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This is Part 2 of the two-part article on quantum computing. Read Part 1 here.

Quantum Cryptography And Post-Quantum Cryptography

Present-day systems are protected by Rivest-Shamir-Adleman (RSA) encryption, which is based on the fact that it is practically impossible for classical computers to factorise large integers.

Peter Shor surprised the world with his polynomial-time quantum algorithm, which made it theoretically possible for a quantum computer to factorise large positive integers, thereby putting present-day encryption, and hence computer and communications systems, at risk.

A quantum computer powerful enough to run the algorithm to factor large integers may be several decades away, but the effort to build the next generation of encryption schemes resistant to a breach using quantum computers is already ongoing.

There are two approaches. The first one, called post-quantum cryptography, is based on constructing classical cryptographic algorithms that are hard for a quantum computer to break.

The other approach quantum cryptography is to use the properties of quantum mechanics itself to secure data.

Quantum cryptography is defined as using quantum mechanical properties for cryptography tasks, such as quantum key distribution (QKD).

Keys are large binary strings of numbers used to provide security to most cryptographic protocols, like encryption and authentication. Though classical key distribution algorithms like Diffie-Helman provide the secure exchange of keys between two parties, they will be vulnerable in the future to quantum computers.

The first QKD scheme was proposed by Bennet and Brassard in 1984. It is called the BB84 protocol and is based on Heisenberg's Uncertainty Principle. The basic idea for this protocol is that Alice can send a secret key to Bob encoded in the polarisation of a string of photons. If an eavesdropper tries to intercept and read it, the state of the photons will change, revealing the presence of the eavesdropper.

Why Are Quantum Computers Hard To Build?

Qubits, unlike classical bits, need to interact strongly among themselves to form entangled states, which in turn form the basis for computation in quantum computers. But to achieve this experimentally is incredibly hard.

We don't want qubits to interact with the environment because it causes decoherence. Decoherence is the phenomenon due to which quantum effects are visible in the microscopic world, but not in the macroscopic world. The main difference between classical information and quantum information is that we cant observe a quantum state without damaging it in some uncontrolled way. We may not look at quantum computers all the time; nature continuously interferes with them. That's why the information the quantum computer is processing needs to be almost isolated from the outside world.

Why We Believe Quantum Computers Can Be Built

Since Richard Feynman's talk 40 years ago, we have come a long way, but the quantum computers present today are not very useful yet.

We are presently in the NISQ era of quantum computers. NISQ stands for 'Noisy Intermediate-Scale Quantum'. 'Intermediate scale' means that the qubit count is greater than 50 and it cannot be simulated using the most powerful classical supercomputers. 'Noisy' means that these devices are not yet error-corrected.

Through the discovery of polynomial-time factorisation and discrete logarithm algorithms by Shor, the interest in quantum computing skyrocketed, but scepticism regarding quantum computing remained, captured in the saying that it is the computer scientists dream [but] the experimenters nightmare".

Again, it was Shor who showed the way. He discovered quantum error-correcting codes and fault-tolerant methods for executing quantum computations reliably on noisy hardware.

In classical error correction, we measure bits to find out errors, but measuring a qubit will destroy the state of the qubit. Shor found a way to detect errors in the qubit without measuring the state of the qubit itself.

The discovery of error-correcting codes showed that we will be able to scale up quantum computers to the degree that they can solve practical problems, but we will need a lot more qubits and a lower inherent error rate before any such correction is useful.

Industry, Governments Are Interested

The promise of quantum computing has propelled major industry players like IBM, Google, Microsoft, Amazon, Honeywell and Alibaba into pouring billions of dollars into quantum computing research.

Google plans to build a full-scale quantum computer by 2029, one that can be used for solving practical business problems. Companies like IBM have laid out technology development milestones to develop a scalable and fault-tolerant quantum computer.

Startups are not falling behind in investment. Several millions of dollars are invested into startups like Rigetti computing, IonQ, Xanadu and PsiQuantum to develop quantum computers.

Governments across the world are pumping billions of dollars into quantum computing research. In 2019, the United States National Science Foundation (NSF) and Department of Energy (DOE) committed to spending $1.2 billion over a period of five years to support quantum computing research.

Similarly, China has included quantum technology as one the high-technology investment areas in its 14th five-year plan. India too has announced a National Mission on Quantum Technologies & Applications (NM-QTA).

While investment of billions of dollars into quantum computing will not immediately result in a practically usable quantum computer, the future promise of the power that quantum computing may deliver has set in motion a flurry of investments into the field.

Quantum Technology In India, In Brief

The Indian government in its 2020 budget announced the Rs 8,000 crore ($ 1.2 billion) NM-QTA. The mission aims to focus on fundamental science and technology development, and to help prepare the next generation of workforce, encourage entrepreneurship, and address issues concerning national priorities.

In India, the Defence Research and Development Organisation (DRDO) and Indian Space Research Organisation (ISRO) have made strides on the quantum communication front.

Not long ago, DRDO demonstrated QKD between Prayagraj and Vindhyachal in Uttar Pradesh over a 100 km fibre optic link.

ISRO, on the other hand, demonstrated quantum entanglement-based real-time QKD over a 300 m atmospheric channel. This is a step towards the development of the planned satellite-based quantum communication (SBQC).

Efforts at building a quantum computer in India presently seem to be limited to academic efforts.

The Future

Noise severely limits the scale of computations in NISQ-era devices. We expect to overcome this issue in the long run using quantum error correction and fault-tolerant quantum computing (FQTC), but the number of qubits required to run these error-correcting schemes is very high and depends on the algorithms we are trying to run and the quality of the hardware.

Present-day quantum computers are not capable enough to replace supercomputers, given the fact that the scaling of the number of qubits remains a challenge.

In 2019, Google demonstrated quantum supremacy using its 53-qubit quantum computer. It means that a programmable quantum device can solve a problem that no classical computer can solve in any physical time. This may give the idea that quantum computers have become more powerful than classical computers, but the problem that was solved in the quantum computer is random in nature and doesnt have any practical significance in real life.

The path to fault-tolerant and error-corrected quantum computers will remain difficult due to the fragile nature of qubits, but the possibilities quantum computers offer makes the pursuit worthwhile.

This concludes the two-part article on quantum computing. Read Part 1 if you haven't already.

This article has been published as part of Swasti 22, the Swarajya Science and Technology Initiative 2022. We are inviting submissions towards the initiative.

Also Read:

Are We Close To Realising A Quantum Computer? Yes And No, Quantum Style

New Quantum Tech Hub At IISER Pune: Quantum Computers, Sensors, Clocks And More On The Cards

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Quantum Computers: Why They Are Hard To Build And Worth The Effort - Swarajya