When it comes to systems for quantum computing, trapped ions rule the roost. Admittedly, I don't think anyone actually likes the idea of a quantum computer based on wafting ions around vacuum chambers so that they can bounce off each other to perform calculations. So the picture is actually considerably more mixed. Ions make a great choice for quantum logic gate operations, but a poor choice for transporting quantum states between logic gates. Light—or more accurately, single photons—make a great choice for transport, but those aren't so great for storage or logic operations.

The point being that different quantum systems will likely play different roles in a useful quantum computer. And that means making these different methods of encoding quantum information talk to each other in an efficient and reliable way. A recent publication in Physical Review Letters proposes a way to link the quantum states of ions to the electrical signals produced by superconducting quantum interference devices (SQUIDs). The upshot is that we get the benefits of long-lived quantum states in the ion and a high-speed connection between ions, via SQUIDs.

What makes ions so special

Ions, in our case, are atoms that have a single electron removed. The charge on an ion makes it easy to use electric fields to hold them in place. Indeed, if you trap a group of ions, they naturally form up in regularly spaced arrays, making it really easy to target individual ions with pulses of light. To make things better, the electronic states of ions have a relatively high energy. This means the jostling thermal motion of the ions in the trap doesn't have too big an influence on the coherence of the electronic quantum states—these are states that are excited by pulses of visible and ultraviolet light. Indeed, even the motion of the ions within the trap is relatively steady and has fixed states, which can also be used for quantum computing. And thanks to the charged state of the ion, the trap motion is quite fast and strong, making it relatively immune to the effects of thermal motion.

All in all, ions are pretty cool to work with. But there are trade-offs. You can have easy but slow, or impossible but fast. Quantum logic operations that use the electronic states of ions take milliseconds to complete, but you only need light and radio frequency pulses to make the operations proceed. The output can be coupled to other ions through pulses of light, as the results of quantum operations are often the emission of a photon.

On the flip-side, the trap-motion states have faster operation times, but you only have the radio frequency fields of the trap to manipulate the ions. So you can only have a single ion in any given trap—the radio frequency pulses would give every ion in the trap the same shove. To make matters worse, the output is also in the form of very weak radio frequency fields, which cannot be transported to other ions easily.

Ions dance with SQUIDs

Another quantum device that operates at radio frequencies is the SQUID, which is a loop of superconducting current. But the frequency of a typical SQUID is in the GHz range, while the motion of ions in a trap is in the MHz range. The difference between these values means that the two barely notice each other when they are in proximity to each other. Yet coupling these two systems together is precisely what a group of Australian physicists have proposed.

In their proposal, the SQUID is a circuit that includes the circuits used to trap the ion. When taken together, all the elements form an inductor and a capacitor, giving the SQUID a particular resonance frequency—this is the frequency of the oscillations of the superconducting current in the loop. This frequency is several GHz, and completely fails to excite the ion.

To couple the ion and the SQUID, an acoustic element is placed underneath the ion trap. When the acoustic element is switched on, the capacitance of the trap changes. This change, however, is not a binary one. Instead, the capacitance continuously varies, oscillating between a maximum and minimum value at the frequency of the acoustic wave.



Quantum superposition Superposition is nothing more than addition for waves. Let's say we have two sets of waves that overlap in space and time. At any given point, a trough may line up with a peak, their peaks may line up, or anything in between. Superposition tells us how to add up these waves so that the result reconstructs the patterns that we observe in nature. Read more…

This matters because the precise resonance frequency of the SQUID depends on that capacitance, so its resonance frequency changes. The result is that the SQUID starts to oscillate on three different frequencies: its natural frequency, one that is the sum of the natural frequency of the SQUID and the acoustic wave, and one that is the difference between the two.

By tuning the acoustic frequency so that the difference between it and the natural SQUID frequency is exactly the same as the ion frequency, the ion's motion becomes coupled to the oscillation of the superconducting current. Between the SQUID and the acoustic element, one can drive the ion into a superposition of trap motion states. Indeed, the SQUID can be turned into a sort of quantum bus by incorporating multiple traps into the same circuit.



Quantum entanglement Quantum entanglement is one of the most misused concepts around. Entanglement is delicate, rare, and short-lived. At its heart, quantum entanglement is nothing more or less than a correlation between two apparently separate quantum objects. Having discovered that, you might ask "so what is all the fuss about?" The answer lies deep in quantum mechanics. Read more…

A nice element is that this setup does not preclude the use of light to manipulate the ion qubits. So, one can use laser light and fiber optics to entangle ions that are sitting in different labs. This provides the best of both worlds: high energy electronic states can be used for readout, long range quantum information transfer, and entangled state generation. The SQUID bus then provides local logic operation control and state transfer.

The paper uses a concrete example: the calculations are based on the physics and engineering of a real trap design. This indicates that the researchers are working to implement the design, but have so far failed to get it to work—experimental demonstrations of a new method are always superior to a paper on a proposed design. I wonder where the difficulties are.

Physical Review Letters, 2012, DOI: 10.1103/PhysRevLett.108.130504

Listing image by Ian Britton