The resiliency of quantum teleportation may help quantum computers function as well as traditional digital ones when doing calculations, according to an article recently published in PNAS. Using sets of six photons, a group of researchers modeled two different kinds of logic gates by teleporting data between the photons. While the components they used are the quantum equivalent of computers that took up whole rooms and used punch cards, the scientists found the system had good transmission fidelity.

Quantum teleportation (not the run-of-the-mill sci-fi teleportation) moves only quantum states. So, instead of having a particle appear at a distance, the system starts with two particles, and places the second in a quantum state that's identical to the first. The procedure relies on entanglement, which can work at a significant distance—up to ten miles, as recent experiments have found—and so it's still subject to the limits that keep entanglement from transmitting information faster than the speed of light.

While quantum teleportation doesn't have perfect fidelity, scientists have become very interested in its resiliency and speed in transmitting information. Teleportation seemed poised in particular to help create a "universal quantum computer," one that can capture and perform all quantum algorithms and operations.

The authors of the PNAS paper wanted to pursue universal quantum computer functionality as simply as possible. They were able to boil down the tools they needed to make quantum logic gates to three things. One is multiparticle entangled states, where four the states of particles were entangled so that reading one would set the other three. Another is Bell-state measurements, where two particles are entangled in one of four known combinations. The final ingredient is single-qubit operations, which can be performed on one qubit (particle) to determine its state.

Of course, these are more actions than tools; the actual tools that accomplished these manipulations were a UV beam, barium-borate crystals, polarizing beam splitters, and wave plates. All the pieces together could create, entangle, and read only six photons acting as qubits, yet the setup was many times larger than that of a standard digital computer chip that can handle a great deal more information. Still, as proof-of-concept, it would do.

The first task the scientists tried to achieve was creating a controlled-NOT gate. The typical function of a c-NOT gate is to look at one of a pair of bits and, depending on whether that bit is 1 or 0, flip the other.

The researchers manipulated data for the gate using two input qubits, one with the state that would ultimately be read, and the other to be flipped. They also had four more qubits all entangled with one another in a special state that they would use to read and output the results of the gate.

To do the gate operation, they took each of the input qubits and paired it with one of the entangled qubits (the two unused qubits were the "output" qubits). Each pair was then read, or measured, with a Bell-state measurement, resolving them to some combination of 1 and 0. This process teleported the data of the two input qubits onto the set of four entangled ones.

Because two of the qubits being measured were entangled with the output qubits, the output qubits were resolved too. All that was left to do was perform unitary operations on the output qubits, depending on how the Bell-state measurement turned out, to get the results of the c-NOT gate. The scientists were also able to model a control gate, which aims to transmit input exactly as it is, using the same principles.

The design isn't without drawbacks; namely, the size is a problem as far as modern comping goes, but the authors hope to develop a chip based on the same principles for use in integrated devices. They also note that some quantum corrections had to be made for time evolutions of the particles. This is standard practice in quantum computing, but still takes extra time and energy. Going forward, they hope to look at more entanglement cases and combinations of spatial modes and polarization, as well as constructing a fault-tolerant gate for their system.

PNAS, 2010. DOI: 10.1073/pnas.1005720107 (About DOIs).