Physical Representation
| Is it possible to have three-dimensional, self-correcting, quantum memory? |
Any two-level quantum system can be used as a qubit. Multilevel systems can be used as well, if they possess two states that can be effectively decoupled from the rest (e.g., ground state and first excited state of a nonlinear oscillator). There are various proposals. Several physical implementations which approximate two-level systems to various degrees were successfully realized. Similarly to a classical bit where the state of a transistor in a processor, the magnetization of a surface in a hard disk and the presence of current in a cable can all be used to represent bits in the same computer, an eventual quantum computer is likely to use various combinations of qubits in its design.
The following is an incomplete list of physical implementations of qubits, and the choices of basis are by convention only.
| Physical support | Name | Information support | ||
|---|---|---|---|---|
| Photon | Polarization encoding | Polarization of light | Horizontal | Vertical |
| Number of photons | Fock state | Vacuum | Single photon state | |
| Time-bin encoding | Time of arrival | Early | Late | |
| Coherent state of light | Squeezed light | Quadrature | Amplitude-squeezed state | Phase-squeezed state |
| Electrons | Electronic spin | Spin | Up | Down |
| Electron number | Charge | No electron | One electron | |
| Nucleus | Nuclear spin addressed through NMR | Spin | Up | Down |
| Optical lattices | Atomic spin | Spin | Up | Down |
| Josephson junction | Superconducting charge qubit | Charge | Uncharged superconducting island (Q=0) | Charged superconducting island (Q=2e, one extra Cooper pair) |
| Superconducting flux qubit | Current | Clockwise current | Counterclockwise current | |
| Superconducting phase qubit | Energy | Ground state | First excited state | |
| Singly charged quantum dot pair | Electron localization | Charge | Electron on left dot | Electron on right dot |
| Quantum dot | Dot spin | Spin | Down | Up |
Read more about this topic: Qubit
Famous quotes containing the word physical:
“For pain is perhaps but a violent pleasure? Who could determine the point where pleasure becomes pain, where pain is still a pleasure? Is not the utmost brightness of the ideal world soothing to us, while the lightest shadows of the physical world annoy?”
—Honoré De Balzac (1799–1850)