Not quite a breakthrough: Quantum computing takes baby step

A breakthrough in quantum computing is trumpeted, but on further review, it's …

A few days ago an article appeared which mangled some quantum mechanics and recent research quite spectacularly. In many ways, what happened is quite understandable. Quantum computing can be a rather esoteric subject, one that doesn't lend itself well to poorly-written press releases that claim huge advances in the "wiring" of quantum computers. Here's the real story.

A quantum computer is not all that different from a classical computer. Logic gates are used to perform operations and the information being operated on must be transported from one logic gate to the next to perform a sequence of operations.* This is a necessary step that must occur before a useful device is developed, but there are a few areas where classical and quantum computers differ.

The first is the nature of the qubit, which will normally be in a state that is both one and zero simultaneously (called a superposition state). The second is a ghostly and not-well-understood correlation between different qubits called entanglement. This property links the value of one qubit with another (or in a practical computer, many others). Finally, to maintain both a superposition and an entanglement, the qubits also need to remain coherent, which essentially means that if you do nothing to them, the qubits will have the same relative relationship over time. When this fails, nature "measures" the qubit, and it becomes a one or a zero.

The paper in question here, which appears in Science, details a scheme to entangle two qubits together in a controllable way. Each qubit is encoded in a ring of superconducting current where the one and zero states are given by the direction of the current flow. The two qubits are coupled through a third superconducting ring that I will call the "coupler" for clarity. The coupler never leaves the equivalent of the zero state, so only the magnitude of current circulating is adjusted. These superconducting current rings all generate a magnetic field; qubit one is linked to the coupler by their mutual inductance—the same mechanism that is used in transformers.

Likewise, the second qubit is linked to the coupler. This means that the strength of the entanglement between qubits one and two can be controlled by the strength of the magnetic field produced by the coupler. In practice, the researchers turn the microwave current going into the coupler up (or down) to control the qubit.

Why is this significant? Controllable entanglement is nothing new, having been demonstrated in optical and ion-based quantum computers before. But this is the first time it has been demonstrated using solid-state qubits, which is worthy of note. However, this is not the "wiring": the researchers did not demonstrate transportation of qubits from one logic gate to another. Furthermore, it is not a huge breakthrough and numerous problems still remain. Individual qubits survive for about 1 microsecond, but the entanglement only lasts for 200 nanoseconds—not much time in which to do calculations. Even if researchers can extend this, no one yet understands why entanglement occurs or why it will disappear even in the absence of disturbances. These really fundamental questions will need to be answered before quantum computers can become practical.

* The computer can also be set up so that the logic gates are sequentially applied to static qubits, in which case there really is no wiring at all.

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