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Although quantum computing is still in its infancy, enough progress is being made for it to look a little more promising than other “revolutionary” technologies, like fusion power or flying cars. IBM, Intel, and Google all either operate or are producing double-digit qubit computers right now, and there are plans for even larger quantum computers in the future. With this amount of inertia, our quantum computing revolution seems almost certain.
There’s still a lot of work to be done, though, before all of our encryption is rendered moot by these new devices. Since nothing is easy (or intuitive) at the quantum level, progress has been considerably slower than it was during the transistor revolution of the previous century. These computers work because of two phenomena: superposition and entanglement. A quantum bit, or qubit, works because unlike a transistor it can exist in multiple states at once, rather than just “zero” or “one”. These states are difficult to determine because in general a qubit is built using a single atom. Adding to the complexity, quantum computers must utilize quantum entanglement too, whereby a pair of particles are linked. This is the only way for any hardware to “observe” the state of the computer without affecting any qubits themselves. In fact, the observations often don’t yet have the highest accuracy themselves.
There are some other challenges with the hardware as well. All quantum computers that exist today must be cooled to a temperature very close to absolute zero in order to take advantage of superconductivity. Whether this is because of a reduction in thermal noise, as is the case with universal quantum computers based on ion traps or other technology, or because it is possible to take advantage of other interesting characteristics of superconductivity like the D-Wave computers do, all of them must be cooled to a critical temperature. A further challenge is that even at these low temperatures, the qubits still interact with each other and their read/write devices in unpredictable ways that get more unpredictable as the number of qubits scales up.
So, once the physics and the refrigeration are sorted out, let’s take a look at how a few of the quantum computing technologies actually manipulate these quantum curiosities to come up with working, programmable computers.
Wire Loops and Josephson Junctions
Arguably the most successful commercial application of a quantum computer so far has been from D-Wave. While these computers don’t have “fully-programmable” qubits they are still more effective at solving certain kinds of optimization problems than traditional computers. Since they don’t have the same functionality as a “universal” quantum computer, it has been easier for the company to get more qubits on a working computer.
The underlying principle behind the D-Wave computer is a process known as quantum annealing. Basically, the qubits are set to a certain energy state and are then let loose to return to their lowest possible energy state. This can be imagined as a sort of quantum Traveling Salesman problem, and indeed that is exactly how the quantum computer can solve optimization problems. D-Wave hardware works by using superconducting wire loops, each with a weakly-insulating Josephson junction, to store data via small magnetic fields. With this configuration, the qubit achieves superposition because the electrons in the wire loop can flow both directions simultaneously, where the current flow creates the magnetic field. Since the current flow is a superposition of both directions, the magnetic field it produces is also a superposition of “up” and “down”. There is a tunable coupling element at each qubit’s location on the chip which is what the magnetic fields interact with and is used to physically program the processor and control how the qubits interact with each other.
Because the D-Wave computer isn’t considered a universal quantum computer, the processing power per qubit is not equivalent to that which would be found in a universal quantum computer. Current D-Wave computers have 2048 qubits, which if it were truly universal would have mind-numbing implications. Additionally, it’s still not fully understood if the D-Wave computer exhibits true quantum speedup but presumably companies such as Lockheed Martin wouldn’t have purchased them (repeatedly) if there wasn’t utility.
There are ways to build universal quantum computers, though. Essentially all that is needed is something that exhibits quantum effects and that can be manipulated by an external force. For example, one idea that has been floated include using impurities found in diamonds. For now, though, there are two major ways that we will focus on that scientists have built successful quantum computers on: ion traps and semiconductors.
In an ion trap, a qubit is created by ionizing an atom of some sort. This can be done in many ways, but this method using calcium ions implemented by the University of Oxford involves heating up a sample, shooting electrons at it, and trapping some of the charged ions for use in the computer. From there, the ion can be cooled to the required temperature using a laser. The laser’s wavelength is specifically chosen to resonate with the ion in such a way that the ion slows down to the point that its thermal fluctuations no longer impact its magnetic properties. The laser is also used to impart a specific magnetic field to the ion which is how the qubit is “programmed”. Once the operation is complete, the laser is again used to probe the ion and determine its state.
The problem of scalability immediately rears its head in this example, though. In order to have a large number of qubits, a large number of ions need to be trapped and simultaneously manipulated by a series of lasers. The fact that the qubits can influence each other adds to the problem, although this property can also be exploited to help read information out of the system. For reasons of complexity, it seems that the future of the universal quantum computer may be found in something we are all familiar with: silicon.
Silicon, in its natural state, is actually an effective insulator. Silicon has four valence electrons which are all perfectly content to stay confined to a single nucleus which means there is no flow of charge, and therefore no current flow. To make something useful out of silicon like a diode or transistor which can conduct electricity in specific ways, silicon manufacturers infuse impurities in the silicon, usually boron or phosphorous atoms. This process of introducing impurities is called “doping” and imbues the silicon with an excess or deficit of electrons in the outer shells, which means that now there are charges present in the silicon lattice. These charges can be manipulated for all of the wonderful effects that we use to create our modern world.
But we can take this process of doping one step further. Rather than introducing a lot of impurities in the silicon, scientists have found a way to put a single impurity, a solitary phosphorus atom including its outermost electron, in a device that resembles a field-effect transistor. Using the familiar and well-understood behavior of these transistors, the single impurity becomes the qubit.
In this system, a large external magnetic field is applied in order to ensure that the electron is in a particular spin state. This is how the qubit is set. From there, the transistor can be used to read the state of this single electron. If the electron is in the “up” position, it will have enough energy to move out of the transistor and the device can register the remaining positive charge of the atom. If it is in the “down” position it will still be inside the transistor and the device will see a negative charge from the electron.
These (and some other) methods have allowed researchers to achieve long cohesion times within the qubit — essentially the amount of time that the qubit is in a relevant state before it decays and is no longer useful. In ion traps, this time is on the order of nano- or microseconds. In this semiconductor type, the time is on the order of seconds which is an eternity in the world of quantum computing. If this progress keeps up, quantum computers may actually be commonplace within the next decade. And we’ll just have to figure out how to use them.