JOVANA
Library Glossary Getting Started Three Levels Fields How it works Mission
Join the mission
All guides

The Qubit as a Circuit

On a superconducting chip, a qubit is not a mysterious particle — it is literally an electrical circuit. Here is how an ordinary LC oscillator becomes a controllable qubit once you add one special component, and why that small change is what makes the whole thing work.

Start with a circuit that rings

Before we touch anything quantum, picture the simplest circuit that does something interesting on its own: an inductor (a coil, the letter L) wired to a capacitor (two plates that store charge, the letter C). Give it a little energy and it does not just sit there — charge sloshes back and forth between the capacitor and the inductor, over and over, at one steady frequency. Engineers call this an LC oscillator, and the back-and-forth is exactly like a child on a swing or a plucked guitar string.

Two design choices set how fast it rings: the size of L and the size of C. Make them small and the circuit oscillates billions of times a second — in the microwave range, the same band Wi-Fi and your kitchen oven live in. That matters because we already know how to make and steer microwave signals with ordinary electronics. On a quantum chip these L and C are not bulky parts; they are thin metal lines and gaps patterned right onto the surface.

One component changes everything

Here is the trick that turns a circuit into a qubit. We replace the ordinary inductor with a tiny special element called a Josephson junction — two superconducting metals separated by an insulating gap so thin that electricity can quietly tunnel across it. You do not need the physics to follow the story; what matters is what it does to the circuit. The junction behaves like a strange, springy inductor whose stiffness changes depending on how much energy is already in the circuit.

  control & readout line
         |
   ======+======
   |           |
  === C       X  <- Josephson junction
   |  (cap)   |    (the nonlinear element)
   ======+======
         |
       ground

  ordinary inductor L  -->  replaced by  X
  result: an artificial atom on a chip
Schematic: an LC circuit with the inductor swapped for a Josephson junction (X). The capacitor still stores charge; the junction provides the nonlinearity that makes a qubit.

Why does a "springy" inductor matter so much? In the plain LC circuit, every rung of the energy ladder was the same height, so the jump from level 0 to level 1 cost the same as the jump from 1 to 2, and from 2 to 3. The junction is nonlinear: it makes those rungs unequal. Now the 0-to-1 jump sits at a slightly different frequency from every other jump. That unevenness has a name — anharmonicity — and it is the whole reason the device becomes usable.

Why uneven rungs let us aim a pulse

To use this circuit as a bit, we only ever want the bottom two rungs: level 0 is our "0", level 1 is our "1". We flip between them by sending in a short microwave pulse tuned to the 0-to-1 frequency. But a pulse is never perfectly pure — it carries a spread of nearby frequencies. If the 1-to-2 jump sat at the very same frequency, the same pulse meant to flip 0-to-1 would also push the system up to level 2, and the qubit would quietly leak out of the only two states we can compute with.

  1. The junction makes the 0-to-1 jump and the 1-to-2 jump land at noticeably different frequencies.
  2. We aim a control pulse precisely at the 0-to-1 frequency.
  3. Because level 2 sits at a different frequency, it barely responds — it is off-resonance.
  4. The pulse therefore flips just 0 and 1, and the qubit stays inside its two-state playground.

A circuit that behaves like an atom

Step back and notice what we built. A real atom has a fixed ladder of energy levels and absorbs or emits light only at specific frequencies. Our little circuit now does the same — except its ladder is designed by us, not handed down by nature, and the "light" it talks to is microwave rather than visible. Engineers call this whole circuit-as-atom way of thinking circuit QED. It is the framework underneath nearly every superconducting quantum chip.

The freedom to design the ladder is the gift of this approach — and also the burden. Because we pick each qubit's frequency by choosing its L and C, two qubits on the same chip can end up with frequencies too close together, which makes them interfere when we try to address them separately. Multiply that across hundreds of qubits and it becomes one of the field's genuine headaches. So this is not a story of effortless power; it is a story of a flexible building block that brings its own hard engineering with it.