Superconducting qubits are tiny electrical circuits that behave like artificial atoms. They are built using materials that, when cooled down to extremely low temperatures, allow electricity to flow with zero resistance — a state called superconductivity.
These circuits are engineered so that they can represent quantum states, such as the 0, 1, or a superposition of both. That makes them function as qubits, the basic units of quantum information.
They are currently one of the most popular and practical types of qubits — used by companies like IBM, Google, Rigetti, and others in real-world quantum computers.
The Basic Idea: An Engineered Atom
Superconducting qubits mimic the quantum behavior of atoms, but instead of using real atoms trapped in vacuum, engineers use macroscopic circuits made of wires and components.
These circuits are cooled to temperatures near absolute zero, often using dilution refrigerators, so they behave according to quantum rules rather than classical physics.
What Are They Made Of?
Superconducting qubits are made using these components:
1. Josephson Junctions
The heart of a superconducting qubit. A Josephson junction is made by placing a thin insulating layer between two superconducting materials.
It allows the current to tunnel quantum mechanically — creating non-linear effects that are essential for qubit behavior.
2. Capacitors and Inductors
These standard circuit components help define how energy flows in the system. Together with the Josephson junctions, they control the energy levels of the circuit.
3. Superconducting Loop
The components are arranged in a loop or other structures that can trap and manipulate electromagnetic fields — these fields encode the quantum states.
How Do They Store Qubit States?
In a typical circuit, current can flow in different patterns (for example, clockwise or counterclockwise in a loop), and those patterns correspond to different quantum energy levels.
The lowest two energy levels are used to represent the 0 and 1 of a qubit. But because it’s quantum, the system can also be in a superposition — a mix of both states at once.
The current doesn’t physically flow in a binary way. Instead, the entire circuit exists in a quantum state, spread across different possibilities — just like an electron in an atom.
Superconducting Qubit Types
There are several different “flavors” of superconducting qubits, depending on how the components are arranged and tuned:
1. Transmon Qubits (Most Popular)
- A refined version of older designs.
- Highly stable and less sensitive to electrical noise.
- Used by IBM and Google.
2. Flux Qubits
- Quantum states are based on the direction of current (clockwise or counterclockwise) in a superconducting loop.
3. Phase Qubits
- Use the phase difference across the Josephson junction to define states.
Each type has strengths and weaknesses depending on how easy it is to manufacture, control, or scale.
Why Superconducting?
Superconductivity is essential because it:
- Removes resistance from the circuit, avoiding energy loss.
- Allows the circuit to behave in a purely quantum way.
- Enables longer coherence times — meaning the qubit remains in a superposition for more time before collapsing into a definite value.
However, they need to be extremely cold — about 15 millikelvin, colder than deep space — which requires complex and expensive cooling systems.
How Are Superconducting Qubits Controlled?
Qubits are manipulated using microwave pulses — similar to those in your kitchen microwave but much more precise.
These pulses can:
- Rotate the qubit to create any superposition.
- Entangle one qubit with another.
- Perform logic operations (quantum gates).
Measurement is also done using microwaves. The circuit’s response to certain probe signals tells whether the qubit is in 0, 1, or a mix.
Entanglement and Coupling
To perform real quantum computations, you need to entangle qubits — make them affect each other’s states.
In superconducting systems, this is done by coupling qubits via special components (like resonators or bus lines), allowing them to share energy and interact.
This is how you implement multi-qubit gates, such as the CNOT gate, which is crucial for complex algorithms.
Challenges
Superconducting qubits are powerful, but they face some hurdles:
1. Decoherence
Even though they are isolated and cold, superconducting qubits eventually lose their quantum properties due to interaction with the environment. This limits how long you can run computations.
2. Noise
External noise (magnetic fields, temperature fluctuations, etc.) can cause errors in qubit operations.
3. Scalability
Adding more qubits without increasing noise or errors is hard. Right now, systems range from a few dozen to a few hundred qubits — but fault-tolerant quantum computing will require thousands or millions.
Why Are They Popular?
Despite the challenges, superconducting qubits are currently the most mature and commercially developed technology because:
- They’re relatively easy to fabricate using existing chip manufacturing techniques.
- They can be precisely controlled using known electronics.
- They have already been used to demonstrate quantum supremacy (Google’s claim of performing a task no classical computer could do in a feasible time).
Summary
Superconducting qubits are at the heart of today’s leading quantum computers. Here’s what you should remember:
- They are artificial atoms built from superconducting circuits.
- Work by manipulating current and energy levels in circuits at extremely low temperatures.
- Josephson junctions give them their quantum behavior.
- Controlled and measured using microwave pulses.
- Powerful and fast but still challenged by noise and scaling.
They represent a fascinating blend of macroscopic engineering and microscopic quantum behavior — and they’re likely to be key players in the race toward practical quantum computing.