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Chris Lee - Oct 28, 2011 12:21 am UTC

I have been writing about quantum computing for a while now. If you look at my recent writing, though, you won't find much about quantum computing. Why? Well, it all felt a little repetitive. The publications were still coming, but each new one seemed very much like the previous one. I'm not being cynical here; sometimes you just burn out on a subject.

In that light, it takes something special to attract my attention. It turns out that making something that looks and feels like a complete quantum computer—albeit on the smallest of scales—will definitely attract my attention. What we have here, ladies and gentleman, is nothing more or less than the first quantum microprocessor.

Quantum computing has turned out to be a challenge because it relies on encoding information in quantum bits (qubits) that have two fundamental properties. The first is coherence, which allows qubit states to naturally change in a syncronized manner. The second is quantum entanglement, which correlates the states of different qubits with one another. When we perform operations and measurements on a qubit that is entangled with another qubit, we automatically learn about and modify the state of its partner. This provides a sort of quasi-parallelism that allows a quantum system to perform some calculations faster than a classical computer.

But a computer is more than its bits. You need a register to hold qubits and perform operations on them. You need a memory, so that you can store qubits between operations. And you need to be able to initialize and readout the qubit so that you can begin and end a calculation. Now, there are groups of researchers who have done all of these separately. And, using trapped ions, some groups can even claim to have done the whole lot together. But I don't think anyone seriously thinks that tables full of optics, lasers, and vacuum systems is the way to quantum computing nirvana.

No, quantum computing nirvana is firmly in the realm of solid-state physics. Unfortunately, this is where the problems begin. Qubits don't last long in the solid state. Entanglement lasts a few hundred nanoseconds and coherence decays away faster than a banking regulation. Yet despite these problems, a group of researchers have managed to make an entire quantum microprocessor out of superconducting qubits.

Admittedly, the computer is rather simple: a two-qubit register made from SQUIDs (superconducting quantum interference devices), two additional SQUIDs that can be used to zero the register (and act as readout), and microwave resonator striplines, which act as memory. The most significant part, however, is a bus that couples the two register qubits together. This bus enables the researchers to program the register to perform different logic operations. That is what makes this something I am willing to call a microprocessor—though it can't load up a sequential set of instructions into a memory element and execute them.

This all works through the magic of magnetic fields. (What, do we understand magnets now?) The microwave frequency that a SQUID likes to operate at depends on the magnetic field it is exposed to. The resonators have a fixed geometry that will only resonate at one microwave frequency. So a memory can be read or written by changing the magnetic field so that it is the same as that as the resonator. The same is true of the zeroing registers. 

As a result, operations are really just a case of ramping magnetic fields up and down. Operations between qubits are performed by applying microwave pulses on the bus between them.

Conceptually, It is all very simple. It also is likely to scale well, since you just need to be able to choose different operating frequencies for each memory element and qubit.

Of course the qubits still don't last very long. Their entangled states last 400ns, and the memory holds its value for four times longer. But the length of microwave pulses required to perform a logic operation are on the order of 30ns, so that 400ns is an absolute age.

No doubt there are plenty of steps, pratfalls, and other interesting hiccups along the way, but this bit of work shows how incremental improvements can come together into something that looks quite spectacular.

Listing image by Photo illustration by Aurich Lawson

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