Quantum leap for digital cable

Dec. 13, 2007
Physicists at the National Institute of Standards and Technology (NIST) have demonstrated a potential component for quantum computers using electronic vibrations on a microfabricated aluminum cable that transfers information between two “artificial atoms.”

The assembly looks like a miniaturized cabletelevision transmission line with some added features, including superconducting circuits with zero electrical resistance and multitasking data bits that obe y the unusual rules of quantum physics.

The cable might someday be used in computers that rely on quantum behavior to break codes, search databases, and carry out other functions exponentially faster than today’s most powerful computers. Moreover, superconducting components could be easier to manufacture and scale up than alternatives such as wires made of individual atoms.

Unlike traditional electronic devices which store information in the form of digital bits that each possess a value of either 0 or 1, each superconducting circuit acts as a quantum bit, or qubit, which can hold values of 0 and 1 at the same time. Qubits could perform far more simultaneous calculations than traditional digital bits. The section of cable shuttling information between two superconducting circuits, or “quantum bus,” could transport data between two or more qubits.

A superconducting qubit is about the width of a human hair. The researchers fabricate two qubits on a sapphire microchip, which sits in a shielded box measuring about 8 cu mm. The 7-mm-long cable, similar to coaxial wiring used in cable television but much thinner and flatter, zigzags around the 1.1-mm space between the two qubits. Like a guitar string, the cable hums, or resonates, at a particular tone or frequency in the microwave range. Quantum information is then stored as energy in the form of microwave particles or photons.

The scientists encoded information in one qubit, transferred it as microwave energy to the cable for a storage time of 10 nsec, and then shuttled the information to a second qubit. “It’s significant because it means we can couple more qubits together and transfer information between them using one simple element,” says NIST physicist Ray Simmonds.

In addition to storing and transferring information, the cable can “refresh” superconducting qubits, which normally maintain the same delicate quantum state for only half a microsecond. Disturbances such as electric or magnetic noise in the circuit can rapidly destroy a qubit’s superposition state.

With improvements, the technology might be used to repeatedly refresh data and extend qubit lifetime more than 100-fold, enough to create a viable shortterm quantum computer memory, says Simmonds. NIST’s cable might also transfer quantum information between matter and light — microwave energy is a low-frequency form of light — and thus link quantum computers to ultrasecure quantum communication systems.

If they can be built, quantum computers — harnessing the unusual rules of quantum mechanics — might optimize complex systems such as airline schedules, make counterfeitproof money, and solve complex mathematical problems.

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