Trapped Ions Make Impeccable Qubits

Abstract

The realization, two decades ago, that quantum mechanics can be a powerful resource to speed up important computational tasks [1] led to intense research efforts to find adequate physical systems for quantum computation. One of the hurdles to a viable technology is the requirement to prepare, manipulate, and measure quantum bits (qubits) with near perfect accuracy: Imperfect control leads to errors that can accumulate over the computation process. Techniques like quantum error correction and fault-tolerant designs can, in principle, overcome these errors. But these strategies can be successful only if the error probabilities are lower than a threshold value. They also increase the complexity of the required quantum hardware, since they require additional qubits. Recent calculations [2] suggest that an error probability of less than 1% would enable fault-tolerant codes, and that lower error probabilities dramatically decrease the number of qubits required for such codes.The quality of qubit manipulation in a number of physical systems has dramatically improved in the past few years [3,4], raising hopes that a quantum computer, at a large enough scale to carry out meaningful computations, might be within reach. Now, Thomas Harty at the University of Oxford, UK, and colleagues [5] are reporting an important contribution to this goal with the demonstration that qubits consisting of trapped 43 Ca + ions can be manipulated with record high fidelities (in quantum information theory, fidelity is a measure of the "closeness" of two quantum states). Their experiments suggest trapped-ion schemes could potentially provide the basic fundamental building blocks of a universal quantum computer.Trapped atomic ions are one of the leading candidate systems to construct a robust quantum computer: they provide a stable and well-isolated quantum system and the strong Coulomb forces between the ions can be used to realize logical gate operations by coupling different qubits. In the last decade, researchers have demonstrated trapped-ion qubits with long coherence times [6], highfidelity state preparation and readout [7], and singleand two-qubit logic gate operations with low error rates [3,8]. Yet each of these properties was demonstrated individually in different systems. The work of Harty and colleagues now poses a combined improvement on all of these fronts in a single experimental system.In the authors' scheme (see Fig. 1), a 43 Ca + ion is confined by radiofrequency electric fields (a so-called "Paul trap") on the surface of a sapphire substrate. Electrodes connected to the structure provide the signals necessary for trapping the ions and driving changes to the qubit states. The choice of the 43 Ca + was crucial to the authors' results: With a modest applied magnetic field (146 gauss), the energy-level separation between the two hyperfine ground-state sublevels of the ion is sufficiently large to become insensitive to small magnetic field fluctuations, abundant in a laboratory environment, that affected the performance of previou...

Universal control of ion qubits in a scalable microfabricated planar trap

“…Trapped ions are attractive for building an extensible quantum system due to their long coherence times and ability to be entangled via photons and phonons [9,10]. Ion trap quantum computer architectures could use transport [11] or photonic interconnects [12] to provide connectivity between disparate regions of the quantum computer.…”

Toroidal nanotraps for cold polar molecules

“…The development of Penning [1] and Paul traps [2] led to unprecedented measurements of fundamental properties of charged particles by isolating individual electrons and ions [3]. Singling out and having the ability to probe individual ions allows laser manipulation of their quantum states to form qubits [4,5], which is capitalized upon in the so called trapped ion quantum computer [6]. Precision measurements benefit from trapping or even slowing down the quantum systems, as reported by Veldhoven, et al, who carried out high resolution spectroscopy by slowing down a pulsed 15 ND 3 molecular beam in a Starkdecelerator [7].…”