Step by step we’re moving closer to useful quantum computing. A big step, announced by the National Institute of Standards and Technology (NIST, USA), was the demonstration of a computing device using two qubits. Previous demonstrations by various researchers have used one qubit. In computing, two computational units are far more powerful than one. This is especially true for quantum computing because each qubit can not only represent the traditional 0’s and 1’s of computing but also a ‘superposition’ that is both 0 and 1.
The processor developed by NIST uses two beryllium ions (electrically charged atoms). They are held in position by an electromagnetic trap and manipulated with ultraviolet lasers. Manipulation includes placing each beryllium ion in a superposition. The additional computational ‘state’ is one of the properties that gives quantum computing and advantage. The two qubit processor can also demonstrate another element of quantum behavior, which is called entanglement. The two qubits can share simultaneous and identical properties, even when physically separated. By satisfying both superposition and entanglement, the NIST processor meets the conditions for a true quantum computing device.
“This is the first time anyone has demonstrated a programmable quantum processor for more than one qubit,” says NIST postdoctoral researcher David Hanneke, first author of the paper. “It’s a step toward the big goal of doing calculations with lots and lots of qubits. The idea is you’d have lots of these processors, and you’d link them together.”
With these capabilities, the NIST team performed 160 different processing routines on the two qubits. Although there are an infinite number of possible two-qubit programs, this set of 160 is large and diverse enough to fairly represent them, Hanneke says, making the processor “universal.” Key to the experimental design was use of a random number generator to select the particular routines that would be executed, so all possible programs had an equal chance of selection.
[Source: Nanotechnology Today]
As with other approaches to a quantum processor, the manipulation of ions is tricky to operate and monitor. (For one thing, quantum states collapse when they are measured.) Typically quantum processors generate a relative high percentage of errors, which must be caught and corrections applied to the results. Reducing the errors – or at least reducing the amount of necessary post-processing – is one of the goals of most quantum computing projects, including this one.
Although very important, the step from one to two qubit processors is in reality a ‘baby step.’ To develop a quantum computer that can solve problems either more quickly, or at all, compared to a traditional digital computer will require combining many (4, 6, 8, 16, 32, 64…etc.) qubit processors. The problems of scale usually associated with increasing the number of bits on a traditional computer, pale in comparison to the difficulties of achieving the same scaling with quantum processors.