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Using the Full Qubit

Brian N. Siegelwax
5 min readMay 19, 2020

Newcomers to quantum computing learn that the states of individual qubits can be represented graphically with Bloch Spheres. And although these unit spheres have x, y, and z axes (called bases), we learn that we can only measure the z basis. Therefore, we can determine the probabilities of measuring a 0 or a 1, giving us the latitude if we think of the sphere as a globe, but we cannot determine the longitude, or the east-west position around the globe.

My workaround draws inspiration from superdense coding and quantum state tomography; my references are listed at the end of this article. In retrospect, the paper should have been sufficient, but I credit the book because credit is due.

Start with any pure state you desire.

If you’re new enough to wonder why I am starting with qubit 2 instead of qubit 0, it’s because the device matters. The specific hardware that I might run this circuit on has qubits arranged in a way in which I could actually start with qubit 0, but this circuit is part of a larger circuit that requires the additional connectivity afforded by qubit 2. You’ll see this measurement circuit again in a separate article after I complete the other part of the full circuit.

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Brian N. Siegelwax
Brian N. Siegelwax

Written by Brian N. Siegelwax

The least qualified person in quantum.

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