Qubit processor goes general purpose

Physicists at the National Institute of Standards and Technology (NIST) have demonstrated what they claim is the first programmable quantum information processor able to run any program allowed by quantum mechanics and not just specific tasks.

The setup has just two quantum bits (qubits) of information but the processor design could form the basis of a module in a future quantum computer.

The NIST demonstration, described in Nature Physics, marks the first time any research group has moved beyond demonstrating individual tasks for a quantum processor—as done previously at NIST and elsewhere—to perform programmable processing, combining enough inputs and continuous steps to run any possible two-qubit program.

The NIST team analysed the quantum processor with the methods used in traditional computer science and electronics by creating a diagram of the processing circuit and mathematically determining the 15 different starting values and sequences of processing operations needed to run a given program.

“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.”

The NIST processor stores binary information in two beryllium ions, which are held in an electromagnetic trap and manipulated with ultraviolet lasers. Two magnesium ions in the trap help cool the beryllium ions.

NIST scientists can manipulate the states of each beryllium qubit, including placing the ions in a superposition of both 1 and 0 values at the same time and can also entangle the two qubits, linking the pair’s properties even when the ions are physically separated.

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 claimed, 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. This approach was chosen to avoid bias in testing the processor, in the event that some programs ran better or produced more accurate outputs than others.

Ions are among several promising types of qubits for a quantum computer, according to the NIST scientists. If they can be built, quantum computers have many possible applications such as breaking today’s most widely used encryption codes, such as those that protect electronic financial transactions. In addition to its possible use as a module of a quantum computer, the new processor might be used as a miniature simulator for interactions in any quantum system that employs two energy levels, such as the two-level ion qubit systems that represent energy levels as 0s and 1s. Large quantum simulators could, for example, help explain the mystery of high-temperature superconductivity, the transmission of electricity with zero resistance at temperatures that may be practical for efficient storage and distribution of electric power.

Each program operated accurately an average of 79 per cent of the time across 900 runs, each run lasting about 37ms. Many more qubits and logic operations will be required to solve large problems. A significant challenge for future research will be reducing the errors that build up during successive operations. Program accuracy rates will need to be boosted substantially, both to achieve fault-tolerant computing and to reduce the computational overhead needed to correct errors after they occur, according to the paper.

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