Quantum computers are not like classical computers. I don’t mean that in the sense that quantum computers perform calculations in any other way, or that they might be faster or smarter. No, I mean that quantum computers have a whole host of problems (read: headache-causing problems) that normal computers don’t.
To mitigate these problems, researchers have hidden quantum information, albeit not very successfully. It turns out that using more than one type of qubit provides a bit more camouflage for quantum information.
Quantum hide and seek
Before we get to the final results, let me paint you a picture of pain. In a quantum computer, calculations are accomplished by manipulating the value of a target qubit – the quantum computer equivalent of a bit – in a way that depends on the value of other qubits. The problem is to do this neatly.
In most cases, all qubits in a system are identical, so if I have a tool that can change one qubit, the same tool will change the adjacent qubits. These tools are inevitably blunt, so changing one qubit has a good chance of changing the neighbors.
Let’s look at a specific example: a quantum computer made up of a series of ions trapped in a trap (an ion is an atom with a missing electron). The ions influence each other by the way they rock back and forth together in the trap.
This collective motion is used to link qubits together, but is very easy to disrupt. Suppose I want to set the qubit state of the central ion. To do that, I have to shine a laser on it; it will (eventually) absorb a photon and change its state. But nothing says it will absorb the first photon it hits. A photon that is not absorbed is scattered, like a bumper pinball machine. That recoil changes the movement of the ion in the trap and disrupts the collective movement of all the ions. This reduces the effectiveness of (and eventually destroys) the collective behavior needed for quantum computation.
But wait – it gets worse. The scattered photon can hit a neighboring qubit and be absorbed. If that happens, you have introduced an error in your calculation. You may have intended to set the status of qubit #3, but you also changed the status of qubit #2.
To solve this problem, a group of researchers showed how you can use a quantum bystander to maintain the state of the qubits for much longer. Instead of using a set of identical ions, the researchers use two different ions. Beryllium ions are used for calculations and between each beryllium ion they place a calcium ion. This protects quantum information in several ways.
The photons scattered by the beryllium ions cannot easily reach other beryllium ions because the calcium ion gets in the way. The calcium ion needs a very different color of light, so the light scattered from the beryllium ion does not change the quantum state of the calcium ion, while the light scattered by the calcium ion does not affect the beryllium ion.
However, these neighbors are not completely isolated from each other. The qubits are still linked by the movement of the ions in the trap. The calcium ion also plays a role in this. When the ions absorb or scatter light, they get a kick that makes their movement in the trap more powerful. This movement must be controlled so that the couplings between qubits remain in check. To do this, the researchers can slow down the calcium ions (using lasers, of course). By slowing down the calcium ion, the researchers suck energy from all the trapped ions, bringing them back under control.
But what’s really cool is how the researchers put it all together in a demonstrator system of three qubits (two beryllium ions and one calcium ion). The researchers brought the calcium ion into a known quantum state and then performed a series of operations on all three qubits. Imperfections mean that eventually there will be some difference between the intended quantum state (i.e, the quantum information) and the intended quantum state. This difference will increase over time because the ions all have a slightly different environment.
This difference is revealed (at least in part) by measuring the state of the calcium ion. This can be done without destroying the quantum state of the beryllium ions.
In response to the measured state of the calcium ion, the trap and state of the beryllium ions are carefully adjusted. Then the calcium ion is cooled and its state is reset. From there, the whole operation of coupling the calcium ions to the beryllium ions can be repeated.
The researchers compared the fidelity of their qubit state (including entangled states) with and without the trick of adjusting the trap and state of the beryllium ions. Without modification, the quantum information stored in the beryllium ions quickly decays. However, with these careful corrections, the researchers were able to perform 50 operations on the beryllium ions without losing the quantum state.
The researchers’ control system isn’t perfect — the information is still decaying, but the decay rate is over 20 times slower than if they used just two beryllium ions.
Best of all, nothing is stopping the researchers from scaling up to more ions. Three qubits is negligible compared to other quantum computers. But achieving more than nine qubits should be possible, which is about the latest state of affairs for ion-based quantum computers. In addition, the cooling and control should make it possible to scale up to even larger numbers of qubits. It’s all quite exciting.
Nature2018, DOI: 10.1038/s41586-018-0668-z (About DOIs)