One of the major problems in performing multiple operations is that the ions heat up after a single operation, in which laser beams, tuned to specific frequencies, adjust the energy level of electrons. Once this happens, explains Jonathan Home, a postdoctoral researcher at NIST, the researchers can’t do any further operations because the qubits can no longer hold both a 1 and a 0. To solve this problem, the researchers added magnesium ions to the mix. These ions are cooled with another set of lasers and, though the cold magnesium ions are not used for computation, they effectively chill the beryllium ions, keeping them in a stable state.
A second challenge when repeating operations inside this type of quantum computer is making sure that the ions are protected from stray magnetic fields that can also cause them to lose their quantum state. To solve this problem, the researchers chose specific energy-levels within which the ions are temporarily impervious to changes in surrounding magnetic fields. This maintains the qubit’s state for up to 15 seconds, plenty of time, says Home, to perform a series of millisecond-long operations. “Our particular choice of levels doesn’t change with the magnetic field,” he says. “We don’t have to worry about the lifetime of the qubits anymore.”
The experiment is a “milestone accomplishment,” says Isaac Chuang, a professor in the electrical engineering, computer science, and physics departments at MIT. “Very much like the early evolution of transistors into calculators, this work demonstrates a complete assembly of basic steps needed for a scalable quantum computer.” Chuang adds that the research “sets the bar” for other quantum computing systems.
In demonstrations, the researchers manipulated two qubits at a time. For ion trap systems, the maximum number of qubits used in varying experiments so far is less than 10. In order to outperform a classical computer, the researchers would need to perform operations on 30 or more qubits, suspects Home, something he thinks could happen in the next five to 10 years. While quantum computers hold promise for breaking ultrasecure encryption codes, he says that early quantum computers will mostly likely be used to simulate physical systems, for example, the electronic properties of materials.
But to get there, the researchers will need to improve their system. Currently, it performs with 94 percent accuracy. For a quantum computer to be reliable enough to use, it must be 99.99 percent accurate. A major factor affecting the accuracy of the system is the intensity fluctuations of the lasers that perform the operations on the ions. However, new, more-reliable, and more-powerful ultraviolet lasers could solve this problem, says Home.