# What is the physical representation of a qubit?

In regular computers, bits may be physically represented using a wide variety of two-state devices, such as polarity of magnetization of a certain area of a ferromagnetic film or two levels of electric charge in a capacitor.

But qubits have a property that they can be in a superposition of both states at the same time. I've seen this question's answers, which explain how can a qubit be represented, or modeled using a regular computer.

So I want to know what can be used (and is used by companies like D-Wave) to represent a qubit in a real physical quantum computer?

This section on Wikipedia collects the most important ongoing attempts to physically implement qubits.

For physically implementing a quantum computer, many different candidates are being pursued, among them (distinguished by the physical system used to realize the qubits):

• Superconducting quantum computing (qubit implemented by the state of small superconducting circuits (Josephson junctions))

• Trapped ion quantum computer (qubit implemented by the internal state of trapped ions)

• Optical lattices (qubit implemented by internal states of neutral atoms trapped in an optical lattice)

• Quantum dot computer, spin-based (e.g. the Loss-DiVincenzo quantum computer) (qubit given by the spin states of trapped electrons)

• Quantum dot computer, spatial-based (qubit given by electron position in double quantum dot)

• Nuclear magnetic resonance on molecules in solution (liquid-state NMR) (qubit provided by nuclear spins within the dissolved molecule)

• Solid-state NMR Kane quantum computers (qubit realized by the nuclear spin state of phosphorus donors in silicon)

• Electrons-on-helium quantum computers (qubit is the electron spin)

• Cavity quantum electrodynamics (CQED) (qubit provided by the internal state of trapped atoms coupled to high-finesse cavities)

• Molecular magnet (qubit given by spin states)

• Fullerene-based ESR quantum computer (qubit based on the electronic spin of atoms or molecules encased in fullerenes)

• Linear optical quantum computer (qubits realized by processing states of different modes of light through linear elements e.g. mirrors, beam splitters and phase shifters)

• Diamond-based quantum computer (qubit realized by electronic or nuclear spin of nitrogen-vacancy centers in diamond)

• Bose–Einstein condensate-based quantum computer

• Transistor-based quantum computer – string quantum computers with entrainment of positive holes using an electrostatic trap

• Rare-earth-metal-ion-doped inorganic crystal based quantum computers (qubit realized by the internal electronic state of dopants in optical fibers)

• Metallic-like carbon nanospheres based quantum computers

The large number of candidates demonstrates that the topic, in spite of rapid progress, is still in its infancy. There is also a vast amount of flexibility.

• Note that $|0/1\rangle$ are not necessarily energy eigenstates ("levels"). In particular when you consider optical modes. – M. Stern Apr 8 '18 at 12:31
• @M.Stern Okay, the previous edit wasn't mine and it seems that had made some unintended change in the language of the post. I have rollbacked the change, and now it should be fine. Thanks. – Sanchayan Dutta Apr 8 '18 at 14:21