How Josephson Junction is the Heart of Quantum Computer?

Quantum computers are expected to tackle problems that classical computers simply can’t solve, for instance, designing next generation drugs for cancer treatment or optimizing complex logistics at scales unmanageable by classical computers.

At the center of many of these powerful machines is something surprisingly tiny: the Josephson junction. While it’s not the only way to build a quantum computer, it’s by far the most successful one to date. In fact, the Josephson junction plays a role in quantum computers that’s very similar to the transistor’s role in classical computers.

To have an anology, a transistor is the core active element in classical computers. It switches between on/off states (0/1), amplifies signals, and forms logic gates. Similarly, the Josephson junction in quantum computers is the active, non-linear element enabling the creation of qubits, allows quantum superposition and entanglement, and enables quantum logic operations and state control.

In classical transistors, you need 0 and 1 state, the system is either in 0 or in 1 state. But in quantum it can be in 0 and 1 and in superposition of 0 and 1 also. To have a quantum bit (qubit), Josephson junction must provide these states. Lets understand what is josephson junction first and then dive into it?

What is a Josephson Junction?

It’s simpler than it sounds. A Josephson junction is made up of two superconductors separated by a really thin layer, which could be an insulator, metal, or semiconductor. And here’s the quantum twist: even though there’s a barrier, Cooper pairs (pairs of electrons bound together at low temperatures) can tunnel through this barrier without resistance. Yep! quantum tunneling in action. The flow of these Cooper pairs is governed by something called the phase difference between the two superconductors, which essentially controls the current.

Anharmonicity is important for qubit formation

Do We Need an LC Circuit?

To form a qubit, we need two energy levels; one ground state and one excited state. In the classical regime, we can get this using an inductor and a capacitor in series. An inductor is an element that stores magnetic field energy, and a capacitor is an element that stores electric field energy. When the capacitor is charged, it dissipates energy into the inductor, and when the inductor releases energy, it charges back the capacitor again. This system, consisting of a capacitor and an inductor, behaves like a harmonic oscillator with a fixed frequency that depends only on the values of the inductance and capacitance. By knowing these values, you can make the system either stay at rest or oscillate at its resonant frequency, that is, at its excited state.

If we bring the system to the nanoscale, quantum mechanics comes into the picture, and the LC system becomes a quantum harmonic oscillator with quantized energy levels. The energy can be given by the following formula:

E_n​=(n+1/2​)ℏω,

where n = 0, 1, 2, 3, …and it tells if the system is in ground state or excited states. ω is the angular frequency, and ℏ is =h/2π

In this case, the energy spacing is equal between levels, which becomes a problem when forming a qubit. That’s because the energy difference between the 0–1 and 1–2 levels is the same (talking about n= 0,1,2), and any energy input that matches that difference could excite the system to either state. To avoid this, we need an anharmonic inductor, which makes the LC circuit anharmonic so the level spacing becomes unequal. This way, we can ensure that the system is only excited from the 0–1 energy level by selecting the right excitation energy.

A Josephson junction can be considered an inductor, and it is naturally anharmonic. By combining it with a capacitor, we can form a qubit, in which we can specifically select the 0–1 energy transition and define the transition from state |0⟩ to |1⟩ as our qubit logic (in quantum mechanics notion, the ground state is designated as |0⟩ and 1st excited state as |1⟩).

To rephrase-

In a regular LC circuit (just an inductor and a capacitor), the energy levels are equally spaced. It’s like a staircase with steps that are all the same height. This system behaves like a harmonic oscillator and isn’t very helpful if you want to isolate two specific energy levels for a qubit.

But introduce a Josephson junction in place of the inductor, and suddenly, you get a nonlinear system. Now the steps on the staircase are uneven. This is called an anharmonic oscillator, and it’s exactly what we need. Thanks to this uneven spacing, we can target just the first two energy levels (which we call |0⟩ and |1⟩) without accidentally exciting higher ones, which was possible in evenly spaced harmonic oscillator. That’s how you build a reliable, controllable qubit. Without the junction’s nonlinearity, you just have a fancy oscillator. With it, you have a quantum processor.

How Do We Excite This Qubit?

Let’s say your qubit has energy levels |0⟩ and |1⟩ separated by E₁−E₀ . To excite it, you need to apply a signal at a frequency exactly equal to (E₁−E₀)/h​​.

This frequency lies in the microwave region of the electromagnetic spectrum. The electric field from the microwave pulse couples to the charge across the capacitor, and if the frequency matches the energy gap, the qubit absorbs one microwave photon and jumps from |0⟩ to |1⟩. This is the excitation process.

If the pulse is shaped just right (in terms of timing and phase), it doesn’t necessarily cause a full jump. It can instead create a superposition, such as: ∣ψ⟩=α∣0⟩+β∣1⟩ (∣ψ⟩ represent the state of the system, and α and β gives the probabilty of individual states). In this state, the qubit is in ‘both states at once’ until you measure it.

How Do We Know It Was Excited?

You can not see the qubit state directly, but the qubit is connected to a readout resonator (a tiny antenna-like structure). The resonant frequency of this readout shifts slightly depending on whether the qubit is in state |0⟩ or |1⟩. You send a probe signal through this resonator and measure its reflection or transmission. Based on the change in the response, you can infer whether the qubit was in |0⟩ or |1⟩.

Coherence Matters!!

Quantum systems are fragile. They lose their quantum behavior (a process called decoherence) at the slightest nudge from the environment. That’s where the Josephson junctions shine again.

When these devices are fabricated carefully and kept ultra-cold (we’re talking millikelvin temperatures), they can preserve their quantum states long enough to run computations and even correct errors.

And they’re fast. Josephson junction based qubits can perform gate operations in tens of nanoseconds, blazing fast in the quantum world. That’s exactly the kind of speed you want before decoherence ruins your quantum party.

Scalable and Fabrication-Friendly

Here’s another big win: Josephson junctions can be made using CMOS-compatible fabrication techniques, the same ones we use for classical chips. That means we can scale up, just like we did with transistors.

In fact, companies like IBM, Google, and Rigetti are already building multi-qubit processors with this exact approach.

Here’s a quick comparison with other types of qubits being developed: Table 1

Qubit Type	Companies	Key Traits
Josephson Junction	IBM, Google, Rigetti, Quantinuum (part)	Fast gates, scalable, needs cooling
Trapped Ion	IonQ, Quantinuum, Oxford Ionics	Long coherence, slower gates
Photonic	PsiQuantum, Xanadu, ORCA	Light-speed, room-temp
Spin (Silicon)	Intel, SQC, Quantum Motion	CMOS-friendly, emerging
Neutral Atom	QuEra, Pasqal	Highly reconfigurable
Topological	Microsoft, Qulabz (early stage)	Fault-tolerant (in theory)

The Josephson junction might sound like just another physics term, but it’s the beating heart of most quantum computers today. It gives us (1). the nonlinearity we need to isolate qubit states, (2). the coherence to keep them stable, and (3). the compatibility to scale them up like classical chips.