# Pandora’s Box of Quantum Computing

Quantum Computing has been the latest hype in the tech market majorly due to its perceived extra-ordinary capabilities compared to classical computers. When I say ‘extraordinary’, you immediately tend to think about exponential enhancements in calculations or computation power. Trust me this is much beyond that! So let’s unveil the pandora’s box to explore the extraordinary.

The basic fundamental building block of a quantum computer is a Quantum bit or Qubit similar to a binary bit in classical computers. The term “classical” implies that these computers follow Newtonian Mechanics and do not defy the laws of nature. The term “quantum” however, implies that the computers follow Quantum Mechanics and defy all possible laws of nature!

**What is a qubit?**

To answer this let’s first know what is a bit (short for binary digit). It can be into states, either 0 or 1. A classical computer only understands the language of bits hence it translates all the data into bits before performing any task. A qubit is kind of a bit but with 0 & 1 and many more possible states.

**Notation: **A qubit in 0 state is represented by

|0> in Braket notation

A qubit in 1 state is represented by

|1> in Braket notation.

The general form of a Quantum bit(qubit) is *c1**|0> + *c2**|1> where *c1*² and *c2²* are coefficients which represent probabilities of attaining |0> & |1> respectively when the system is measured. Hence *c1²*+*c2*² = 1 should be satisfied to ensure that the system will definitely collapse to some state.

So c1 and c2 can take any values and hence, a single qubit can be a linear combination of |0> and |1> states also. Now you know how can a qubit exist in multiple states. This is the first “extra-ordinary” capability of the qubit that differentiates the operation of the quantum computer from a classical computer.

**Measurement**

What does it mean when I say, measure the system? To answer this question, I would like to recollect Schrodinger’s famous cat in the box experiment which paved the way for Quantum Mechanics. He placed a cat in a box with radioactive material. As the radio-active material starts to decay, it releases poison and the cat would die inhaling it. But we do not know when the material would release the poison. So, without opening the box, we could not tell the state of the cat.

If you put the cat in the box, and if there’s no way of saying what the cat is doing, you have to treat it as if it’s doing all of the possible things — being living and dead — at the same time,” explains Eric Martell, an associate professor of physics and astronomy at Millikin University. “If you try to make predictions and you assume you know the status of the cat, you’re [probably] going to be wrong. If, on the other hand, you assume it’s in a combination of all of the possible states that it can be, you’ll be correct.

So in quantum computers, the qubits can exist in any of the states at any time. Only when we try to measure the system, will we come to know the current state of the system. So, when we measure the system, the state with maximum probability will be seen after measuring.

**Quantum Superposition**

If you take 2 classical bits together, the total possible states will be 2² i.e. Each bit can be either 0 or 1. Therefore total possible states are 00,01,10,11 and only one of these cases will exist at a one-time instance.

In the case of 2 qubits, we know that each qubit can exist as c1|0>+c2|1>. Therefore, if we take both the qubits together, a generalized system would be created as shown below.

(c1*|0>+c2*|1>) (c3*|0>+c4*|1>) = c1’*|00> + c2’*|01> +c3’*|10>+c4’*|11>

The above result on the right-hand side says that the system of 2 qubits will exist as a ‘superposition’ of these 4 states i.e. |00>, |01>, |10>, |11> at any time. Can we say which state is this system currently in? Not until we measure the system.

The two qubits are said to ‘superimpose’ with each other to create this state. There isn’t really a concept of which qubit goes first or when does this qubit enter the system. This is the real crux of the quantum computing theory. The operations are done on the entire set of states in a parallel manner, a concept called Quantum Parallelism. This is the second “extra-ordinary” capability of the qubits that differentiates the operation of the quantum computer from a classical computer.

**Quantum Entanglement**

An entangled system is defined to be one whose quantum state cannot be factored as a product of states of its local constituents. It is an irreversible state.

Quantum entanglement is a physical phenomenon that occurs when pairs or groups of particles interact or share spatial proximity in ways such that the quantum state of each particle cannot be described independently of the state of the others, even when the particles are separated by a large distance. This means that, if two qubits are in an entangled state, and if we measure one of the qubits, we can be definitely sure about the state of the second qubit. This is still an unexplained phenomenon but has many applications in this domain. This capability of qubits can never be replicated on a classical system, hence is very special and “spooky” for Quantum Computers.

**Problems with Quantum Computers**

Although this new technology seems very exciting and innovative, it comes with an equally challenging application of Quantum Physics and Chemistry. And one could only imagine the actual cost of building a quantum computer. A figure around 15 million$ I would say.. MNCs like IBM, Google, Microsoft have made significant progress in establishing these technologies. D-Wave, Rigetti Computing are some new startups primarily focusing on Quantum Computers. Now let's see some of the major issues involved with establishing a Quantum Computer.

**Debugging Challenges**

You can’t examine the wave function of a quantum system or qubit — you can only measure or observe a single observable, which then causes the rest of the wave function to collapse. You can’t examine the probability amplitudes for the basis vectors of a qubit. You can’t set breakpoints in a quantum program, examine quantum state, possibly even change the quantum state, and then continue execution. It will be difficult to successfully run a quantum algorithm end-to-end.

**Quantum Coherence**

One of the most challenging problems involved in quantum computations is that the system should be strictly coherent while the calculations are performed. “Coherence” means that the environment where the system of qubits would be deployed would need a strict sub-zero temperature to show-case their extra-ordinary quantum capabilities as discussed above. Any kind of interaction between the heat waves or noise from the environment with the qubits will cause the entire system to collapse or decohere. So more the number of qubits in the system, the greater the probability of them being affected by these noises.

**Quantum Circuit Simulator**

One may wonder that if these quantum computers have only recently been established, how are the researchers and quantum experts building their quantum algorithms and where do they test it? Quantum circuits is that place. A quantum circuit is an intuitive drag and drop simulator where qubits can be added, gates and operators are merely draggable objects which can be attached to the circuit. The flow of the qubits through these gates and respective transformations, the probabilities and amplitudes of the resulting quantum states can be easily visualized. It is a great replica of the computations in real-world quantum computers. It makes life much easier in terms of creating and testing a new quantum algorithm. There have been several establishments in this area. IBM Q , Forest, Quirk, to name a few. Several python integrated libraries have been created to enable researchers to try and test their algorithms.

**Conclusion**

It can be conceived that if these roadblocks are tackled through technological advancements, soon Quantum Computing will be shaping a new era in the world of technology. Researchers from all around the world have been working to mitigate these challenges and make quantum computers accessible to the public.

Currently, some startups have been really successful in running quantum algorithms on Quantum computers. Cloud-based service will be provided to corporations where they can really leverage the capabilities of a quantum computer. This concept of Hybrid Quantum Computing looks very promising and investments and tie-ups from governments, associations, institutions, and banks have already started pouring in. It will be only time when this mammoth of technology will transform the way people think about computers and their capabilities.

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