What is Quantum Computing?

Quantum computing
is the area of study focused on developing computer technology based on
the principles of quantum theory. The quantum computer, following the
laws of quantum physics, would gain enormous processing power through
the ability to be in multiple states, and to perform tasks using all
possible permutations simultaneously.

A Comparison of Classical and Quantum Computing

Classical computing relies, at its ultimate level, on principles expressed by
Boolean algebra. Data must be processed in an exclusive binary state at
any point in time or bits. While the time that each transistor or
capacitor need be either in 0 or 1 before switching states is now
measurable in billionths of a second, there is still a limit as to how
quickly these devices can be made to switch state. As we progress to
smaller and faster circuits, we begin to reach the physical limits of
materials and the threshold for classical laws of physics to apply.
Beyond this, the quantum world takes over. In a quantum computer, a
number of elemental particles such as electrons or photons can be used
with either their charge or polarization acting as a representation of 0 and/or 1. Each of these particles is known as a quantum bit, or qubit, the nature and behavior of these particles form the basis of quantum computing.

Quantum Superposition and Entanglement

The two most relevant aspects of quantum physics are the principles of superposition and entanglement.

Superposition:
Think of a qubit as an electron in a magnetic field. The electron’s
spin may be either in alignment with the field, which is known as a
spin-up state, or opposite to the field, which is known as a spin-down
state. According to quantum law, the particle enters a superposition of
states, in which it behaves as if it were in both states simultaneously.
Each qubit utilized could take a superposition of both 0 and 1.

Entanglement:
Particles that have interacted at some point retain a type of
connection and can be entangled with each other in pairs, in a process
known as correlation. Knowing the spin state of one
entangled particle — up or down — allows one to know that the spin of
its mate is in the opposite direction. Quantum entanglement allows
qubits that are separated by incredible distances to interact with each
other instantaneously (not limited to the speed of light). No matter how
great the distance between the correlated particles, they will remain
entangled as long as they are isolated. Taken together, quantum
superposition and entanglement create an enormously enhanced computing power.

Where a 2-bit register in an ordinary computer can store only one
of four binary configurations (00, 01, 10, or 11) at any given time, a
2-qubit register in a quantum computer can store all four numbers
simultaneously, because each qubit represents two values. If more qubits
are added, the increased capacity is expanded exponentially.

Difficulties with Quantum Computers

Interference
— During the computation phase of a quantum calculation, the slightest
disturbance in a quantum system (say a stray photon or wave of EM
radiation) causes the quantum computation to collapse, a process known
as de-coherence. A quantum computer must be totally isolated from all external interference during the computation phase.Error correction
— Given the nature of quantum computing, error correction is ultra
critical — even a single error in a calculation can cause the validity
of the entire computation to collapse.Output observance — Closely related to the above two, retrieving output data after a quantum calculation is complete risks corrupting the data.

The Future of Quantum Computing

The biggest and most important one is the ability to factorize a very large
number into two prime numbers. That’s really important because that’s
what almost all encryption of internet applications
use and can be de-encrypted. A quantum computer should be able to do
that relatively quickly. Calculating the positions of individual atoms
in very large molecules like polymers and in viruses. The way that the
particles interact with each other — if you have a quantum computer you
could use it to develop drugs and understand how molecules work a bit
better.

Even though there are many problems to overcome, the breakthroughs in the last 15 years, and especially in the last 3, have made some form of
practical quantum computing possible. However, the potential that this
technology offers is attracting tremendous interest from both the
government and the private sector. It is this potential that is rapidly
breaking down the barriers to this technology, but whether all barriers
can be broken, and when, is very much an open question.

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