Noisy Intermediate-Scale Quantum (NISQ) technology will be available in the near future. Quantum computers with 50-100 qubits may be able to perform tasks which surpass the capabilities of today's classical digital computers, but noise in quantum gates will limit the size of quantum circuits that can be executed reliably. NISQ devices will be useful tools for exploring many-body quantum physics, and may have other useful applications, but the 100-qubit quantum computer will not change the world right away --- we should regard it as a significant step toward the more powerful quantum technologies of the future. Quantum technologists should continue to strive for more accurate quantum gates and, eventually, fully fault-tolerant quantum computing.

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Quantum Computing in the NISQ era and beyond

The statistical error in any estimation can be reduced by repeating the measurement and averaging the results. The central limit theorem implies that the reduction is proportional to the square root of the number of repetitions. Quantum metrology is the use of quantum techniques such as entanglement to yield higher statistical precision than purely classical approaches. In this Review, we analyse some of the most promising recent developments of this research field and point out some of the new experiments. We then look at one of the major new trends of the field: analyses of the effects of noise and experimental imperfections.

Advances in quantum metrology

Noisy Intermediate-Scale Quantum (NISQ) technology will be available in the
near future. Quantum computers with 50-100 qubits may be able to perform tasks
which surpass the capabilities of today's classical digital computers, but
noise in quantum gates will limit the size of quantum circuits that can be
executed reliably. NISQ devices will be useful tools for exploring many-body
quantum physics, and may have other useful applications, but the 100-qubit
quantum computer will not change the world right away --- we should regard it
as a significant step toward the more powerful quantum technologies of the
future. Quantum technologists should continue to strive for more accurate
quantum gates and, eventually, fully fault-tolerant quantum computing.

/pdf/quantum-computing-in-the-nisq-era-and-beyond-3utyn832f7.pdf

The widespread use of controlled molecular-level motion in key natural processes suggests that great rewards could come from bridging the gap between the present generation of synthetic molecular systems, which by and large rely upon electronic and chemical effects to carry out their functions, and the machines of the macroscopic world, which utilize the synchronized movements of smaller parts to perform specific tasks. This is a scientific area of great contemporary interest and extraordinary recent growth, yet the notion of molecular-level machines dates back to a time when the ideas surrounding the statistical nature of matter and the laws of thermodynamics were first being formulated. Here we outline the exciting successes in taming molecular-level movement thus far, the underlying principles that all experimental designs must follow, and the early progress made towards utilizing synthetic molecular structures to perform tasks using mechanical motion. We also highlight some of the issues and challenges that still need to be overcome.

Synthetic molecular motors and mechanical machines.

Brownian motors: noisy transport far from equilibrium

Universal logic gates for two quantum bits (qubits) form an essential ingredient of quantum computation. Dynamical gates have been proposed in the context of trapped ions; however, geometric phase gates (which change only the phase of the physical qubits) offer potential practical advantages because they have higher intrinsic resistance to certain small errors and might enable faster gate implementation. Here we demonstrate a universal geometric pi-phase gate between two beryllium ion-qubits, based on coherent displacements induced by an optical dipole force. The displacements depend on the internal atomic states; the motional state of the ions is unimportant provided that they remain in the regime in which the force can be considered constant over the extent of each ion's wave packet. By combining the gate with single-qubit rotations, we have prepared ions in an entangled Bell state with 97% fidelity-about six times better than in a previous experiment demonstrating a universal gate between two ion-qubits. The particular properties of the gate make it attractive for a multiplexed trap architecture that would enable scaling to large numbers of ion-qubits.

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Experimental demonstration of a robust, high-fidelity geometric two ion-qubit phase gate

Experimental demonstration of a robust, high-fidelity geometric two ion-qubit phase gate*

The highest two-qubit gate fidelities have been demonstrated in two experiments that use scalable trapped ion platforms.

High-Fidelity Quantum Logic Gates Using Trapped-Ion Hyperfine Qubits.

Qubits based on trapped ions can be prepared and manipulated with record-breaking accuracy, offering a promising scalable platform for quantum computing.

https://physics.aps.org/featured-article-pdf/10.1103/PhysRevLett.113.220501

High-Fidelity Preparation, Gates, Memory, and Readout of a Trapped-Ion Quantum Bit.

We demonstrate single-shot qubit readout with a fidelity sufficient for fault-tolerant quantum computation. For an optical qubit stored in $^{40}\mathrm{Ca}^{+}$ we achieve 99.991(1)% average readout fidelity in ${10}^{6}$ trials, using time-resolved photon counting. An adaptive measurement technique allows 99.99% fidelity to be reached in $145\text{ }\text{ }\ensuremath{\mu}\mathrm{s}$ average detection time. For $^{43}\mathrm{Ca}^{+}$, we propose and implement an optical pumping scheme to transfer a long-lived hyperfine qubit to the optical qubit, capable of a theoretical fidelity of 99.95% in $10\text{ }\text{ }\ensuremath{\mu}\mathrm{s}$. We achieve 99.87(4)% transfer fidelity and 99.77(3)% net readout fidelity.

/pdf/high-fidelity-readout-of-trapped-ion-qubits-g2w52nm5h2.pdf

D. M. Lucas

Papers

Experimental demonstration of a robust, high-fidelity geometric two ion-qubit phase gate

Experimental demonstration of a robust, high-fidelity geometric two ion-qubit phase gate*

High-Fidelity Quantum Logic Gates Using Trapped-Ion Hyperfine Qubits.

High-Fidelity Preparation, Gates, Memory, and Readout of a Trapped-Ion Quantum Bit.

High-fidelity readout of trapped-ion qubits.