The one thing everyone knows about quantum mechanics is its
legendary weirdness, in which the basic tenets of the world it describes seem
alien to the world we live in. Superposition, where things can be in two states
simultaneously, a switch both on and off, a cat both dead and alive. Or
entanglement, what Einstein called “spooky action at distance” in which objects
are invisibly linked, even when separated by huge distances.
But weird or not, quantum theory is approaching a
century old and has found many applications in daily life. As John von
Neumann once said: “You don’t understand quantum mechanics, you just get used
to it.” Much of electronics is based on quantum physics, and the application of
quantum theory to computing could open up huge possibilities for the complex
calculations and data processing we see today.
Imagine a computer processor able to harness super-position,
to calculate the result of an arbitrarily large number of permutations of a
complex problem simultaneously. Imagine how entanglement could be used to allow
systems on different sides of the world to be linked and their efforts
combined, despite their physical separation. Quantum computing has immense
potential, making light work of some of the most difficult tasks, such as
simulating the body’s response to drugs, predicting weather patterns, or
analyzing big datasets.
Such processing possibilities are needed. The first
transistors could only just be held in the hand, while today they measure just
14 nm – 500 times smaller than a red blood cell. This relentless shrinking,
predicted by Intel founder Gordon Moore as Moore’s law, has held true for
50 years, but cannot hold indefinitely. Silicon can only be shrunk so far, and
if we are to continue benefiting from the performance gains we have become used
to, we need a different approach.
Quantum fabrication
Advances in semiconductor fabrication have made it possible
to mass-produce quantum-scale semiconductors – electronic circuits that exhibit
quantum effects such as super-position and entanglement.
The image, captured at the atomic scale, shows a
cross-section through one potential candidate for the building blocks of a
quantum computer, a semiconductor Nano-ring. Electrons trapped in these rings
exhibit the strange properties of quantum mechanics, and semiconductor
fabrication processes are poised to integrate these elements required to build
a quantum computer. While we may be able to construct a quantum computer using
structures like these, there are still major challenges involved.
In a classical computer processor a huge number of
transistors interact conditionally and predictably with one another. But
quantum behavior is highly fragile; for example, under quantum physics even
measuring the state of the system such as checking whether the switch is on or
off, actually changes what is being observed. Conducting an orchestra of
quantum systems to produce useful output that couldn’t easily by handled by a
classical computer is extremely difficult.
The basic element of quantum computing is known as a qubit,
the quantum equivalent to the bits used in traditional computers. To date,
scientists have harnessed quantum systems to represent qubits in many different
ways, ranging from defects in diamonds, to semiconductor nano-structures or
tiny superconducting circuits. Each of these has its own advantages and
disadvantages, but none yet has met all the requirements for a quantum
computer, known as the DiVincenzo Criteria. The most impressive progress has
come from D-Wave Systems, a firm that has managed to pack hundreds of qubits on
to a small chip similar in appearance to a traditional processor.
Quantum secrets
The benefits of harnessing quantum technologies aren’t
limited to computing, however. Whether or not quantum computing will extend or
augment digital computing, the same quantum effects can be harnessed for other
means. The most mature example is quantum communications.
Quantum physics has been proposed as a means to prevent forgery
of valuable objects, such as a banknote or diamond, as illustrated in the image
below. Here, the unusual negative rules embedded within quantum physics prove
useful; perfect copies of unknown states cannot be made and measurements change
the systems they are measuring. These two limitations are combined in this
quantum anti-counterfeiting scheme, making it impossible to copy the identity
of the object they are stored in.
The concept of quantum money is, unfortunately,
highly impractical, but the same idea has been successfully extended to
communications. The idea is straightforward: the act of measuring quantum
super-position states alters what you try to measure, so it’s possible to
detect the presence of an eavesdropper making such measurements. With the
correct protocol, such as BB84, it is possible to communicate privately,
with that privacy guaranteed by fundamental laws of physics.
Quantum communication systems are commercially available
today from firms such as Toshiba and ID Qauntique. While the
implementation is clunky and expensive now it will become more streamlined and
miniaturized, just as transistors have miniaturized over the last 60 years.
Improvements to Nano scale fabrication techniques will greatly
accelerate the development of quantum-based technologies. And while useful
quantum computing still appears to be some way off, its future is very exciting
indeed.
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