I seem to write a lot of articles about quantum computing. You might even think I was obsessed or something. But, I believe that, once we have a useful quantum computer, it will be one of those technologies that we wonder how we ever did without. And by we, I mean scientists. One of the big applications for quantum computers is to solve quantum mechanical problems. And quantum problems are things like the properties of molecules, solids, and liquids.
Without a quantum computer, we will never really know what is possible in terms of new materials. But, to make a quantum computer possible, we need a quantum bit (qubit) that can be protected from unwanted disruptions. To make this possible, a group of researchers from the University of Science and Technology in China have shown that dressing an atom may be the way to go.
I'm sorry, we only allow dressed states in here
To understand what the researchers have done, we need to know what a dressed state is. From a physicist's point of view, an atom is simply a set of energy states. An atom can absorb certain, discrete amounts of energy, depending its current state. On absorbing energy, the atom enters an excited state, and some time later, emits a photon and changes state again. The states it changes between are natural energy states of the atom.
But you can't excite the atom without shoving some energy its way. This energy is in the form of radiation with an energy that matches the difference between two of the atom's sates. Now, let's shine a light field on our atom, but instead of choosing a photon energy that matches a transition, we will choose an energy that is just slightly wrong. Now, according to our naive explanation above, the atom will simply ignore this light. But that is not what actually happens.
The electric field of the light drives the electrons to oscillate anyway, which modifies the energetic structure of the atom. As the electrons oscillate, their shifts periodically reduce and increase the energy required to make the transition. The size of these changes depends on the difference between the frequency of the applied field and the transition, and the strength of the applied field.
The result is that the single state is split into two states, one for each side of the oscillation. These are referred to as the dressed states of the atom. When we excite the atom, we never know which of the dressed state the atom enters, so, on excitation, it automatically enters a superposition state of the two possibilities. In other words, as soon as we turn on the light, the atom enters both states simultaneously.
Why is this important? The way these states change with time is dominated by the strong applied fields, so dressed states can be set up so that they are a bit indifferent to the environment, allowing superposition states to survive longer. In other words, we increase the predictability, or coherence, of the atom in the superposition state.
A return to your regular programming
The qubit implementation that the researchers chose was nitrogen vacancy (NV) centers in diamond. The basic idea is that carbon likes to have four partners, while nitrogen only likes three. When you put nitrogen in a diamond, one of the carbon atoms is left without enough partners. The result is that there is an electron floating around in that gap with its electronic structure.
In the past, researchers have used the electron's spin states—spin states are the orientation of the electron's magnetic field—in an applied magnetic field. But, the problem was that nearby C13 (normal carbon is C12) atoms would add a whole lot of noise to the magnetic field. Effectively, you can think of the surrounding magnetic field as dressing the spin states of the NV, so that the energy levels randomly split, with the splitting changing over time.
The result is that the superposition state, which we need to be swinging back and forth between the two spin states, stops swinging very fast. In the language of quantum mechanics, it loses coherence very fast.
In the past, researchers showed that this can be avoided by applying pulses of light—microwaves pulses, in this case—at just the right time. This gives the swing a push, and keeps the nitrogen vacancy center in the superposition state.
So, that is one qubit. How do you do that trick with two that are entangled? In this case, the two qubits will be swinging at different rates. To keep them both coherent, and entangled with each other, requires carefully timing your pulses so that the pulse gives both qubits a push at the right time. Now, imagine three? Or trying to perform a quantum operation on them? It quickly becomes impossible to determine when one should apply the sequence of pulses that both stop our qubits from falling apart, and perform the necessary logic operation.
This is the problem that the researchers are trying to solve. Each of the spin states gets split by a pair of applied microwave fields. The cool thing is that, if you choose the splitting value right, the dressed states become immune to the noise in the magnetic field. More precisely, the splitting in the energy levels is, for any given magnetic field strength, independent of small changes to the magnetic field.
Instead of using the two spin states as the logic one and zero, the two energy levels created by the microwave field are used as logic one and zero. Since the energy difference between these two states is rather small, it means that quantum operations are achieved with radio frequency pulses instead of microwave pulses.
The researchers showed that the coherence of the qubit was extended from about 1µs to just about 20µs, which is a respectable increase. More importantly, they show that, by applying radio frequency pulses, they can perform qubit operations without destroying the qubit coherence. In fact, it seems that by applying quantum operations, the coherence is preserved better than if the qubit is left to its own devices.
Although they didn't show it, this setup should also allow multiple entangled qubits to be manipulated in a fairly straightforward manner, rather than having to figure out some complicated sequence of light pulses.
Catches and consequences
I have to say that this is a very cool bit of work, and it offers hope for one of the bigger problems in quantum computing. But it comes with its own set of gotchas. For instance, qubit logic operations must now be performed by radio frequency pulses. These pulses have to be longer, because the rate at which the qubit changes is much slower. So, even though the coherence is longer, the operations take longer, so it is unclear if you actually have enough time to perform more operations.
The other thing that makes this difficult is that the microwave fields have to be individually tailored to the NV center. To make matters worse, the radio frequency fields cannot be focused that well, so you end up addressing every qubit at the same time. This takes away one of the big advantages that NV centers have: lots of potential qubits in a single crystal. Instead, you may be limited to one qubit per crystal.
Nevertheless, I can see this being applied in other systems where these problems are not so great.
Physical Review Letters, DOI: 10.1103/PhysRevLett.109.070502