Today in the journal Science Advances, researchers from the ion trapping group of the University of Sussex in the U.K. Aarhus University in Denmark, Siegen University in Germany, Google Inc and Riken in Japan have proposed a fundamentally new architecture for an ion-trap quantum computer. I was a part of this research and am very excited to work on a method for ion-trap quantum computing that can form the basis of a large-scale machine.
Ion-trap quantum computers have been one of the leading technologies for large-scale quantum computing. The underlying technology is very mature and was developed initially to be used as very accurate atomic clocks. When quantum computing was initially developed in the 1980's and 1990's, ion traps were one of the first technologies to experimentally demonstrate individual quantum bits (qubits) and since then, technology development has been pronounced.
In an ion-trap quantum computer, individual qubits are ionised atoms. Some systems use Calcium, some use Beryllium and some use Ytterbium. As the atom is ionised (i.e. carries a net positive charge) it can be trapped by an electromagnetic field, holding it in place. The ion qubit is then held in an electromagnetic field inside a ultra-high-vacuum container. This vacuum is required to make sure that the ion is not knocked out of the trap due to collisions with other atoms flying around inside the system. The qubit itself is defined by the quantum state of a single electron of the ion. Two stable electronic states are chosen to represent the binary zero and one states and these states can be manipulated via lasers or by manipulating the magnetic field environment of the ion.
Manipulation of a single ion qubit is now routine in laboratories around the world. Injecting and trapping an ion, performing single qubit quantum gates and reading out individual qubits can be done with extremely low error rates, in multiple systems, and many small-scale tests and protocols have been demonstrated over the past decade and a half.
Operations on multiple qubits are also possible through coupling ions through motional degrees of freedom between two (or more) trapped ions. Because individual ions are positively charged, if they are placed in the same trap, they will experience a mutual repulsion due to their respective positive charges. This mutual repulsion changes slightly when the electronic configuration changes between each individual ion and hence can be used to enact quantum logic gates between two qubits. Again, through careful control of the system, experimentalists have enacted logic operations between qubits and realised small-scale programmable ion-trap quantum computers.
The question that physicists and engineers are now addressing is scalability, namely how do we increase the number of qubits in the system to enact complex and required error correction protocols and scale the system to sufficient size to perform quantum algorithms that cannot be realised on even the most powerful classical supercomputers?
Scaling ion-trap computers to the level of millions (if not billions) of qubits requires very careful design. Luckily, ion-trap computers have a rather unique property: qubits can be moved (shuttled) around, they are not fixed in place. By manipulating electromagnetic fields that are used to trap individual ions, they can be moved and shuttled around the computer. This allows us to trap ions separately and move them around to inject or "load" them into the computer, measure them in dedicated readout zones and to entangle them with other ions in the computer, fast and with very low error rate.
Even with the very low error rates that experimentalists can achieve with ion-trap technology, they are still not good enough for large-scale algorithms such as Shor's factoring algorithm or Grover's search algorithm. Active error correction codes are still needed. The ion-trap architecture is consequently designed around a class of topological error correction codes, known as surface codes. Surface codes are a desirable method for large-scale, error-corrected quantum computers as they are amenable to system design and have very good performance. Surface codes only require error rates for each physical operation in our computer to be below approximately 1% before they begin working effectively. Error rates at 1% or lower are already experimentally achievable in ion-trap systems.
In other designs for ion-trap computers, physicists have imagined building small mini-computers, each containing anywhere between 10-100 physical ion qubits. These mini-computers would then be linked together with photons and optical fiber. This would allow scale-up by connecting together separate and comparatively small ion-traps to form a larger computer. unfortunately, the downside to this approach is that establishing an optical connection between separated ion-traps is both very slow and very noisy, two things that are detrimental to a functional and useful quantum computer.
In our approach, we decided that a monolithic design for an ion trap is better. The X-junction shown above allows an individual ion to interact with its four neighbours, hence to scale the computer to arbitrary size, we just physically connect may X-junctions together and shuttle ion qubits between X-junctions to perform gates.
We define a module that consists of an array of 36x36 X-junctions, each junction containing a single qubit in our quantum computer. This module contains all the control structures necessary to manipulate the qubits in the ion-trap. Below the surface of the trap (where each individual qubit hovers about 100 micro-meters above the electrodes) there are layers of electronic control and cooling. Finally, the module is fabricated to a set of piezo-actuators and then fabricated to a support frame. The piezo-actuators are used such that two modules can be aligned together and ions transported across the junction between two modules. Our analysis showed that provided each module was aligned to less than 10 micro-meters in either the x,y or z direction, we could still reliably shuttle ions between modules.
If this module can be built, scaling the quantum computer to arbitrary size simply requires fabricating more and more modules and connecting them together. In this way, the ion-trap quantum computer can operate as fast as possible with very low error rates and does not require us to build and integrate in additional quantum technology such as photonic interconnects which have so far proven to be difficult to build reliably, with good performance.
Scaling an ion-trap quantum computer will require some very high quality engineering. Each module contains enough X-junctions to accomodate 36 ion qubits and occupies a physical space of 90mm x 90mm, this is a comparatively large footprint for a quantum computer. We can envisage a much larger system, as illustrated, which contains 2.2 million X-junctions in a series of connected vacuum chambers (hence 2.2 million qubits). The size of each chamber is 4.5m x 4.5m, about the size of a mid-sized office. Additionally, the entire quantum computer must maintain an ultra-high vacuum inside for the length of time necessary to run a quantum algorithm (which may be anywhere from seconds to weeks).
While the engineering challenges are significant, they are not impossible and much of the research in the ion-trap community is focused on these issues. One significant adaptation that we made in this architecture is the elimination of a significant amount of laser control. In more traditional ion-trap quantum computers, every operation on ion qubits (except for shuttling) is mediated by precisely focused laser beams. For a system containing millions of qubits, the amount of laser control would be significant and potentially very costly to the design of a large-scale machine.
In 2016, the ion-trap group at Sussex University (who lead the work on this paper) demonstrated a new technique to control and manipulate ion qubits. Instead of using tightly focused laser beams, the group use a microwave pulse that was broadcast over the entire ion-trap. The ions that they wanted to react to this microwave pulse were "tuned in" via precise control of the magnetic field environment around a particular ion. In this way you could use one microwave pulse to enact operations on large numbers of qubits simultaneously by tuning in the relevant qubits by changing local magnetic fields. This eliminates the need to have selective laser control for every ion qubit in the machine. Controlling the local magnetic field to each X-junction is performed with wires embedded underneath the surface of the ion-trap. By controlling electrical current through these wires, we can alter the magnetic field near a particular ion and "tune them in" to global microwave control pulses applied over the entire computer.
We believe that this model of an ion-trap quantum computer may be significantly easier to engineer and ultimately build than other designs. Many of the components of this monolithic design have already been demonstrated experimentally and much of the challenge left is to put all these pieces together and to slowly scale the system to first 10's of qubits then to 100's, 1000's and hopefully millions in the not too distant future.
The future of ion-trap quantum computing looks very bright and this technology is a direct competitor to superconducting quantum computing designs pioneered by places like IBM and Google. Both technologies maturing at the same time gives us tremendous flexibility in how we adapt quantum computing technology to specific commercial tasks in this new and exciting technology sector.
- Simon Devitt, co-founder of h-bar quantum consultants.