Question inspired by this article from IEEE Spectrum on blocks containing different polarization filters for use in classrooms, and my previous question on representing the three-polarizing-filter experiment in quantum computing terms. Here I want to go the other way.

Do there exist any easily-purchasable educational quantum computing toys, such as those a physics teacher might use in a classroom? I am imagining here a set of polarizing filters or beam splitters with which you can (in conjunction with a laser) create very simple quantum circuits.

I am especially interested in ways to make a CNOT gate.

up vote 5 down vote accepted

Quantum computers are, unfortunately, quite hard to build. Experiments with polarizing filters or beam splitters would be able to demonstrate quantum effects, but I know of no way to make simple quantum circuits for multiple qubits unless you have single photon sources and detectors.

Alternatively, you could use current cloud-based devices. The IBM Q Experience has a simple GUI interface that would be suitable for students (after some introduction), and will then run the circuit on real hardware. If you students would be able to make circuits programmatically, they can use more quantum IBM hardware as well as hardware by Rigetti, with other companies also in the pipeline.

For a 'single qubit' experiment, you could perhaps just use polarizing filters. The $|0\rangle$ and $|1\rangle$ states of the qubit could be associated with horizontal and vertical polarization, and the $|+\rangle$ and $|-\rangle$ states could be associated with angles of $45^{\circ}$ and $135^{\circ}$. Then just by holding up a filter, you can turn sunlight into a stream of single qubits in a given state.

With a second filter, you can similarly measure in the $|0\rangle / |1\rangle$ basis (by holding it horizontally or vertically, and seeing if any light comes out) or the $|+\rangle / |-\rangle$ basis (by holding it diagonally). With multiple filters you could chain these measurements, and show the how the measurement bases are complementary. You could even remake the game I made to run on quantum computers: Battleships with complementary measurements.

This would be a single qubit example, despite the fact you have many qubits, because they are always the same state and they never interact. So you just have many samples of a single qubit process, that just happen to be shining down on you all at once.

Disclosure: I work for IBM, and Rigetti once gave me a t-shirt

  • So we can do simple unitary transformations on a single laser beam with waveplates and polarizing filters, but I imagine something like a CNOT between two laser beams is far out of the realm of possibility? – ahelwer Oct 9 at 16:47
  • The disclosure about the "t-shirt" made me chuckle :) – Blue Oct 9 at 20:54
  • @ahelwer A CNOT would require a controlled interaction between a well defined pair of photons. So it would be too complex. – James Wootton Oct 11 at 11:15

You can get the sort of optical bench that is typically used for classrooms.

For a couple examples:

3B Scientific

School Speciality

I think the one I have taught with before was from 3B, but I don't know about any of the others so research them yourself rather than taking a product recommendation from me. There are several options and this choice will be dependent on your quality/cost requirements.

Those will be for experiments about lenses and diffraction rather than polarization so you will have to get the polarizers separately. An example:

Edmund Optics

But you see how all the pieces are put on mounts along a track so you can easily slide them around. That is the sort of setup to look for so that lining everything up along the beamline is easier.

In teaching the three polarizer experiment, we would also have a detector to measure intensity, but you wouldn't need that if this is more like a toy rather than teaching fitting the data to something like $A \sin^2 (B \theta + C)$ with error analysis.

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