What is Quantum Computing?
Quantum Computing is the application of Quantum mechanics concept in computer design, computer manufacturing and all possible Quantum theories usage in the scope of computer .
Quantum mechanics is a fundamental theory in physics which describes nature at the smallest scales of energy levels of atoms (原子) and subatomic particles (亞原子粒子).
In traditional computer, we use ' bit' as unit to describe a status of computing result, each individual status called pattern, and generally understand as 0 or 1.
In Quantum computer, we use the energy level (能階) for an electron in an atom concept in quantum mechanics to represent the status. The lowest energy level in Quantum mechanics is called Ground State(基態), which is 0 in traditional computer concept; and Excited state (激發態) is represent of certain energy level, which could be understand as 1 in traditional computer concept.
However, the difference of Quantum Computer to Traditional Computer is that there are many "pattern" in between 0 and 1 because the energy level of Excited state could be many. Therefore, if we define the lowest energy level or say ground state as 0, a certain mount of energy level or say excited state as 1, then the range of energy level between ground state 0 and defined excited state 1 could be an individual pattern. We could therefore have three patterns, 0 pattern ,1 pattern and 0/1 pattern in Quantum computer. With such computing design, quantum computer could achieve a larger amount and more efficient calculation in compare with traditional computer.
Superposition(疊加原理) is a principle that states while we do not know the state of an object at a given time, it is possible that it is in all states simultaneously, as long as we do not look at it to check its state. The way that energy and mass become correlated to interact with each other regardless of distance is called entanglement(量子纏結).
Superposition and entanglement
Concept of Spin (自旋)
Stern 與 Gerlach 於 1921 年發現銀原子束在不均勻磁場中分為兩道。 Pauli 為解釋原子光譜的多重態，於 1924 年提出電子的「兩值性」（ two-valuedness ）。後來 Goudsmidt 與 Uhlenbeck 提議電子有 spin （一般直譯為「自旋」），即刻被 Ehrenfest 與 Pauli 指出：不可能！因為若從電子的磁偶極矩估計，自轉的速度會違背相對論。 Pauli 是對的，因為後來 Dirac1928 年發現，電子的這一性質來自相對論與量元物理的結合，與自轉根本無關。筆者因此主張不可以訛譯訛──不能說電子有自旋。那要如何說呢？說電子有「兩儀性」 [1] ──它在外加磁場中表現「上儀」及「下儀」。為描述這一性質，我們定電子的「儀數」為 1/2 。
這一說法可以推廣：質子與中子的儀數也是 1/2 ，光元的儀數是 1 ，介子的儀數是 0 ，等等。 Pauli 復於 1940 年從量元場論（ quantum field theory ）證明：儀數為整數的粒子都是合群粒子（ bosons ），儀數為半整數的粒子都是不合群粒子（ fermions ）。一群相同粒子（ identical particles ）在溫平衡下的分布依其群性（ statistics ）而不同。 1933 年， Heisenberg 更提出質子與中子是同一種粒子的不同狀態，另有「同種儀」（ isospin ）的性質，它們的「同種儀數」為 1/2 。
Reference:
Fundamentals Of Quantum Computing | All about circuit
Quantum Superposition And Entanglement Explained | Clerro
Quantum Superposition And Entanglement Explained | Clerro
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