Qubit Primer

Some useful notes on Quantum Computing

From Russ Fein,  quantumtech.com

Quantum Computing Modalities – A Qubit Primer Revisited  ... 

In December 2021, in an early iteration of this Blog, I Russ Fein described the various qubit modalities in use by some of the Quantum Computing (QC) hardware players.  A lot has happened since that post, so I thought it would be constructive to revisit the topic.

When that earlier post was published (click here if interested in reviewing), it described 10 leading quantum hardware companies focusing on four core qubit types (superconducting, trapped ions, photonics and quantum dots).  Today there are dozens of quantum hardware companies, a few additional common modalities (notably neutral atoms) and significant advances made across the spectrum.

Qubit Dynamics

While many articles describing and comparing QCs focus on the number of qubits, this core number belies the complexity in comparing actual QC performance due to additional limitations described below.  Qubit count is the equivalent of only using horsepower to describe a car.  While horsepower is an important metric, most car buyers are equally if not more focused on comfort, handling, fuel economy, styling, etc.  Some effort has been made to “consolidate” these variables for QC into a single performance metric (such as Quantum Volume, CLOPS (circuit layer operations per second) or QED-C’s Benchmarks), although no single measurement has yet been adopted by the broad QC ecosystem.  For the casual reader, I’d caution you to not focus too much on the number of qubits a given QC has.  While “more is better” is generally a useful mantra, as you’ll see below, it is not that simple.

As you may know or recall, placing qubits in a superposition (both “0” and “1” at the same time) and entangling multiple qubits where one is dependent on the status of the other (entanglement) are two fundamental quantum properties which help empower Quantum Computers and allow them to perform certain calculations that can’t easily be executed on traditional computers.  Before we review the various types of qubits (i.e., quantum hardware platforms), it may be helpful to summarize some of the limitations faced when placing qubits in superposition and/or entangling multiple qubits, and discuss the key metrics used to measure these properties. ...