Quantum Computing

CS 301: Assembly Language Programming Lecture, Dr. Lawlor

Quantum computing has gotten a ton of press, due to its enormous theoretical promise.  Unfortunately, it's possible that like fusion power, which is theoretically superior to all other forms of power generation but currently intractable in practice, quantum computing is "the technology of the future, and always will be".

  Ordinary Computer Quantum Computer
Goal Get 'er done! Speedups up to exponential:
e.g., search n values in sqrt(n) time, factor integers in polynomial time.
Data storage 1's and 0's (bits) Vector with axes 1 and 0 (qubits)
Not just 1 or 0: both at once!
Assignments? Yes No (violates laws of physics)
Reversible? No Yes, except for "collapse" operation
Swap? Yes Yes
Logic gates AND, OR, NOT CNOT, Hadamard rotate 45 degrees
Instructions Reversible Quantum Operations,
and Irreversible Collapse
Clock Square wave Limited by coherence time
When? Now ???
Limits Heat & power, software
How many bits can you keep coherent?

Many things, like photons or electrons, display several very odd mechanical properties with mystical sounding quantum names.  The key point here is that everything is made of waves.

The wave equation for a particle is the Schrödinger equation i \hbar
        \frac{\partial}{\partial t}\Psi = \hat H \Psi.  

Restating this equation in small words, "a particle's wave changes with time depending on the particle's energy."

I prepared some photographic examples of wave properties visible in laser light.  Hopefully the in-class demos will work too!  Waves are very strange; see IBM's atomic force microscope images of electrons bouncing off atoms, compare to the Falstad ripple tank applet (Java) for an interactive version.  

A quantum computer is based on "qubits", which you can think of as a dot in 2D space: the X axis means a "0", the Y axis means a "1".  Normal bits must lie on one axis or another, but qubits can live between the axes.  For example, 

Since coordinates (0,0) means "not a 0, and not a 1", and we don't know what that means, we usually require the qubit to live on a unit sphere--it's either going to be a zero or going to be a one, so the probabilities (equal to the square of the amplitude) must add up to one. 

There are many ways to implement qubits, such as via photon polarization, where "0" means horizontally polarized, "1" means vertically polarized, and a superposition is unpolarized.  You could also implement the qubits as possible positions of an electron, where "0" means in the left box, "1" in the right box, and superposition means it's sitting in both boxes at once.

Tons of research groups have built single-bit quantum computers, but the real trick is entangling lots of bits together without premature "collapse", and that seems to be a lot harder to actually pull off, typically requiring the system to be electrically isolated (for example, inside a superconductor shell), mechanically isolated from vibrations, and kept near absolute zero.

Quantum Programming

Just like a classical computer uses logic gates to manipulate bits, the basic instructions in a quantum computer will use quantum logic gates to manipulate individual wavefunctions.  Because you don't want to destroy any aspect of the data (this causes decoherence), these are linear operators, so you can represent any coherent quantum logic gate with a matrix that manipulates the qubit values. 

1 qubit
2-entry state vector
2 qubits
4-entry state vector
3 qubits
8-entry state vector
n qubits
2n-entry state vector

One entry in a state vector is a complex number, combining amplitude, which when squared gives a probability, and a phase.

Again, think of a qubit like a little vector.  One qubit is represented as a 2-entry vector representing a linear combination of the states zero |0> and one |1>.  These are not exclusive, so a probability of (0.5,0.5) is a perfectly plausible qubit.

probability that qubit is 0:
magnitude of |0>
probability that qubit is 1:
magnitude of |1>

For example, a "Pauli-X" gate is represented by this 2x2 rotation matrix:

0 1
1 0

Plugging in the input and output coefficients, we have:

  a=0 a=1
output a=0 0 1
output a=1 1 0

The probability for a=1 on the input becomes the probability for a=0 on the output, and vice versa--this is just a NOT gate!

