Portland, Ore. -- The National Institute of Standards and Technology (NIST) has demonstrated error-free quantum communication by pairs of entangled atoms, promising secure quantum computers.
Entanglement is a quantum phenomenon in which two particles take on identical internal states in close proximity. If conditions are right, the synchronization persists even after the particles become separated, enabling the quantum information processed by one member of the pair to be simultaneously processed by the other. "If you can have entangled pairs in different locations, that is a sufficient resource for transporting totally secure quantum information and for doing universal quantum computations," said Dietrich Leibfried, a member of the technical staff at NIST (Boulder, Colo.).
The quantum state lets a particle encode a 0 and a 1 simultaneously (called a superposition of states), permitting quantum calculations to be performed in a single clock cycle. As long as the calculations do not disturb the quantum bits (qbits), the superposition enables simultaneous quantum calculations on all the superimposed logic states. Since entangled ions are synchronized, the superposition of states encoded in one member of the pair is automatically mirrored in the other, enabling secure communications. Almost any operation performed to determine the state of the entangled particle also destroys the synchronization, thus enabling the receiver to detect eavesdropping. The downside is that destroying the synchronization halts the communication.
Thus far, schemes to sidestep this catch-22 have centered on photons. Researchers make multiple entangled pairs of photons, perform the same operations on each pair and then destructively read out from only one pair.
Now NIST has demonstrated a method that lets entangled atoms communicate information nondestructively, potentially enabling long quantum calculations in which intermediate results could be obtained without disturbing their quantum states. The algorithm traps charged ions in four electromagnetic traps spaced only a few microns apart on the surface of a chip. Ultraviolet lasers then entangle two pairs of ions.
"You have to frame your question in such a way that you do not disturb the quantum state," said Leibfried.
The NIST data "purification" procedure used two pairs of beryllium ions. The procedure progressively entangled two ion pairs, performed a quantum processing step, then read out the results from one pair while maintaining the integrity of the information in the other pair.
The first step uses laser pulses to set the spin state of each member of each pair. If one member of a pair is encoded spin "up," then the other member is encoded spin down. "The trick was that we measured something that did not give us complete information about the state we wanted to preserve; we only measured certain properties that told us the ions were entangled," said Leibfried. The second step entangles each atom with one member from its own pair and one member from the second pair.
The purification procedure--which NIST said worked in one out of three attempts, compared with one in a million attempts with photons--employed special error correction procedures that made the information more secure at each iteration.
Next, NIST plans to build chips with as many as a hundred ion traps. "In principle you can iterate this purification process to create entangled pairs of any fidelity, asymptotically approaching perfect entanglement on as many pairs of ions as you need," said Leibfried. Another group at NIST is experimenting with entangling two types of ions on the same chip.
The project was funded by the U.S. Disruptive Technology Office and the Commerce Department.