Scientists are constantly looking for new candidates for Qubit. A Qubit does not have absolute state of either 0 or 1 like a classical bit rather it is a superposition of both states. Qubit can be 0, 1 and any value in between at the same time, until an act of measurement is performed which forces it to relinquish all possible states except for one. Act of measurement turns the Qubit into a classical bit and all the Quantum mechanical advantage is lost. This is where property of Entanglement helps out. Quantum Entanglement is the phenomena under which properties of two particles that have previously interacted are inextricably linked in such a way that any change in the state of one particle simultaneously changes the state of other, and this holds true even if the two particles are at opposite ends of universe. Superposition of Qubit is maintained while figuring out its state by performing the measurement on its Entangled pair from which state of Qubit under consideration is inferred. Quantum Entangled bits have higher correlation than two classical correlated bits as established by Bell's inequality. While performing measurements through Entanglement, high correlation between the two Qubits is desirable for faithful results. Correlation giving a fidelity of 96-97% has been achieved by Professor Andrea Morello and his team.
Fact that Qubit can be in two state at same time allows for performing millions of iterations simultaneously making Quantum computers astonishingly superior than classical ones. Quantum computers use sequence of Qubits. A Qubit can be 0, 1 and all points in between at same time. Quantum computer with two Qubits can be in 4 different states at same time. Quantum computer with n Qubits can be in 2^n different states at the same time on the other hand a classical computer can only be in one of these 2^n states at any given time. Thus with every additional Qubit computing power grows exponentially. A Qubit has 2 key states denoted as 0=(1 0) and 1=(0 1) known as basis states. Quantum computer uses these states to perform iterations according to unitary matrix transformation. Theory and logic of Qubit computations is getting developed and numerous methods have been submitted to realize Quantum computing. One such method could be Quantum Annealing as indicated by T Lanting et al. Scientists so far have achieved successful multiplication of two integers using a Quantum computing method.
Quantum computers are based on behavior of matter at Quantum level. Manner in which the spin, energy or speed of these particles changes on interaction is used to create the logic based on which Quantum logic gates are created which are then used to operate on a set of Quantum inputs so called Qubits to yield an output. Qubits are Input to Quantum computers but output comes in form of classical bits because act of getting an output forces particles to relinquish all states they can be in except for one. Binary computer uses binary code as input which is then operated upon using logic circuits made of binary logic gates such as AND, OR, XOR. A binary logic gate is a realization of binary operators such as AND or NOT operator and are made using transistors such as a CMOS transistor. Architecture of Quantum computer is different from architecture of binary computers. In a Quantum computer, Quantum transistors made of controlled Qubits are used to create Quantum gates in order to realize basic Quantum logical operators as in unitary matrix which are then used to create Quantum logic circuits designed to solve real world problems. Code corresponding to such circuits is developed and used for programming. Quantum algorithm is used for creating codes for solving problems using Quantum computers. Based on the definition of Qubit, Quantum transistors have been created such as single atom transistor wherein by controlling the state of Qubit, conduction path can be opened or closed. Using single electron Qubit, Quantum logic gates have been created such as the CNOT gate, a two Qubit gate wherein target Qubit flips its spin when control Qubit is pointing down and maintains its spin when control Qubit is pointing up. Here spin of electron serves as Qubit and control is exercised through microwaves. This 2 Qubit gate alongwith single Qubit operations can be used to create any other gate set. It gives us a way of creating Quantum computers with 100s of Qubits. For sheer processing power, Quantum computer with 300 Qubits will have more computing ability than all binary computers on Earth combined. Quantum computer with 300 Qubits in Entanglement will have processing power equivalent to 2300 bit conventional computer. 2300 is about the number of particles in observable universe.
