December 2, 2004, Gaithersburg, MD--A practical method for automatically correcting data-handling errors in quantum computers has been developed and demonstrated by physicists at the National Institute of Standards and Technology (NIST).
Described in the Dec. 2, 2004, issue of the journal Nature, the NIST work is the first demonstration of all the steps of error correction for quantum computers. The method was implemented using ions as quantum bits (qubits). Ions are arguably the leading candidate for use as qubits in a quantum computer.
Conventional computers use electronic switches that are either on or off to represent 1s and 0s that then can be stored or manipulated to make calculations. Quantum computing would use the quantum states of matter (such as magnetic properties) as 1s, 0s¿or even both at once. Specific applications could include code-breaking of unprecedented power, faster database searching, fraud-proof digital signatures and optimization of everything from communications systems to airline schedules. But unless data-handling errors are corrected, "noise" caused by environmental disturbances, such as fluctuating magnetic fields associated with electrical equipment, could diminish any gains over today's computers.
The new NIST method helps to ensure the correctness of data during computations by creating redundant data sets, or what might be called quantum backup copies. "The basic concept is a familiar one: If someone doesn't understand what you say, you repeat it several times, and eventually they'll get it," explains physicist Dietrich Leibfried, who developed the approach and helped to demonstrate its feasibility in NIST's Boulder, CO, laboratories.
Direct copying of qubits is prohibited by the rules of quantum mechanics. Like all known quantum error correction methods, the NIST method gets around this obstacle by exploiting a famously spooky (the term used by Einstein) feature of quantum mechanics that allows the "entanglement" of physically separated atoms to link their quantum properties in predictable ways. The atoms also are prepared in a special "superposition" state in which they represent both 1 and 0 at the same time.
The demonstration used three beryllium ions as qubits. One "primary" ion is entangled with two "helper" ions as part of a series of encoding steps. The primary qubit is essential to the computation; the other two are expendable. Because the three are entangled, errors in one affect the others, a condition that is reflected in the joint quantum state of all three qubits. If the quantum state of the primary qubit is accidentally changed, the mistake can be detected and corrected by reversing the steps to decode the data, and then measuring the values of the two extra qubits.
Unlike other demonstrations of quantum error correction, the NIST approach makes corrections based on actual measurements, allows qubits to be "reset" on the fly, and could be scaled up for use in quantum computers of practical size and utility. Previous demonstrations by other groups have involved correction of errors in qubits made of molecules in a liquid, without the ability to measure or reset and reuse the extra qubits needed to detect errors. The ability to "empty the trash bin," rather than simply storing mistakes somewhere in the computer, makes the NIST approach more practical.