As I’m looking back at the few articles I’ve written, I was thinking to myself that I may have jumped the gun a bit in terms of the topics presented. I wanted to write a series of articles to get back to basics and present the more fundamental parts of quantum computing, such as the qubits that are being used in quantum computers today so we can get a more well-rounded perspective about the true nature of quantum computing (QC) at its current state.

As you may have glanced from our article, “differences between a quantum computer and a classical one” qubits are the fundamental building block for current QC. However, there are a variety of technologies that are currently being employed to actually make these so-called qubits. Some of the most prominent technologies used by the likes of Google and IBM include superconducting loop qubits, trapped ion qubits, and quantum dot qubits. Some potential and up and coming technologies include topological qubits, photonic qubits, and diamond vacancy qubits. Here we will go over more in depth on the prominent ones, as some of these technologies have achieved qubit counts of up to 50+, which is where most top-of-the-line companies working on QC are currently navigating at.

Superconducting Loops

These types of qubits are intriguing because they can rely on a lot of the existing semiconductor factories that are already built to create traditional central processing units. The technology utilizes transistors which effectively create josephson junctions for the superconducting task. A microwave signal excites the current in the circuit loop to a superposition state and can change the states of single qubit gates. According to IBM, two qubit gates use tuned microwave pulses called cross resonance gates to be able to achieve a similar qubit state change.

Some of the challenges with scaling up this technology include 1) the difficulty in arranging the qubits in a way that allows for precise microwave signals to be delivered and 2) crosstalk interactions of the qubits that are present like unintended microwave transmissions and noise coming from buses coupling two qubit gates 3) general defects with the qubit hardware itself due to the inherent manufacturing techniques.

Trapped Ions

These qubits use electrically charged atoms, more commonly known as ions which have quantum energies dependent on the location of the electrons. Most commonly lasers are used to manipulate the ions in the trap and for state preparation. Trapped ion processors make use of Penning or Paul traps to lock the ions in a plane. Both of these traps make use of electric fields, with penning traps also using magnetic fields to achieve a 3-dimensional ion trap. However, Paul traps are the most prominent area of research in trapped ion qubit technology. Below is a picture from the NIST which is a three-dimensional Paul trap with multiple areas for ion traps and therefore locals for experiment procedures.

Some of the challenges for scaling up this technology include 1) the unprecedented amount of precision and accuracy needed with lasers that are interacting with the qubits themselves via waveguides for routing, state change, and readout 2) the errors that come up due to light scattering within the on-chip readout 3) similar manufacturing defects as that of superconducting loops which can further increase light scattering and other errors.

Spin/Quantum Dots

The two previously mentioned qubit technologies are the most commonly being used and scaled up by large companies general QC. However, I thought it pertinent to also include spin/quantum dot (QD) qubits because of their existing infrastructure in the semiconductor industry and the use of quantum dots for high definition TV applications. To be clear, the qubit is formed not via the quantum dots itself but because of the holes these QDs form which can then be inhabited by an electrons to act as a qubit, with the fluctuation of charge in the dot being the readout for the information encoding. The number of electrons or overall charge in a dot is measured by a quantum point contact or another quantum dot.

Some of the difficulties with scaling include 1) environmental charge noise that affects the qubit readout 2) the sensitivity of charge sensors that are reading the qubit information 3) again, general manufacturing defects that come from the tiny scales that these semiconductor gates have to be constructed in.

References:

Trapped-Ion Quantum Computing: Progress and Challenges: Colin D. Bruzewicz (April 9, 2019)

Lighting up the ion trap | MIT Lincoln Laboratory Kylie Foy (October 21, 2020)

Quantum Computing with Trapped Ions | NIST Daniel Slichter (April 7, 2023)

Eagle’s quantum performance progress | IBM Research Blog Oliver Dial (March 23, 2022)

Quantum Supremacy Using a Programmable Superconducting Processor – Google Research Blog John Martinis (October 23, 2019)

Quantum Dots/Spin Qubits | Oxford Research Encyclopedia of Physics Shanon P. Harvey (February 24, 2022)

Semiconductor Qubits in Practice Anasua Chatarjee (2021)

A Brief Introduction to Quantum Computing | Bitcoin Insider Tanisha Bassan (June 09, 2018)