**Quantum computers — the doom of blockchain?**

Blockchain technology is barely 10 years old but it’s not too soon to ask if blockchain security will be possible in a post-quantum world. Let me be more direct: Will quantum computers render blockchain technology obsolete?

The theory behind quantum computing was first laid out in the 1980s. Yet, it was not until recently that practice caught up with theory, enabling the construction of the first quantum computers. An unchallenged pioneer in this technology is the Canadian company D-Wave Systems. Its clients include the CIA and the National Security Agency (NSA), many research institutes, NASA, and businesses including Google and Lockheed Martin. The European Union plans to allocate a billion euros to quantum research. Tech companies are developing their own technologies anticipating diverse applications for the awesome computational power that can be derived from quanta, the fundamental building blocks of matter.

**The evening of Moore’s Law**

Why is so much being spent on quantum computing? Why is it such a huge breakthrough?

Today’s processors are made up of billions of transistors a few nanometers in size, packed into a very small space. **According to ****Moore’s Law****, the number of transistors that fit into a microprocessor doubles roughly every two years.**Unfortunately, or inevitably, increases in the processing power of chips have been plateauing. We are approaching the technological limits of how many transistors can be jammed into such a small space. **The borderline that cannot be crossed is a transistor the size of a single atom with a single electron used to toggle between the states of 0 and 1.**

The simplest way to demonstrate the advantages of the quantum computer is to compare it with the classical machine. The familiar device we know from our daily work relies for all its operations on basic information units called bits. These, however, can only represent two states: 0 or 1.

In quantum computing, it’s possible to use intermediate, non-binary states that liberate us from the bondage of 0 and 1, two opposing values. The qubit(or quantum bit), which is what the information units used by quantum devices are called, can assume the values of 0 and 1 *simultaneously*. In fact, qubits can assume an infinite number of states *between*0 and 1, achieving what is referred to as the superposition. Only when the value of a qubit is observed does it ever assume either of the two basic states: 0 or 1.

This may seem like a minor difference, but a qubit remaining in superposition can perform multiple tasks at the same time. We are helped here by the operation of two fundamental laws of quantum physics. Physically, a qubit can be represented by any quantum set to two different basic states: two energy levels in an atom, or two levels of photon polarization, vertical or horizontal. Therefore, while a bit in a classical computer holds one of two values (0–1 or 1–0), and two bits hold one of four values, and so on, two qubits hold not two but four values at any given time while 16 qubits may hold as many as 65536 values simultaneously, or 16 squared. The number of possibilities doubles for every qubit added, allowing a quantum machine to process far more data than can a binary computer in an incredibly short time.

Imagine a volume of data so big it would take millions of years to process by means of a classical computer. This would not be a problem for a quantum machine. It can process data hundreds of thousands and, ultimately, millions of times faster than machines made up of even the most sophisticated silicon components. **The difference in capacity between ****quantum and conventional computers****can theoretically amount to an astounding 1:18 000 000 000 000 000 000 times!**

Such a computer could sift through and recognize objects in a giant collection of photographs. It would be perfect for big number processing, encryption and code breaking.

Or, blockchain breaking.

**The kiss of death for cryptocurrencies**

According to some researchers, once quantum computers rise and spread, they could be used to crack the cryptographic protections responsible for the operating model and security of blockchain technology — the technology on which cryptocurrencies are based.

Collectively, on January 3, 2018, cryptocurrencies were worth an estimated USD 700 billion. This certainly makes them worth fighting for. **What makes blockchain technology vulnerable to the threat of quantum computers?**Blockchain architecture is protected by two types of security keys: private and public. To make a cryptocurrency transaction, the buyer shares a public key with its seller, while the latter uses a private keyto acknowledge receipt. Should anyone other than the seller or buyer acquire the private key, they would gain control of the transaction. The private key can either be stolen or broken by the brute force of enormous computational power. The emergence and spread of quantum computers will render the blockchain technology’s algorithms useless. **A holder of a quantum computer will be able to calculate the private key using the public key.**This will give the code holder unfettered access to all world’s wallets holding all the world’s cryptocurrencies.

However, even though it can crack a private key in minutes, the cost of a quantum computer will make that a very expensive operation.

But $700 billion is a powerful incentive.

**Not all is lost**

The easiest way to secure keys in the face of quantum computing would be to have the cryptocurrency community adopt a more sophisticated set of cryptographic standards. The technology to do so is out there. However, any modifications require the consent of the entire cryptocurrency community, with separate consents for each cryptocurrency. Considering that a recent attempt to get all users to agreeto an increase in the volume of bitcoin (BTC) blocks — from 2MB to 4MB — has failed miserably, reaching a consensus for upping security standards may prove equally elusive. The blockchain protocol requires 80% of currency users to approve any change. Since doubling the bandwidth, and significantly accelerating transactions would benefit everyone, that would appear to be a no-brainer. And yet, as it turned out, not everyone saw it that way.

On the other hand, by the time quantum computers become widely available, the cryptocurrency community may well recognize the threat and begin to see eye to eye on updating cryptographic standards. That would keep blockchain and the cryptocurrency technology secure from quantum computers well into the future.

**Devilishly fast but not unlimited**

A quantum computer requires a control system (the equivalent of an operating system), algorithms to make quantum calculations and proper calculation software. The development of quantum algorithms is very difficult as they need to rely on the principles of quantum mechanics. The algorithms followed by quantum computers rely on the rules of probability. What this means is that by running the same algorithm on a quantum computer twice, one may get completely different results as the process itself is randomized. To put it simply, to arrive at reliable calculation with a quantum computer, one must factor in the laws of probability — a complex process indeed!

Quantum computers are suited for very specialized and specific calculations as well as algorithms that help harness all their powers. In other words, quantum computers will not appear on every desk or in every home. However, regardless of how much time is needed to generate a given result by means of an algorithm, we can imagine, even today, a situation in which a quantum machine, and only a quantum machine, could solve a problem that mankind desperately needs to solve.

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