iv | NewScientist | 6 July 2013

At temperatures of a few kelvin, electrons in some superconducting materials form entangled pairs, known as Cooper pairs, which flow without resistance and are peculiarly resilient against decoherence.

Superconducting quantum interference devices (SQUIDs) already exploit this effect to make incredibly sensitive measurements of electromagnetic fields. But electron movements and magnetic field states within a SQUID can also be manipulated using external fields to form the bits of a quantum logic device. SQUID qubits offer good initialisation and decoherence times, typically 10 or so times greater than the time taken to switch a gate.

With large numbers of qubits, however, heating due to the external fields used for manipulation becomes an issue, and the largest verified system remains at just three qubits. The company D-Wave Systems in Burnaby, British Columbia, Canada, has claimed since 2011 to have a 128-qubit computer in operation (pictured, right); but it remains controversial whether this device is fully quantum, and if it can implement all quantum logic operations.

QUBIT: Superconducting statesCollections of many hundreds of atoms might make for good qubits when trapped, cooled and arranged using lasers in a two-dimensional array known as an optical lattice. The energy states of these atoms can encode information that can be manipulated using further lasers, as with trapped ions (see opposite page). We’ve mastered the basic techniques, but making a true quantum computer from cold atoms awaits establishing reliable entanglement among these aloof bodies.

QUBIT: Cold atoms

Nuclear spin states manipulated using magnetic fields were among the first qubits explored. In 1998, the first implementation of Grover’s algorithm (see “Number crunching”, page vi) used two nuclear magnetic resonance qubits to seek out one of four elements in a database.

The great advantage of spin states is that they make qubits at room temperature, albeit with a very low initialisation accuracy of about one in a million. But the disrupting effects of thermal noise on entanglement means that nuclear-spin computers are limited to about 20 qubits before their signal becomes washed out.

A variant on the spin theme exploits nitrogen impurities in an otherwise perfect diamond (carbon) lattice. These introduce electrons whose spin can be manipulated electrically, magnetically or with light – but scaling up to anything more than a couple of spins has proved difficult.

QUBIT: Nuclear spins

The position, polarisation or just number of photons in a given space can be used to encode a qubit. Though initialising their states is easy, photons are slippery: they are easily lost and do not interact very much with each other. That makes them good for communicating quantum information, but to store that information we need to imprint photon states on something longer-lived, such as an atomic energy state.

If we can nail that, quantum computing with photons is a promising concept, not least because the

processing can be done at room temperature. In 2012, a team at the University of Vienna, Austria, used four entangled photons to perform the first blind quantum computation. Here a user sends quantum-encoded information to a remote computer that does not itself “see” what it is crunching. This may be a future paradigm – totally secure quantum cloud computing.

QUBIT: Photons

Cavity electrodynamics is a quantum computing approach that aims to combine stable cold atoms with agile photons. Light is trapped inside a micrometre-scale cavity and atoms sent flying through, with logical operations performed through the interactions of the two.

Initialisation is highly efficient, and the decoherence time allows 10 or so gate operations to be performed – although scaling the technology up awaits reliable ways of entangling trapped cold atoms. Serge Haroche of the Collège de France in Paris, one of the pioneers of this approach, shared the 2012 Nobel prize in physics with trapped-ion researcher David Wineland (pictured, right).

QUBIT: Atom-light hybrids

This promising basis for a quantum computer has yet to get off the theoretical drawing board, because it depends on the existence of particles confined to two dimensions called anyons. These “topological” particles are peculiarly impervious to environmental noise, in principle making them excellent qubits. Particles such as Majorana fermions that fulfil some of the requirements of anyons have been fabricated in certain solids, but whether they are useful for practical quantum computing is still debatable.

QUBIT: Topological states

Quantum computing pioneers David Wineland (left) and Serge Haroche shared the 2012 Nobel prize in physics

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6 July 2013 | NewScientist | v

There are many ways of making the “qubits” for a quantum computer to crunch, from polarising light to cooling atoms to taming the collective motions of electrons. But any qubit must fulfil some stringent criteria, particularly in proving robust, or “coherent”, in the face of buffeting from its surrounding classical environment. No single sort of qubit has yet ticked all the boxes

Building a quantum computer

trapping ions is perhaps the most advanced method of making a quantum computer’s qubits. positively charged ions are caught in electromagnetic fields and cooled to a nanokelvin or so to reduce their vibrations and limit decoherence. information is then encoded in the ions’ energy levels and manipulated using laser light. that brings excellent initialisation success (99.99 per cent), accuracy (over 99 per cent) and stable memory storage (years).

in 1995 david Wineland and his colleagues at the uS national institute of Standards and technology in Boulder, colorado, used trapped ions to create the first quantum logic gate – a controlled not (c-not) gate for disentangling entangled ions. in 2011, physicists from the university of innsbruck, austria, developed a 6-qubit trapped-ion computer that fulfilled the specifications for a universal quantum simulator that richard Feynman had set out in 1981.

decoherence and scalability remain interrelated problems, however. With a few entangled qubits the decoherence time is 1000 times the gate-switching time, but this rapidly reduces as qubits are added.

quBit: trapped ions

in 1997, david diVincenzo of iBm wrote down some desirable conditions that remain a rough, though not exhaustive, checklist for what any practical quantum computer must achieve.Scalability to out-gun a classical computer, a quantum computer must entangle and manipulate hundreds of qubits. quantum computers built so far have just a handful. Scaling up is a big hurdle: the larger the system, the more prone it is to “decohere” in the face of environmental noise, losing its essential quantumness.initialiSation We must be able to reliably set all the qubits to the same state (to zero, say) at the beginning of a computation.coherence the time before decoherence kicks in must be a lot longer than the time to switch a quantum logic gate – preferably, several tens of times. in most practical implementations so far this requires an operating temperature near absolute zero to limit the effects of environmental interference.accuracy the results of manipulations must be reproduced accurately by the qubit, even when many manipulations are applied in sequence.Stable memory there must be a reliable way to set a qubit’s state, keep it in that state, and reset it later.

WHat maKeS For a good quBit?

” To out-gun a classical computer, we must entangle hundreds of qubits. So far we have managed a handful”

Using lasers to trap ultracooled atomic ions is a well-developed way to make a small-scale quantum computer

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