Quantum computing is one of the current big things in both physics and computer science circles. But there is a serious divide between what we think might be possible and what we can, in fact, do. There are theorists out there working themselves into a frenzy, trying to show that quantum computing will make a smoother latte. On the experimental side, many researchers are still in various stages of single gate operations. It is like the difference between trying to make a valve and knowing what you can do with lots of valves once you have them.
In a recent paper, published in Applied Physics Letters, researchers from the UK and Australia have demonstrated that quantum computing gates with very low error rates, based on integrated optical circuits, are now feasible. This might pave the way for multi-gate optical quantum computers.
Quantum computing is, as the name might suggest, a merger between classical digital computers and the quantum freakiness that permeates the world around us at the smallest scales. In a classical computer, a bit can have two values: logic one and logic zero. When we perform operations on a string of bits, we either leave them unchanged or flip them, depending on some control bits. It is important to realize that the value of a bit at any particular time does not depend on any of its partner bits.
If we add a dash of quantumness to the mix, we can do two things. First, logic elements, now qubits, are no longer logic one or logic zero; instead, they are both at the same time. When we read out the result from a program, we obtain a definite one or zero, but during the computation, the qubit really is in both states. Operations don't necessarily flip bits. Instead, they modify the probability of a measurement returning a one or a zero. The second element added to the mix is correlations between qubits. When we perform an operation on one qubit in a string of them, we are actually performing an operation on all the qubits.
There are good and bad aspects to this. A quantum computer doesn't always return the right answer, but some operations, like factoring or database searches, can be sped up. Not returning the right answer comes from two factors. There is an intrinsic uncertainty associated with measurement—it's the price we pay for being in a quantum universe. There are also instrumental imperfections, which, at the moment, play a major role in limiting quantum computing.
This is where Laing and colleagues come in. They focused on the construction of near perfect circuity. In the case of optical quantum computing logic, this corresponds to making perfect beam splitters and interferometers.
These aren't the normal optics you might find in a microscope, which makes things both easier and more difficult. For instance, in a waveguide, a beam splitter is replaced by a directional coupler, where two waveguides are brought into close proximity. Over a certain length, light from one waveguide will leak into the adjacent waveguide. The amount of light that transfers depends on how close the two waveguides are and the distance they remain close. So, in principle, it is very easy to design a perfect beam splitter. In practice, fabrication uncertainty makes this a bit of a lottery—the usual procedure is to make quite a few, test them all, and pick the good one to report on.
Interferometers are similar, in that they involve splitting and recombining light beams. However, in addition to requiring two perfect beam splitters for the interferometer, one also needs to carefully control how far the light must travel between the two. In other words, the fabrication tolerances on the two different light paths are quite tight.
However, once you have these two elements, you can make a controlled NOT gate—a gate that inverts the quantum state of one qubit, depending on the state of the controlling qubit—which is a logic element from which all other logic elements can be constructed. That is exactly what this paper demonstrates. They show that they have very low loss waveguides, and that they can make beam splitters with a splitting ratio within a couple percent of their design ratio.
To illustrate this, they showed data obtained from the quantum interference between single photons passing through their beam splitter. The error bars on the data are tiny, so within the uncertainty of their measurements, they have a perfect instrument.
Likewise, Laing and colleagues show a controlled NOT gate that gets it right 97 percent of the time. "Right" being a relative thing here—this is the fidelity, which means it takes into account the fact that quantum measurements have a finite chance of getting the wrong answer irrespective of the quality of the equipment. From this, they calculate that, at worst, they have an error rate between one part in 100 and one part in 1000. The latter figure is probably good enough to start thinking about multiple gate operations.
As you can see, I'm not reporting on anything startling here, just a good solid bit of technology that is necessary for optical quantum computers to do anything useful. I do wonder, however, how many of the circuit elements on the wafer were functional, because that is probably the limiting factor now. One thing missing in all optical implementations of quantum computers is programmability, because that involves switching light paths around. In integrated optic implementations, like this one, switches could be fast, and if the losses are low enough, programmability might well be on the horizon.
The bigger problem on the horizon is multi-qubit calculations. To perform a calculation represented by a register of eight qubits, every one of those qubits has to be entangled with every other qubit, and that ain't easy.
Applied Physics Letters, 2010, DOI: 10.1063/1.3497087