By achieving entanglement in silicon, researchers have brought quantum computing a step closer.
Ambitions to build a solid-state quantum computer received a boost this week from a team of researchers based at the University of Oxford, UK, who reported the simultaneous entanglement of 10 billion pairs of quantum bits, or qubits, in a highly purified crystal of silicon1.
Qubits are analogous to the bits used for programming in today's computers, but are more useful because they can be placed in a 'superposition' of several quantum states at the same time. It has been shown theoretically that by running calculations in parallel, using many quantum states in superposition, a quantum computer could solve problems that would take a classical computer an infinite amount of time, for example, running Shor's algorithm, which factors large numbers into primes and could be used, for example, to crack the most powerful encryption algorithms on the Internet.
Building a useful quantum computer will involve creating more than 1 million such qubits and entangling them together, so that the states of any one qubit are intrinsically linked with the states of others. The current record for the number of qubits all entangled together is 12.
The authors' next step — which they have yet to undertake — will be to entangle the pairs of qubits with one another, forming a massive-scale quantum computer. By working with billions of qubits at once, "we're trying to hopscotch the middle ground that is holding up a lot of groups", says PhD student Stephanie Simmons, an author on the paper.
With her supervisor, John Morton, and other colleagues, Simmons entangled the pairs of qubits by passing a sequence of radio and microwave pulses into a silicon crystal embedded with 10 billion phosphorus atoms and cooled to less than three degrees above absolute zero. The sequence of pulses flipped the magnetic orientation, or spin, of each phosphorus nucleus and one of its electrons until the pair became entangled with each other, forming a two-qubit system. Simmons and her colleagues confirmed the entangled state by detecting a microwave signal emitted from the crystal.
A sure start
Frank Wilhelm, who works on the theory of quantum-computing devices at the Institute for Quantum Computing in Waterloo, Canada, says that the work represents an advance over the more common method of qubit creation — the use of liquid nuclear magnetic resonance, which entangles the spins on molecules in fluids using a magnetic field. This method holds the current record of 12 for the largest number of entangled spins, but because the starting state of each spin is random, there has been debate among researchers over whether calculations done using the bits are genuine quantum algorithms or just simulations, he says. By contrast, Morton and his colleagues' method means that the starting state of each spin is known to be in either the up or the down orientation.
Wilhelm adds that the work represents progress towards a goal outlined in Nature more than a decade ago by Bruce Kane, then at the University of New South Wales in Sydney, Australia, of one day building a quantum computer using phosphorus-doped silicon2. "The promise is that because it's silicon technology and the semiconductor industry has worked on this, progress will accelerate," he says.
Although the work does move researchers closer to this goal, Morton cautions that the type of silicon used the experiment was not standard commercial-grade; the sample is an extra-pure crystal of the isotope silicon-28, from which atoms of silicon-29 have been removed. Silicon-29 is magnetic and would interfere with the entanglement procedure.
"It's nice, impressive work," says Jeremy O'Brien, a quantum-computing specialist at the University of Bristol, UK. But what is really needed, he says, is the ability to do the additional nanofabrication to put electrodes on the silicon chip to address each individual nucleus and electron pair, a technology that will be needed to get more than two spins entangled together in silicon. "That would be really impressive," he says.
Morton says that recent work by Andrea Morello at the University of New South Wales and his colleagues shows that it will be possible to address individual phosphorus nuclei as O'Brien suggests3. However, he estimates that it may take three to five years to reach the next step of persuading electrons in the crystal to hop from one site to the next, extending the entanglement to many bits. "That will be the real test for getting this scheme to work," says Morton.
Simmons, S. et al. Nature doi:10.1038/nature09696 (2011).
Kane, B. E. Nature 393, 133-137 (1998).
Morello, A. et al. Nature 467, 687-691 (2010).
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Reich, E. Silicon quantum computer a possibility. Nature (2011). https://doi.org/10.1038/news.2011.29