trapped Atomic Ions %A Kenneth R. Brown %A Jaewan Kim %A Christopher Monroe %X The first generation of quantum computers are on the horizon, fabricated from quantum hardware platforms that may soon be able to tackle certain tasks that cannot be performed or modelled with conventional computers. These quantum devices will not likely be universal or fully programmable, but special-purpose processors whose hardware will be tightly co-designed with particular target applications. Trapped atomic ions are a leading platform for first generation quantum computers, but are also fundamentally scalable to more powerful general purpose devices in future generations. This is because trapped ion qubits are atomic clock standards that can be made identical to a part in 10^15, and their quantum circuit connectivity can be reconfigured through the use of external fields, without modifying the arrangement or architecture of the qubits themselves. In this article we show how a modular quantum computer of any size can be engineered from ion crystals, and how the wiring between ion trap qubits can be tailored to a variety of applications and quantum computing protocols. %8 2016/02/09 %G eng %U http://arxiv.org/abs/1602.02840