Programmable quantum information processor using two qubits of information
Qubit: Just like a bit is the basic unit of computer information a quantum bit or qubit is a basic unit of quantum information i.e. a qubit is a quantum analogue of the classical bit. A classical computer has a memory made up of bits, whereas a quantum computer maintains a sequence of qubits.
Each bit represents either a one or a zero where as a single qubit can represent a one, a zero, or any quantum superposition of these. The difference between a bit and a qubit is that whereas a bit must be either 0 or 1, a qubit can be 0, 1, or a superposition of both. Also multiple qubits can exhibit quantum entanglement.(For quantum entanglement see: Entanglement and quantum images). The states a qubit may be measured in are known as basis states or vectors. In general a quantum computer with n qubits can be in an arbitrary superposition of up to 2 to the power of n different states simultaneously whereas a normal computer with n bits can only be in one of these 2 to the power of n states at any one time. Because of this fundamental difference between bits and qubits, quantum computers can solve certain problems much faster than any of our current classical computers. However at present it is felt that the gain is only in efficiency as the quantum computers do not allow one to compute functions that are not theoretically computable by classical computers. One of the possible applications of a quantum computer is breaking today’s most widely used encryption codes, such as those that protect electronic financial transactions.
Programmable quantum information processor using two qubits of information
Physicists at the National Institute of Standards and Technology (NIST) have demonstrated the first “universal” programmable quantum information processor using two qubits of information. This is the first time any research group has made a programmable quantum processor for more than one qubit. The research group has demonstrated the ability to perform programmable processing, combining enough inputs and continuous steps to run any possible two-qubit program. NIST postdoctoral researcher David Hanneke says, “This is the first time anyone has demonstrated a programmable quantum processor for more than one qubit. 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.”
In the NIST processor the two qubits of information is stored in two beryllium ions that are cooled to very low temperatures and held in a an electromagnetic trap and manipulated with ultraviolet lasers. 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. Scientists also can “entangle” the two qubits.
With these capabilities, the NIST team performed 160 different processing routines on the two qubits. According to the scientists although there are an infinite number of possible two-qubit programs, this set of 160 is large and diverse enough to fairly represent them and thus 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.
The programs did not perform easily described mathematical calculations. Rather, they involved various single-qubit “rotations” and two-qubit entanglements. As an example of a rotation, if a qubit is envisioned as a dot on a sphere at the north pole for 0, at the south pole for 1, or on the equator for a balanced superposition of 0 and 1, the dot might be rotated to a different point on the sphere, perhaps from the northern to the southern hemisphere, making it more of a 1 than a 0.
Each program operated accurately an average of 79 percent of the time across 900 runs, each run lasting about 37 milliseconds. To evaluate the processor and the quality of its operation, NIST scientists compared the measured outputs of the programs to idealized, theoretical results. They also performed extra measurements on 11 of the 160 programs, to more fully reconstruct how they ran and double-check the outputs.
According to the researchers a significant challenge for future research will be reducing the errors that build up during successive operations. Also 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.
The new processor made by the NIST scientists 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.
Source: http://www.nist.gov/public_affairs/releases/quantumprocessor_111609.html
November 16, 2009