One of the leading technologies for building a quantum computer is trapped atomic ions (single atoms with one electron removed), and a principal goal of the NQIT project is to develop the constituent elements of a quantum computer based on these ions.
Professor David Lucas of Oxford's Department of Physics said: “Each trapped ion is used to represent one ‘quantum bit’ of information. The quantum states of the ions are controlled with laser pulses of precise frequency and duration. Two different species of ion are needed in the computer: one to store information, a ‘memory qubit’, and one to link different parts of the computer together via photons, an ‘interface qubit’.”
The report demonstrates the all-important quantum 'logic gate' between two different species of ion - in this case two isotopes of calcium, the abundant isotope calcium-40 and the rare isotope calcium-43.
Prof Lucas explained: “The Oxford team has previously shown that calcium-43 makes the best single-qubit memory ever demonstrated, across all physical systems, while the calcium-40 ion has a simpler structure which is well-suited for use as an interface qubit. The logic gate, which was first demonstrated for same-species ions at NIST, Boulder in 2003, allows quantum information to be transferred from one qubit to another; in the present work, the qubits reside in the two different isotopes, stored in the same ion trap. The Oxford work was the first to demonstrate that this type of logic gate is possible with the demanding precision necessary to build a quantum computer.”
From this technological achievement, the researchers were able to perform a ‘Bell test’, by first using the high-precision logic gate to generate an entangled state of the two different-species ions, then manipulating and measuring them independently. This is a test which probes the non-local nature of quantum mechanics; the fact that an entangled state of two separated particles has properties that cannot be mimicked by a classical system. This was the first time such a test had been performed on two different species of atom separated by many times the atomic size.'
Prof Lucas added: “The significance of the work for trapped-ion quantum computing is that we show that quantum logic gates between different isotopic species are possible, can be driven by a relatively simple laser system, and can work with precision beyond the so-called ‘fault-tolerant threshold’ precision of 99% - the precision necessary to implement the techniques of quantum error correction, without which a quantum computer of useful size cannot be built.'
In the long term, it is likely that different atomic elements will be required, rather than different isotopes. In related work, the NIST Ion Storage group has demonstrated a different type of quantum logic gate using ions of two different elements (beryllium and magnesium).
