Phase qubit

Key takeaways

Phase qubit belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Phase qubit to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Phase qubit from memory before moving on to harder problems.

Reference excerpt

In quantum computing, and more specifically in superconducting quantum computing, the phase qubit is a superconducting device based on the superconductor–insulator–superconductor (SIS) Josephson junction, designed to operate as a quantum bit, or qubit. The phase qubit is closely related, yet distinct from, the flux qubit and the charge qubit, which are also quantum bits implemented by superconducting devices. The major distinction among the three is the ratio of Josephson energy vs charging energy (the necessary energy for one Cooper pair to charge the total capacitance in the circuit):

For phase qubit, this ratio is on the order of 106, which allows for macroscopic bias current through the junction; For flux qubit it's on the order of 10, which allows for mesoscopic supercurrents (typically ~300 nA); For charge qubit it's less than 1, and therefore only a few Cooper pairs can tunnel through and charge the Cooper-pair box. However, transmon can have a very low charging energy due to the huge shunt capacitance, and therefore have this ratio on the order of 10~100. The quantization based on phase of the Josephson junction was first demonstrated experimentally by John Clarke, Michel Devoret and John M. Martinis in 1985, for which they were awarded the Nobel Prize in Physics in 2025.

Introduction A phase qubit is a current-biased Josephson junction, operated in the zero voltage state with a non-zero current bias. A Josephson junction is a tunnel junction, made of two pieces of superconducting metal separated by a very thin insulating barrier, about 1 nm in thickness. The barrier is thin enough that electrons, or in the superconducting state, Cooper-paired electrons, can tunnel through the barrier at an appreciable rate. Each of the superconductors that make up the Josephson junction is described by a macroscopic wavefunction, as described by the Ginzburg–Landau theory for superconductors. The difference in the complex phases of the two superconducting wavefunctions is the most important dynamic variable for the Josephson junction, and is called the phase difference δ {\displaystyle \delta } , or simply "phase".

Main equations describing the SIS junction The Josephson equation relates the superconducting current (usually called the supercurrent) I {\displaystyle I} through the tunnel junction to the phase difference δ {\displaystyle \delta } ,

I = I 0 sin ⁡ δ {\displaystyle I=I_{0}\sin \delta } (Josephson current-phase relationship) Here I 0 {\displaystyle I_{0}} is the critical current of the tunnel junction, determined by the area and thickness of the tunnel barrier in the junction, and by the properties of the superconductors on either side of the barrier. For a junction with identical superconductors on either side of the barrier, the critical current is related to the superconducting gap Δ {\displaystyle \Delta } and the normal state resistance R n {\displaystyle R_{n}} of the tunnel junction by the Ambegaokar–Baratoff formula

I 0 = π Δ 2 e R n {\displaystyle I_{0}={\frac {\pi \Delta }{2eR_{n}}}} (Ambegaokar–Baratoff formula) The Gor'kov phase evolution equation gives the rate of change of the phase (the "velocity" of the phase) as a linear function of the voltage V {\displaystyle V} as

V = ℏ 2 e d δ d t {\displaystyle V={\frac {\hbar }{2e}}{\frac {d\delta }{dt}}} (Gor'kov-Josephson phase evolution equation) This equation is a generalization of the Schrödinger equation for the phase of the BCS wavefunction. The generalization was carried out by Gor'kov in 1958.

McCumber–Stewart model The model to describe the potential was derived by Dean McCumber, and independently by W. C. Stewart in 1968. The alternative and direct current Josephson relations control the behavior of the Josephson junction itself. The geometry of the Josephson junction—two plates of superconducting metal separated by a thin tunnel barrier—is that of a parallel plate capacitor, so in addition to the Josephson element the device includes a parallel capacitance C {\displaystyle C} . The external circuit is usually simply modeled as a resistor R {\displaystyle R} in parallel with the Josephson element. The set of three parallel circuit elements is biased by an external current source I {\displaystyle I} , thus the current-biased Josephson junction. Solving the circuit equations yields a single dynamic equation for the phase,

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Phase qubit

Start with the simplest possible case. Write down what Phase qubit claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Phase qubit before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Phase qubit ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Phase qubit

In research: Phase qubit appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Phase qubit in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Phase qubit is common in secondary-school and first-year university syllabi. It links to neighbouring topics Quantum electronics, Quantum information science, Superconductivity, so understanding it makes those chapters shorter.
In everyday life: Look for Phase qubit outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Phase qubit in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Phase qubit means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Phase qubit out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Phase qubit in simple terms?

In quantum computing, and more specifically in superconducting quantum computing, the phase qubit is a superconducting device based on the superconductor–insulator–superconductor (SIS) Josephson junction, designed to operate as a quantum bit, or qubit. The phase qubit is closely related, yet distin…

Why does Phase qubit matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Phase qubit?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Phase qubit.