(10/05/2017)

Qubits are the basic unit of data in quantum computing.

Qubits (two-state quantum-mechanical systems) are the basic unit of data in quantum computing. But what are they, exactly?  How do Qubits work? Here are seven things you should know about qubits.

1. The origin of the name

The term “qubit” was coined by Ben Schumacher in a 1993 paper (published in 1995). The paper credits Schumacher’s “intriguing and valuable conversations” with Bill Wootters as the source of the name.

2. Qubits vs. bits

The computers that we’re familiar with use bits as their basic unit of data.  Each bit can be either true or false, on or off.  Quantum computers use qubits.  Like bits, qubits can be in one of two states when measured, but that’s where the similarities end. Qubits us quantum mechanical phenomena like superposition and entanglement to exist in multiple states at the same time until measured and to hold up to two bits worth of data.

3. Superposition

Just as Schrödinger’s cat had some, all, or none of its nine lives left when he placed it into that box, other objects (like qubits) can exist in a superposition of multiple states. A standard bit can hold either a zero or a one. Two bits can hold one of four values at any time: “00,” “10.” “01,” and “11.” At 2 billion positions per second, a standard 64-bit computer would take around 400 years to cycle through all its possible values.

Qubits, on the other hand, can hold a zero, a one, or any proportion of both zero and one at the same time. An array of qubits can use superposition to represent all 2^64 possible values at once, allowing a quantum computer to solve problems that are practically impossible for standard computers.

4. Entanglement

Entanglement is a physical relationship between two or more qubits in which one qubit seems to know what happens to another, even when they are a large distance apart. Entangled qubits become a system with a single quantum state. If you measure one qubit (i.e., collapse its superposition to a single state), you will have the same impact on the other qubits in the system.

Thanks to entanglement, qubits can hold up to two bits of data and transmit data between qubits up to 1400 meters apart (as of the writing of this post).

Physically, qubits can be any two-level system. For example, the polarization of a proton, or the spin of an electron. Under a strong magnetic field, an electron will polarize with the spin pointing down. Hitting the electron with microwaves will increase its energy and make it spin upward. If the microwave pulse is stopped between positions, the spin is left in a superposition.

Researchers have implemented qubits in several physical representations, including:

• The spin of atomic nuclei
• The energy state of an electron suspended by magnetic fields and stimulated by a laser (part of the aptly-named “trapped ion” quantum computer)
• An electric current which exists in superposition between clockwise and anticlockwise in a superconducting wire.

6. Qubits can be used to execute quantum algorithms

Mathematicians have developed advanced algorithms that can only be run using quantum computers full of qubits. Algorithms such as Shor’s algorithm for integer factorization (given an integer, find its prime factors) and Grover’s algorithm for searching an unstructured database. These algorithms will be able to solve certain problems much faster than the algorithms our current computers can execute.  Problems like breaking public-key encryption schemes.

7. The challenges of qubit storage

Qubits require very low temperatures and precise physical conditions to function. In 2008, a team of researchers transferred data for 1.75 seconds between two qubits. Since then, that time has been extended to 39 minutes at room temperature and 3 hours at low temperature–a huge step forward in the development of quantum computers.

Qubits are still young, but active research projects around the world grow our knowledge every day. A major quantum computing breakthrough is just around the corner. Let’s get ready to change the world!