Quantum computing is one of the first great technologies of the 21st century, and it will change everything someday.

Welcome to the Quantum Edge newsletter. Here you will learn more than just: “quantum computing works because of superposition and entanglement.” The Quantum Edge newsletter will tell you what goes with superposition and entanglement and what those terms actually mean.

Read about the physics, chemistry, and all sciences that create the foundation for quantum computing. Join me in my quest to translate the mysteries of the quantum world to the language of the dinner table and the coffee shop.

In today’s newsletter: The basics of quantum computing

We’re going to jump to a time when we have an early adopter hobbyist desktop quantum computer. If this were the digital computer age, we would be sitting in 1975 with something like the MITS Altair 8800. But this is the quantum computing era and it’s the year 2035. We’ll call our hypothetical desktop quantum computer the Strange, model One. The name is a double entendre homage to the strange way quantum computers work and the strange quark, one of our 17 primary elemental particles.

Our quantum computer, the Strange One, has at its heart a 10,000 transmon qubit quantum processing unit (QPU). It’s really more of a co-processor than a standalone computer. To program, manage, and operate it, I have a powerful conventional laptop connected to it via a highspeed USL optical conduit. The Strange One sits to the side and occupies a box about the size of a large personal printer. The box has a logo, a power cord, the USL port, and a small display. Other than that and cooling vents it is unadorned black.

The Strange One’s QPU relies on high-temperature superconductors so, while it doesn’t need liquid helium, it does contain a number of high-delta semiconductor cooling units. These take up the lion’s share of the box volume. The QPU operates at -120° C (about 153° K). A hot startup (from no power) requires about four hours of cooldown and thermal soak. Standby mode keeps everything at -40 and only requires 45 minutes.

Once at operating temperature, the unit is ready to be configured. As it sits, all 10,000 qubits are non-initialized (not in superposition). All coding and configuration is done in the laptop. I don’t need the Strange One to be powered up while creating the quantum configuration, so I keep it in standby to save power.

The first quantum computer lesson starts with quantum circuit software on the laptop. This example only uses a few of the available 10,000 qubits. The remainder will sit idle. The example configuration will initialize just four qubits (q0, q1, q2, and q3) that will end up as two entangled pairs.

The Hadamard gate (H-gate) is the fundamental quantum gate that puts a qubit into superposition. Figure 1 shows a QPU configuration timeline. The operations go from left to right. They can go backwards too, but during setup here, it’s following a standard left to right progression.

Each horizontal black line represents one qubit in the QPU. We don’t have control over which of the 10,000 qubits are used here. The supervising computer (the laptop) figures out the most efficient location.

Four qubits are selected and two have a Hadamard gate (H) applied to them. That puts q0 and q2 into superposition as step one of the process.

In step two, a CNOT (controlled NOT) gate is applied to q0 and q1 and, simultaneously to q2 and q3. The result is two entangled pairs of qubits in superposition: pair A and pair B.

The formal name for a maximumly entangled pair of qubits in superposition is a Bell pair, named after physicist John Stewart Bell. You might also see it referred to as an EPR pair. Entanglement had been somewhat predicted before Bell figured out what was going on. However, it didn’t make sense to the pre-quantum physicists. The label EPR (Einstein, Podolsky, Rosen) comes from an early thought experiment by physicists Albert Einstein, Boris Podolsky, and Nathan Rosen. Bell state is the most common of the two labels but both monikers refer to an entangled pair of qubits in superposition.

Newsletter issue 24 discussed the hardware construction of a transmon qubit. I’ve included figure 2, above, from that issue. The hypothetical Strange One quantum computer QPU has 10,000 qubits so it’s a lot denser than this one, and the control circuitry is considerably more sophisticated, but it’s fundamentally the same as this.

The four plus-sign like objects within the area B callout in figure 2 are transmon circuits. The quantum circuit software on the laptop will give instructions to control circuitry inside Strange One to initialize these four transmons into operating qubits. After that initialization, they can be called q0, q1, q2, and q3 to match the qubits labeled the same in figure 1.

Transmon qubits are tiny RC (resistor/capacitor) circuits. An RC circuit takes electrical energy and, under the right conditions, oscillates between charged and discharged states. The act of oscillating is commonly called resonating. The resistor and capacitor values determine how fast the circuit charges and discharges, and thus the tuned frequency. A typical quantum computer transmon is tuned such that the charge/discharge frequency is in the neighborhood of four to six gigahertz (GHz).

As we learned earlier in this newsletter series, changing electric energy levels (charging and discharging) causes an electromagnetic field. This field, when it is as small as it is in a transmon structure, can exhibit the quantum properties of superposition and entanglement.

The transmons in our QPU have control lines coming into them (figure 2, C). The supervising computer instructs the control electronics within the Strange One quantum computer to send very short microwave pulses to the control lines for q0 and q2.

The transmons are tuned to resonate at a frequency of 5.1 GHz. That means the microwave pulses sent down the control lines are 5.1 GHz signals. Sometimes the frequency will be varied slightly depending on what must be done to the qubit.

The pulse area, which is defined as the duration (or length of time the pulse lasts) in combination with the amplitude (what the voltage/intensity is) will determine what the pulse does to the qubit. One level of pulse area might shift the field in the transmon qubit such that the value of the qubit shifts from |0⟩ all the way to |1⟩. A different pulse area might shift it halfway. The shape of the microwave pulse and the phase also have an impact on how the pulse affects the transmon wave that makes up the qubit.

Variations in amplitude, frequency, and phase have a different impact on the qubit. The packets of microwave energy will typically be sent as a small pulse as illustrated below in figure 4.

Along with frequency, phase and amplitude, the shape of the pulse can be varied to have different effects on the qubit. Determining the specific values of the amplitude, frequency, length, and shape comes down to some complex math that I won’t be delving into here. It’s only important to know that these types of variations illustrated in figure 3 and 4 can be utilized to do different things with qubits.

The Hadamard gate is the pulse sent to the qubit to put it into superposition. It has a specific set of attributes that bump the qubit into superposition.

I wrote about conventional computer gates in newsletter issue 25. Figure 1 in that issue showed the schematic diagram for a conventional NOT gate. Conventional computer gates are pieces of hardware consisting mostly of transistors and resistors wired up together to form a physical circuit. A conventional gate is a wired up circuit that you send signals through. The gate differs in function based on the components and the wiring within the gate.

In a quantum computer, a gate is the action taken. A single microwave pulse, or one or a few pulses sent in succession, to a qubit is the gate. The attributes of the pulse(s), as discussed above, determine what will happen and are thus the main factor in what the gate does. The gate differs in function based on the pulse parameters.

In the next issue, I’ll go into more detail about how the different attributes of the controlling microwave pulse act to control the qubit. And I’ll cover the CNOT gate too - how a CNOT entangles qubits and how it can be used to move entangled qubits around on the QPU.

In the meantime put your Strange One into standby mode to conserve electricity. You can use the laptop for other things while you wait for the next newsletter issue.

That’s all for today.

New to the Quantum Edge newsletter?

Thinking about re-reading it but want a more transportable format?

I’ve wrapped the first ten issues of The Quantum Edge newsletter into book form. The collection, called “The Quantum Computing Anthology, Volume 1”, is now available in Kindle and paperback on Amazon. The book collects newsletter issues 1 through 10 and has some additional material and edits for continuity and clarity.

You can order the Volume 1 Kindle or paperback editions on Amazon today: The Quantum Computing Anthology, Volume 1

Check your email box Thursday - probably. (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.