Stable quantum bits, capable both of storing quantum information for macroscopic time scales and of integration inside small portable devices, are an essential building block for an array of potential applications. We demonstrate high-fidelity control of a solid-state qubit, which preserves its polarization for several minutes and features coherence lifetimes exceeding 1 second at room temperature. The qubit consists of a single ^(13)C nuclear spin in the vicinity of a nitrogen-vacancy color center within an isotopically purified diamond crystal. The long qubit memory time was achieved via a technique involving dissipative decoupling of the single nuclear spin from its local environment. The versatility, robustness, and potential scalability of this system may allow for new applications in quantum information science.

/pdf/room-temperature-quantum-bit-memory-exceeding-one-second-u89ucnzjiq.pdf

Room-Temperature Quantum Bit Memory Exceeding One Second

Remarkable progress towards realizing quantum computation has been achieved using natural and artificial atoms as qubits. This paper presents a brief overview of the current status of different types of qubits. On the one hand, natural atoms (such as neutral atoms and ions) have long coherence times, and could be stored in large arrays, providing ideal 'quantum memories'. On the other hand, artificial atoms (such as superconducting circuits or semiconductor quantum dots) have the advantage of custom-designed features and could be used as 'quantum processing units'. Natural and artificial atoms can be coupled with each other and can also be interfaced with photons for long-distance communications. Hybrid devices made of natural/artificial atoms and photons may provide the next-generation design for quantum computers.

https://iopscience.iop.org/article/10.1088/0034-4885/74/10/104401/pdf

Natural and artificial atoms for quantum computation

Trapped ions are among the most promising systems for practical quantum computing (QC). The basic requirements for universal QC have all been demonstrated with ions, and quantum algorithms using few-ion-qubit systems have been implemented. We review the state of the field, covering the basics of how trapped ions are used for QC and their strengths and limitations as qubits. In addition, we discuss what is being done, and what may be required, to increase the scale of trapped ion quantum computers while mitigating decoherence and control errors. Finally, we explore the outlook for trapped-ion QC. In particular, we discuss near-term applications, considerations impacting the design of future systems of trapped ions, and experiments and demonstrations that may further inform these considerations.

Trapped-ion quantum computing: Progress and challenges

How can the electric noise in the vicinity of a metallic body be measured and understood? Trapped ions, known as unique tools for metrology and quantum information processing, also constitute very sensitive probes of this electric noise for distances from micrometers to millimeters. This paper presents various models for the origin of the electric noise, provides a critical review of the experimental findings, and summarizes the important questions that are still open in this active research area.

Ion-trap measurements of electric-field noise near surfaces

Individual laser-cooled 24Mg+ ions are confined in a linear Paul trap with a novel geometry where gold electrodes are located in a single plane and the ions are trapped 40 microm above this plane. The relatively simple trap design and fabrication procedure are important for large-scale quantum information processing (QIP) using ions. Measured ion motional frequencies are compared to simulations. Measurements of ion recooling after cooling is temporarily suspended yield a heating rate of approximately 5 motional quanta per millisecond for a trap frequency of 2.83 MHz, sufficiently low to be useful for QIP.

/pdf/a-microfabricated-surface-electrode-ion-trap-for-scalable-d00fxldn0n.pdf

A microfabricated surface-electrode ion trap for scalable quantum information processing

We describe a new electrode design for a surface-electrode Paul trap, which allows rotation of the normal modes out of the trap plane, and a technique for micromotion compensation in all directions using a two-photon process, which avoids the need for an ultraviolet laser directed to the trap plane. The fabrication and characterization of the trap are described, as well as its implementation for the trapping and cooling of single Ca+ ions. We also propose a repumping scheme that increases ion fluorescence and simplifies heating rate measurements obtained by time-resolved ion fluorescence during Doppler cooling.

https://iopscience.iop.org/article/10.1088/1367-2630/12/5/053026/pdf

Implementation of a symmetric surface-electrode ion trap with field compensation using a modulated Raman effect

We describe single-shot readout of a trapped-ion multiqubit register using space- and time-resolved camera detection. For a single qubit we measure $0.9(3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ readout error in $400$ $\ensuremath{\mu}$s exposure time, limited by the qubit's decay lifetime. For a four-qubit register (a ``qunybble''), we measure an additional error of only $0.1(1)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ per qubit, despite the presence of $4%$ optical cross-talk between neighboring qubits. A study of the cross-talk indicates that the method would scale with a negligible loss of fidelity to $~$$10 000$ qubits at a density of $\ensuremath{\lesssim}$$1$ qubit/$\ensuremath{\mu}$m${}^{2}$, with a readout time of $~1$ $\ensuremath{\mu}$s/qubit.

Scalable simultaneous multiqubit readout with 99.99% single-shot fidelity

We describe the fabrication and characterization of a new surface-electrode Paul ion trap designed for experiments in scalable quantum information processing with Ca+. A notable feature is a symmetric electrode pattern which allows rotation of the normal modes of ion motion, yielding efficient Doppler cooling with a single beam parallel to the planar surface. We propose and implement a technique for micromotion compensation in all directions using an infrared repumper laser beam directed into the trap plane. Finally, we employ an alternate repumping scheme that increases ion fluorescence and simplifies heating rate measurements obtained by time-resolved ion fluorescence during Doppler cooling.

A. H. Burrell

Papers

Implementation of a symmetric surface-electrode ion trap with field compensation using a modulated Raman effect

Scalable simultaneous multiqubit readout with 99.99% single-shot fidelity

Implementation of a symmetric surface electrode ion trap with field compensation using a modulated Raman effect