Vector representation is used to present Quantum states mathematically. Mathematically a Qubit can be presented in terms of its basis vector states as
2 Qubits in terms of their 4 basis vector states can be shown as
Models of Quantum computing include Adiabatic, one way, Quantum gate array and Topological. Many methods are available for implementing a Quantum computer such as Nuclear magnetic resonance, Fullerine based ESR, Linear optical, Trapped ion and Quantum dot. Logic of Quantum computing is far more complex than classical binary logic and requires simplification. Apart from this Scientists have to deal with Decoherence while designing these Quantum computers. Decoherence is the characteristic of getting into disorderly and unorganized state due to external interference or internal causes. Quantum state of Qubit changes with slightest of disturbance. To do calulations its essential to maintain their state that is up spin or down spin state for example, for entire duration of calculation. Scientists try to work around this problem by keeping Qubits in super cooled, ultra vacuum environment causing them to get in Quantum mechanical ground state. Liquid Nitrogen or liquid Helium is used for cooling. Preservation of Quantum state has been achieved for a maximum period of upto 2-3 hours so far. With more and more research in Quantum error correction, factors that can affect the state of Qubits are being recognized and ways of neutralizing those factors are getting created by research teams around the world. We only have a certain probability of getting the expected result when computing using quantum mechanical properties because state of particles at Quantum level cannot be measured with absolute certainty, we can only have a probability of one result or other determined according to Heisenberg uncertainty principle. This is why Quantum algorithms have to be run several times in succession to get result expected as per Quantum logical operators (unitary matrix transformations).
Researchers at the National Institute of Standards and Technology (NIST) have now managed a significant breakthrough by ‘teleporting’ or transferring, quantum information from one photon to another over a distance of 100 km of optical fiber. In this experiment as shown in their infographic they created a Photon and then split it in two using a special crystal generating a pair of entangled photons whose states are identical. One of the Entangled Photon is transported to receiving end through a spool of Optical Fiber. Then they generate the input Photon and select its state either early, late or a superposition of both. Input and helper Photon are made to meet at a beam splitter with a 50/50 chance of getting straight through or reflecting at an angle. Detectors developed at NIST and based on superconducting nanowires made of molybdenum silicide are placed suitably to detect the arrival of Photons. When one detector clicks early and the other late, it means Photons are out of phase. Detectors at receiving station measure the state of output Photons from which the state of input photon can be inferred. Thus teleportation of Quantum information is achieved. NIST’s Marty Stevens says “Only about 1 percent of photons make it all the way through 100 km of fiber”.This is nothing like the general idea of Teleportation but it’s a great progress towards achieving the same. After all this is how the first conventional computers were developed and over 50 years of constant improvements have given us the computer and internet as we know it. Just remember the early experiments on Silicon and Germanium crystals in order to develop the very first transistors. We know that teleportation in principle is achievable because of the insight provided by the team of C.H. Bennett, G. Brassard, C. Crepeau, R. Jozsa, A. Peres, and W. Wootters. They pointed that complete teleportation of quantum information can be achieved in theory at least. Say you want to teleport object A. You can do this by scanning the quantum state of A and another object B together. The unscanned part is transferred to another object C through B which is Entangled with C. Using the scanned data of original Quantum state of object A, one of many treatments can be applied to C in order to recreate the complete original Quantum state of A, transforming C into A. This theory has been used by many scientists to demonstrate quantum teleportation at numerous occasions.
Anton Zeilinger and his team are carrying out Photon teleportation between the Islands of La Palma and Tenerife, over a distance of 143 km through open space. In their experiments they create two identical Photons (Heralded Single Photon), one of which is transported to Tenerife over a high energy Laser. A third Photon which they are going to teleport is brought close to the Photon at La Palma and their interactions are observed. Due to entanglement the state of distant Photon changes with the state of the Photon at the sending station. The observations made at sending station are used to convert the photon at Tenerife into an exact copy of the third Photon. A number of Photons have already been teleported using this method.
References:
1) https://arxiv.org/pdf/1501.00011.pdf
2) http://newsroom.unsw.edu.au/news/quantum-computing-taps-nucleus-single-atom
3) http://researcher.watson.ibm.com/researcher/files/us-bennetc/BBCJPW.pdf
4) https://arxiv.org/pdf/quant-ph/9511027.pdf
4) https://arxiv.org/pdf/quant-ph/9511027.pdf
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