In the previous tutorial ,we talked about that how it is impossible to precisely calculate position and momentum of a sub atomic particle because of Heisenberg uncertainty principle.

Now the whole premise of using qubit -was to store entire spectrum of information because of its wave like behavior ,but till now what we understood ,that if we will try to measure the qubit(or subatomic particle) ,its wave function collapses and some how even if we will measure it ,we can’t measure it more precisely than the limit set by Heisenberg uncertainty principle .Then how existing quantum computers are working.

To understand this first we need to understand -that how qubits are made

There are many kind of qubits people are working on, but what are the certain properties ,which need to be considered before making a qubit

- As we understood till now ,the power of qubit lies in its wavefunction and its wavefunction is comprised of super positioned particles ,so for a qubit holding this superposition is really important ,otherwise it will behave just like a normal bit. Qubit can lose its superposition because of environment changes, that’s why quantum devices generally have to be supercooled to absolute zero (-273℃) to keep their atoms from being distributed.
- These atoms also have to be kept away from basically every kind of electromagnetic wave. That includes any heat, including the microwaves from your cell phone, which can cause a qubit superposition state to collapse, which scientists call decoherence.
- Other important thing ,there should be way to measure this superposition of qubits without destroying the wave function
- Time to measure the qubit should be lesser than the qubit can maintain the superposition ,this seems a little bit strange now ,but it will be important ,when we understood that current qubits can’t maintain the superposition/coherence for very long time .
- Qubits should be scalable ,means we can add more qubits into a quantum system ,without introducing too much of error

These days companies are working on different kind of qubits ,some are good in maintaining coherence/superposition for longer time ,some are more scalable .

Below is the list of some popular qubits mentioned on Qubit wiki https://en.wikipedia.org/wiki/Qubit

Physical support | Name | Information support | |0> | |1> |

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 dotpair | Electron localization | Charge | Electron on left dot | Electron on right dot |

Quantum dot | Dot spin | Spin | Down | Up |

Gapped topological system | Non-abelian anyons | Braiding of Excitations | Depends on specific topological system | Depends on specific topological system |

Among these most popular qubits ,which are supported by many companies and research group are as follows .This list is taken from this blog – https://www.autodesk.com/products/eagle/blog/future-computing-quantum-qubits/

Qubits | Companies | Coherence/Superposition time | Pros | Cons |

Superconducting | Google, IBM, and Quantum Circuits | To date, the longest superposition state achieved with superconducting loops was a measly .000005 seconds with 9 entangled qubits | Superconducting loops are based on existing technologies used in the semiconductor industry and are one of the fastest performers around. | They lose their superposition state easily and have to be kept at near zero temperatures at all times. |

Trapped Ions
| quantum computing startup ionQ | Trapped ions have the greatest success to date, remaining in a superposition state for more than 1000 seconds with 14 entangled qubits. | Trapped ions possess a high degree of stability when in a superposition state. | It’s also the slowest of all the qubit types in development, and requires a bunch of compact lasers to remain stable. |

Silicon Quantum Dots | Intel | The longest superposition state achieved by one of Intel’s qubits was .03 seconds with 2 qubits entangled. | Like the stable lineup of processors that Intel develops, their silicon quantum dots are known for being extremely stable and are founded on the company’s existing semiconductor and silicon technologies. | Like other qubits, this one also has to be kept at near zero temperatures at all times to maintain a superposition state. |

Diamond Vacancies
| Quantum Diamond Technologies | To date, this qubit has achieved a superposition state lasting 10 seconds, with 6 qubits entangled. Second place! | Unlike other qubits which need to operate at near zero temperatures, diamond vacancy qubits can work at room temperatures. | On the flip side, these qubits are also tough to entangle, does this have something to do with the temperature perhaps? |

Topological Qubits | Microsoft and bell labs | No Information on this as of now | No Information on this as of now | This qubit is based on the use of quasiparticles, which means that it resides on the boundary between two particles |

In next blog we will talk about superconducting qubits ,because these are most popular currently ,We will discuss that how to make these qubits, how to measure them,how to store information in these qubits and how error is corrected in these qubits.

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