While this all sounds great in theory, building a useful quantum computer turns out to be pretty tricky. There are two challenges in particular that will need to be overcome before qubits are actually applied on our desktops or in our mobile phones.
When performing our operations, we introduce well-characterized perturbations to the qubits, which lead to specific changes in the composition of the superposition state. Coherence is a term that refers to the pristine relationships between the component states. In addition to the well-defined operations that we apply, other perturbations can creep in, at random, from the outside. Coherence can be degraded (or destroyed) by such processes, and this decoherence can lead to errors in the computations. As an extreme case, an unintentional measurement provoked by some outside factor will collapse the superposition state entirely. Decoherence can also be thought of as destructive interference acting between the wave functions of the component states.
Decoherence arises from the fact that our spins can never be perfectly isolated from the outside world. For this reason, a real-world quantum computer will have to use some clever technique to isolate the qubits from their environment. This often means cooling the qubits down to very low temperatures to avoid the random (coherence-killing) effect of heat.
The second major challenge facing real-world quantum computers is the ability to make larger and larger systems that utilize more and more qubits. The problem with such scaling lies in the difficulty of increasing the number of qubits in a carefully controlled manner and without losing coherence. For example, a two-qubit system could be made from just two electrons bound within a small molecule. Slightly larger molecules would be needed to make quantum computers with a handful of qubits.
DEFINITION
Coherence describes the well-defined relationships between the component quantum states that make up a superposition state.
Decoherence refers to the unintentional loss of coherence through the influence of random, external factors.
Scaling is the process of building larger quantum computers based on more and more qubits.
To make a quantum computer with dozens or even hundreds of qubits, however, you would need larger and larger molecules. And, as the size of the molecules increases, so do the chances that uncontrolled effects (say, electromagnetic attraction between all the electrons and protons in the system) will lead to decoherence. Fortunately, over the past two decades, experimental physicists have come up with a number of clever ways to combine more and more qubits and keep them isolated from decoherening factors long enough to perform some basic computations.
The first demonstration of a quantum computer, in 1998, applied the principle of nuclear magnetic resonance. This demonstrated a two-qubit system based on two nuclear spins within small organic molecules. A uniform magnetic field was applied to establish the distinct spin states, and then rapidly oscillating magnetic fields were applied to initialize the qubits, perform operations, and then read out the answer. Larger qubit systems have since been demonstrated using nuclear magnetic resonance in molecules, however it is unlikely this technique could be used for systems with more than a dozen or so qubits due to decoherence effects.
QUANTUM LEAP
Since the best use of quantum computers is made from using special quantum algorithms, they hold promise for solving specific types of problems. A good example is the factoring of large numbers (nice big ones of, say, a few hundred digits). This is especially true for odd numbers that are formed by the product of two prime numbers. Other useful tasks for quantum computers include complex database searchers, random number generation, and even the simulation of molecules or other quantum systems.
A second notable example involved the use of traps for either positive or negative ions. Positive ions, for example, are simply atoms that have acquired a positive charge due to the loss of some of their electrons. Ion traps are contraptions that can isolate ions in space through the clever use of electromagnetic fields. In a quantum computer based on this system, probe lasers are applied to initialize the ions (the qubits), perform operations, and read out the final answer.
In 2000, this technique was used to demonstrate a four-qubit system based on beryllium ions. Due in part to this advance, American physicist David Wineland and his French counterpart Serge Haroche were awarded the 2012 Nobel Prize in physics. This technique is promising since an additional set of lasers can be applied to cool the ions to very low temperatures, and therefore mitigate decoherence. Scaling up such an ion-trap quantum computer, however, will be a challenge since ions repel each other and it’s difficult to keep lots of them trapped.
QUANTUM QUOTE
[I]t seems that the laws of physics present no barrier to reducing the size of computers until bits are the size of atoms, and quantum behavior holds sway.
—Richard Feynman
A third potential type of quantum computer, also demonstrated in 2000, used physical systems that resemble traditional computer chips. Using standard semiconductor technology, small silicon systems called “quantum dots” can be created that effectively corral single electrons. The electrons in neighboring quantum dots interact weakly with one another, allowing for the creation of superposition states. In this case, scalability is thought to be achievable given the talents of the semiconductor industry. The key to making a real-world quantum dot computer will therefore be the taming of decoherence. This will likely impose the inconvenient requirement that the qubits be cooled to temperatures far below everyday values.
Although there are both fundamental and technical challenges, these early demonstrations give physicists hope that quantum computation could one day become viable. There seem to be physical systems out there that could be well isolated, easily manipulated, and scaled up from the single qubit stage to produce large-scale systems. So, although the idea of quantum computing may sound exotic and remote, scientists are optimistic about breakthroughs in the not-too-distant future.
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