CNOT gate

A controlled NOT takes two bits as input.  Two qubits makes a 22=4 float vector with these components:

a=0 && b=0 |00>
a=0 && b=1 |01>
a=1 && b=0 |10>
a=1 && b=1 |11>

The CNOT gate's matrix is basically a 2x2 identity, and a 2x2 NOT gate, stuck together:

1 0 0 0
0 1 0 0
0 0 0 1
0 0 1 0

Again, putting in the input and output vectors, we can see what's going on:

  a=0 a=1
  b=0 b=1 b=0 b=1
a=0 && b=0 1 0 0 0
a=0 && b=1 0 1 0 0
a=1 && b=0 0 0 0 1
a=1 && b=1 0 0 1 0

If a=0, nothing happens--b's probabilities are exactly like before.  If a=1, then b gets inverted, just like a NOT gate.  If a is in a superposition of states, b is in a superposition of being inverted and not being inverted.

The basic programming model with a quantum computer is:

  1. Initialize your quantum registers with a superposition of 0 and 1: these hence contain every possible answer.
  2. Run a series of instructions to selectively amplify the answer you're looking for, or attenuate the answers you're not looking for.  For example, you can arrange "wrong answers" so they cancel each other out.  Each instruction must be a reversible operation, but in theory can be arbitrarily complex and there's no theoretical limit on the number of instructions.   However, in practice, the machine only works if you can keep the whole register entangled in a coherent superposition: accidental collapse or "decoherence" is currently the limiting factor in building big quantum computers.
  3. Finally, look at the register.  The act of looking will "collapse" to a particular set of 1's and 0's, hopefully representing the right answer.  If there are several right answers, physics will pick one randomly.

People started to get really interested in Quantum Computers when in 1994 Peter Shor showed a quantum computer could factor large numbers in polynomial time.  The stupid algorithm for factoring is exponential (just try all the factors!), and though there are smarter subexponential algorithms known, there aren't any non-quantum polynomial time algorithms known (yet).  RSA encryption, which your web browser uses to exchange keys in "https", relies on the difficulty of factoring large numbers, so cryptographers are very interested in quantum computers.  In 1996, Lov Grover showed an even weirder result, that a quantum search over n entries can be done in sqrt(n) time. 

Quantum Hardware

At the moment, nobody has built a popular, easy to use quantum computer, but there are lots of interesting experimental ones.  The biggest number a quantum computer has factored is now 56,153 (until 2012 it was 21=3*7, after piles of science; see readable poster description of process; or try the Quantum Computing Playground version).   But *if* the hardware can be scaled up, a quantum computer could solve problems that are intractable on classical computers.    Or perhaps there is some physical limitation on the scale of wavelike effects--the "wavefunction collapse"--and hence quantum computers will always be limited to a too-small number of bits or too-simple circuitry.  The big research area at the moment is "quantum error correction", a weird combination of physics-derived math, experimental work on optics benches, and theoretical computer science.  At the moment, nobody knows if this scaling of hardware and algorithms will combine to enable useful computations anytime soon.

There's a different "Adiabatic Quantum Computer" design used by British Columbia based quantum computer company D-Wave that they hope will scale to solve some large problems.  The adiabatic part of the name refers to the adiabatic theorem, proved in 1928, that says a system in quantum ground state, moved slowly (adiabatically) to a new state, is still in ground state.  In practice, this allows the quantum solution of reasonably large problems, up to 1024 bits on their current hardware, but only those that fit this model and the company's choice of wiring. This is not a general-purpose quantum computer, but it has shown speedups on real-world optimization problems.  They published some results in Nature (described here), and have some fairly big contracts from Amazon, Lockheed Martin, Google, etc.  

Basically every big tech company has some people working on quantum computing, from Microsoft to Google.

Thus, the future of quantum computers is currently in a superposition between two outcomes:

This superposition may collapse sometime in the next few years.  Or maybe not!