I can explain digital logic down to the electron in a MOSFET, but I can't come close to the same with quantum computing. This newsletter is a journal of my quest to learn the fundamentals of quantum computing and explain them on a human level.

Welcome to the Quantum Edge newsletter. Join me in my year-long journey into the weirdness that is quantum computing.

Issue 9.0, May 3, 2025

In today’s newsletter: Looking at bits and qubits.

Benson’s law of useable-for-computing states that in order for something to be useable for calculation or computing, it must 1) have different states, 2) those states must be stable, 3) the states must be readable, and 4) there must be a way to change the state. For conventional computers, memory cells (called bits) have these four properties and thus comply with my law.

A Bit of Information, or What Do I Mean by “State”?

In a conventional computer, the smallest piece of information is called a bit. The value of a bit is either one or zero. Those are the two states: one and zero. Some people call the states “on” and “off” instead of one and zero, and that’s fine. On and off work well as labels because you can point to a light switch and by flipping the switch, the light ends up either in the state of being on or the state of being off. On is one (one light) and off is zero (zero light).

A system with the ability to be either on or off is a binary system. A computer bit has two possible states (on/off, AKA one/zero) so we say it is a binary system. The word binary has roots in Latin that mean “two together” and old English meaning duality, or a pair. For the purposes of computer information, binary just means that a bit can have two possible states: one and zero which is synonymous with on and off. You can say either and be correct.

A bit is held in a single memory cell, which, in a conventional computer, is made of a transistor and a capacitor. If a voltage charge is applied, the bit will hold that charge, which can be measured. The capacitor holds the charge (kind of like a small battery) and the transistor is the device that gives the computer access to the state of charge in the capacitor.

The charge will be some voltage level greater than zero - usually anything between 0.5 and 1.5 volts. If the charge is zero or near zero (below 0.5 volts), the bit is said to have a value of zero. If is has a charge, it is said to be a value of one. Logic circuits connected to that bit can set it to zero, set it to one, make sure it’s stable, or read it. It meets the four criteria I gave in the first paragraph.

Figure 1 shows the circuit diagram for a pair of memory cell bits. The capacitor in the cell on the left is charged with electrons (blue dots) and has a voltage level giving it a value of 1. The cell on the right is not charged and therefore has a value of 0. Put a billion of these bits together and you have a gigabit of computer memory.

Figure 2 shows a grouping of eight memory cells, holding eight bits. Six of the cells have no charge and two of them are charged. To write that out in binary, you would put down: 00001010. For those not familiar with the insides of a computer, this is how numbers are stored and worked on in a computer. In computer terms, we call 8 bits a byte.

To store different numbers, you just need to be able to store different patterns of 1 and 0. Two bits, as we have in figure one, can make up four different patterns of 1 and 0.

That’s 00, 01, 10, and 11. That’s four different patterns made out of two binary bits. If you have three bits, you can make eight patterns and count from zero to seven: 000 = 0, 001 = 1, 010 = 2, 011 = 3, 100 = 4, 101 = 5, 110 = 6, and 111 = 7.

Pulling out a little math, you can start to make larger numbers. There are two bits in the table above, with two possible values each, so the number of patterns is 2 × 2 = 4. You can also write 2 x 2 as 2 to the power of 2, or 2² .

If you put three bits in your group of memory cells, each of the three bits, again, can be either 1 or 0. This gives you 2 × 2 × 2 = 8. This is 2 to the power of 3, or 2³ . The eight-bit byte in figure 2, is 2 to the power of 8, or 2⁸ , which spelled out is 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2. That equals 256 possible different patterns of 1 and 0.

Two bytes (or 16 bits) can have 2¹⁶ , or 65,535 combinations. Four bytes (32 bits) is 2³² , or 4,294,967,296. You can store some pretty big numbers in a conventional computer with just a few bits of information. Most modern computers do their calculations with a 64-bit “word.” That’s eight eight-bit bytes for a total of 2⁶⁴ different possible patterns of 0 and 1.

Moving from a Conventional Computer Bit to a Quantum Computer Qubit

So far in this issue, we have discussed the smallest part of a conventional computer (the bit) and how it can be grouped into bytes and words used to represent numbers. The smallest part of a quantum computer is very different than a conventional computer. Quantum computers start with the qubit.

A Qubit? What’s a Qubit?

A qubit is the smallest part of a quantum computer. Though a qubit is very different than a bit, the qubit and bit take a roughly equivalent role in their respective computer types.

A qubit is made out of a single subatomic particle. Subatomic particles are things smaller than atoms, as we have discussed in early newsletter issues. The most common particle used to create a qubit is the electron. Back in school, we were taught that electrons hold a negative charge and that they orbit around the nucleus of an atom. What we weren’t taught (or at least I wasn’t taught, or maybe I dozed off during the lesson) was that electrons are more complex than just being a negatively charged particle.

In addition to charge, electrons have a property called spin (see issue 5). They can be in a state of spin up or spin down. The spin up and spin down property is what allows an electron to store information. An electron is always a negatively charged particle regardless of what the spin state is. The spin state can be changed and read by the use of magnetic fields and microwave energy.

Figure 3 shows a pair of electrons, one spin up and the other spin down. The lines extending outward represent the magnetic field that goes along with the spin state. The magnetic field allows the spin state to be changed and read.

Does it Qualify as a Computational Device Under Benson’s Law of Useable-for-Computing?

1. Electrons have two states (spin up and spin down)

2. Are the states stable? Sort of. That’s a subject for another time

3. An electron’s state can be read

4. An electron’s state can be changed

An electron, as a qubit, clearly meets three of my four criteria. The last criteria - stability - is one of the big hold ups in today’s quantum computing world. Qubits, with today’s equipment, are just not stable enough for broad use. Without stability, data changes over time and you can’t trust an answer. However, a great deal of time and money is being put toward finding ways to make qubits stable. Qubits already have enough stability for some experimental work. Hopefully, within the next few years, they will be stable enough for real work.

Stay tuned. Next week we introduce quantum superposition.

In Summary…

Check your email box Thursday. (Okay, some of these weekly issues have come out on Friday, or not at all. But, in a quantum world, how can you tell?)

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Positive Edge is the consulting arm of Duane Benson, Tech journalist, Futurist, Entrepreneur. Positive Edge is your conduit to decades of leading-edge technology development, management and communications expertise.