A couple of years ago, D-Wave Systems reported that qubits within its quantum computers were tunnelling. This would permit the qubits to, in effect, explore the energy of all the possible states of a system significantly faster than a traditional system, speeding up certain kinds of computations. At a recent computing workshop the company claimed to have also found evidence for entanglement with systems of two and eight qubits. Entanglement is a requirement for its machine to truly operate as a quantum computer, but not proof that it does. D-Wave’s newest machine, a 512 qubit processor due to come out this year, is not a universal computing machine. It does, however, promise to provide up to 10,000 times speed-up compared to classical computers for certain kinds of problems.

The qubits in D-Wave’s computer are implemented as an array of chilled, superconducting niobium (formerly the element columbium) loops. To perform a computation, a physical problem involving energy minimization, for example, is mapped onto this qubit landscape. D-Wave was recently able to solve a problem in the folding of proteins, which tend to adopt the lowest energy configuration. While floating in solution, the string of amino acids comprising a protein may in reality fold a little bit differently than a string sequentially built up naturally by the ribosomal presses. In many cases, though, their structure can be predicted.

In a paper published at the beginning of the year in Physical review B, D-Wave researchers described a new technique known as qubit tunneling spectroscopy, which it used to measure the energies of its qubits. This method allows the company to determine whether the system is at least compatible with entanglement. D-Wave’s results depend on certain assumptions about the energy spectrum of its system, which leaves it open to criticism. More recently, other researchers have corroborated D-Wave’s claims, by providing additional evidence of entanglement using purely mathematical tests.

While entanglement is necessary for quantum computation, if it does not result in actual performance gains there would seem to be little point in going through with all the trouble of making a quantum computer. To be commercially viable, D-Wave needs to demonstrate performance gains that scale as the number of qubits increases, to the point where problems of non-trivial complexity can be addressed. D-Wave’s methods are fairly new, and while its theory shows speed-up would definitely occur at a temperature of absolute zero, its computer runs slightly hotter. D-Wave has promised new data, to be be published later this year, and many folks anxiously await the results, particularly those who have already bought its machines.

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