- atomic particles have discrete energy states (levels) - they can be nudged between states through interaction with other particles. For example a hydrogen atom might go from a high-energy state to a low energy state if struck by a photon ("particle" of light). The presence of two states provides the basic need of the binary number system on which conventional computers operate (in their case the states are "on" and "off" voltages).
- (This is the really strange one!) A particle that is isolated from the rest of the world is effectively in all possible states (called "superposition") - it is only when it interacts with the world (through another particle) that its state becomes "fixed". The act of measuring the state of a particle is an interaction that fixes its state - until the measurement is taken it could be in any one of the possible states.
- "Quantum-Mechanical Computers" by Seth Lloyd, Scientific American, October 1995.
- "Quantum Computing with Molecules" by Neil Gershenfeld and Isaac Chuang, Scientific American, June 1998.
- "The Feynman Processor: An introduction to quantum computing" by Gerard Milburn, Frontiers of Science series, Allen & Unwin, Sydney.
- Stanford-Berkeley-MIT-IBM Quantum Computing Project
- 29 April 1999: Nature article "Quantum beep: computing the future"
- 30 August 1999: Scientific American Quibit Chip
- 17 Nov 1999: String theory and quantum mechanics - another example of the orchestra analogy.
- 26 Nov 1999 Nature: Demonstrating the viability of universal quantum computation using teleportation and single-qubit operations by DANIEL GOTTESMAN AND ISAAC L. CHUANG
- Feynman links.
- 16 Mar 2000 Nature: Quantum information and computation by CHARLES H. BENNETT AND DAVID P. DIVINCENZO
- 10 Aug 01 ABC: Quantum computing a leap closer.
- 22 Sep 01 SciAm: Scientists Create Double Quantum Dot for Computing.
- 27 Nov 02 New Scientist: Quantum computing making 'tremendous progress.
- 29 Nov 02 SpaceDaily: Energy Needs May Limit Size, Ability Of Quantum Computers.
- 7 May 04!: Ultrafast Quantum Computing A Step Closer To A Superposition Dot.
- 8 Oct 04 SpaceDaily: Next Step To The Quantum Computer
- 23 Feb 05 Scientific American: What makes a quantum computer so different (and so much faster) than a conventional computer?
- 25 Feb 05 Science (regn): The Road to Quantum Computing.
- 23 Feb 06 Nature: Counterfactual quantum computation through quantum interrogation !
- 24 Mar 06 New Scientist (subs): Quantum computers: March of the qubits.
- 4 Jan 07 Science: Nondestructive Optical Measurements of a Single Electron Spin in a Quantum Dot

"These results represent progress toward the manipulation and coupling of single spins and photons for quantum information processing." - 25 Feb 08 Scientific American ($): The Limits of Quantum Computers
- 19 Sep 09 New Scientist: Quantum computers are coming – just don't ask when.
- 17 Oct 12 The Australian ($): Australian engineers write quantum computer 'qubit' in global breakthrough

- 17 Jul 13 New Scientist: Instant Expert 33: Quantum information.
- 24 Apr 14 SMH: Aussie quantum computing star Michelle Simmons elected member of prestigious society.
- 6 Oct 15 SMH: Australian researchers make quantum computing breakthrough, paving way for world-first chip.
- 28 Oct 19 New Scientist: It’s official: Google has achieved quantum supremacy.

by Michael Paine.

In October 1998 I described the DNA computer, which uses the techniques of molecular biology to manipulate strands of specially prepared DNA and solve difficult mathematical problems. "The DNA computer provides enormous parallelism... in one fiftieth of a teaspoon of solution approximately 10 to the power 14 DNA 'flight numbers' were simultaneously concatenated in about one second".

There is another line of research that also has the potential for enormous parallelism (possibly conducting billions of calculations simultaneously) and that is the Quantum-Mechanical Computer. The field of research is known as Quantum Computing. Whereas DNA computers operate with molecules quantum-mechanical computers operate with fundamental atomic particles such as atoms, electrons and photons.

Quantum mechanics describes the interactions between such
particles
and these interactions are very strange. One of the founders of quantum
mechanics, Neil Bohrs said "Anyone who can contemplate quantum
mechanics
without getting dizzy hasn't properly understood it.". Thankfully, for
the purpose of understanding the basic operation of a quantum computer,
we do not need to "understand" quantum mechanics - just two key points:

Conventional computers work with logic gates. For example, the AND gate takes two inputs and the output is 1 if both inputs are 1, otherwise the output is 0. Any arithmetic task can be achieved using a combination of these logic gates. It turns out that quantum particles can be manipulated in ways that achieve the same logic and this is how scientists are building the first quantum-mechanical computers. The task of making a useful device is going to be very difficult but some simple demonstration devices have already been made.

Why bother working with such tiny objects? It is the second property of quantum particles that is so alluring. Provided that we don't try to take a peek during the computations, the nature of a quantum-mechanical computer is such that it will calculate the results for all possible combinations of input - remember this is done in parallel using just one sequence of interactions between particles. Thus the uncertainty about the state of individual particles is the strength of the quantum-mechanical computer. Since the state of a particle represents a "bit" of information, the uncertainty about its state also applies to the unit of information, which is called a "qubit" in quantum computing. A qubit is a bit of information that is in a twilight world of (generally) two states.

We can think of qubits working their way through the quantum-mechanical computer and at the end of the calculation all possible "answers" are available. The last step is to convert qubits back to bits so they make sense - this might involve observing photons emitted by atoms to determine the state of these atoms. Note that if this was attempted part-way through the calculation the qubits would be destroyed and the calculation disrupted.

Seth Lloyd has likened the quantum-mechanical computer to a symphony orchestra. Energy states of atoms can be thought of as waves, much like the sound waves. An atom in a single state is like a single note (tone) from a single instrument. A superposition of states of one atom is like a musical chord. The quantum computation is like the sound from a whole orchestra. Taking this analogy further, if you wanted to know what note a particular instrument was playing you would need to isolate it in a sound-proof room in order to take an accurate measurement - in doing so you would ruin the symphony. In the same way measuring the state of a particle during a quantum computation will ruin the computation.

Several classes of "intractable" computing problems could be
addressed
by quantum computing. One is the task of finding the factors of a large
number - important for computer security because factors are used for
encryption
systems. In brief, a binary number consisting of 50 digits has 2^{50}
combinations (2 to the power of 50, or about
10,000,000,000,000,000,000,000,000
!) and a conventional computer would need to work through all of the
possible
combinations - the most powerful conventional computer available
(perhaps
physically possible!) would take over a million years to do this. In
theory,
a quantum computer could achieve the same result with just a few
hundred
qubits (hydrogen atoms!). A similar efficiency could apply to finding
an
item in a randomly sorted database. And how about debugging software -
every possible pathway tested in one go!