In a step that may help in the development of memory for quantum computers, researchers teleport a quantum state between two atoms that are separated by a meter using a technique that should scale to much larger distances.
The past several years have seen a number of advances towards the goal of creating a scalable quantum computer. Because quantum objects can be in a number of states simultaneously, these computers could sample a large solution space in an instant, providing solutions to certain problems that are currently very computationally expensive. But it's not simply enough to have something that can perform quantum computations; the other parts of a traditional computer, such as memory and communication busses will also be needed. Researchers have now demonstrated the teleportation of quantum states between two ions that are a meter apart, a development that has applications in both quantum computing and communications.
Teleportation, from a quantum perspective, doesn't mean the same thing as it does to everyone who immediately thinks of Star Trek's transporters. In general, it involves two entangled quantum objects, a sender and a receiver, and the quantum state of the former is sent to the latter. A measurement can be performed on the sender that, thanks to the entanglement, changes the state of the receiver. The results of the measurement of the sender can then be used to manipulate the receiver, placing it into the same state as the sender.
In this system, the sender and receivers are ions, which start out a meter apart and are unentangled. To change this, the researchers used a process called entanglement swapping. both ions were irradiated with a laser, placing them in an excited state. Each then emitted a photon that its state was entangled with. These two photons were then combined at a beam splitter (a partially reflective mirror), which swapped the entanglement between the ions and photons so that the two ions were entangled. The color of the light coming from the beam splitter let the researchers know if they succeeded with the entanglement swapping.
Successfully entangling the two ions at a distance is only the first step. The researchers then measured the state of the sender and used that to determine which of two microwave pulses to shine on it. Once that pulse was absorbed, the receiver was set in the sender's unknown quantum state.
Teleportation of quantum states has been observed before, but the new approach has a number of distinct advantages. Long-distance teleportation had been achieved in the past with photons, but photons are great at carrying information, yet not so handy for manipulating information. Atoms and ions are much better for that--especially these ions, which were nearly stationary at temperatures just above absolute zero. Teleportation of atomic states had been achieved in the past, but only over very short distances because of the need for the atoms to interact. The new technique provides a "best of both worlds" situation--entanglement can be created over arbitrarily long distances (far longer than the one meter used here) rapidly by photons, but the actual storage of the quantum state happens on ions that sit in a well-defined location.
That said, the technique is not without its issues. The key limiting factor appears to be the harvesting and interaction of the single photons at the beam splitter, which the authors say succeeds with a probability of about 2.2 x 10-8. Since the system takes a measurable amount of time to recycle through a ground state before each attempt, the net result is that it takes an average of 12 minutes just to perform a successful entanglement. A 0.0014 baud quantum modem is not exactly going to set the world on fire. The authors suggest a number of ways to improve the efficiency of this process, primarily by using equipment that's already in use.
Another issue is that the process is only about 90 percent accurate. The authors argue, however, that this isn't a result of any inherent indeterminacy in the system. Instead, several pieces of instrumentation cause a series of small (generally less than four percent) errors to creep in that, in sum, influence the accuracy of the outcome. Again, it's possible that changes in instrumentation can limit the problems here.
The authors are well aware of the potential uses of their approach. "The teleportation scheme demonstrated here could be used as the elementary constituent of a quantum repeater," they wrote. "Moreover, the entangling gate implemented in this protocol may be used for scalable measurement-based quantum computation." There's no guarantee this scheme will show up in a future quantum computer, but it certainly provides those who are thinking about building these devices with a useful tool to consider.
Science, 2009. DOI: 10.1126/science.1167209
Spooky memory at a distance with quantum teleportation - Ars Technica
