Frequently Asked Questions on Quantum Computing…Part 3

What about quantum advantage?

Quantum advantage is when a quantum computer can do a practical task sufficiently better than a classical computer to warrant switching between the technologies. Some argue chasing quantum supremacy is mostly a useless stunt since it has no practical application. It seems businesses hoping to sell quantum computers or access quantum computers — incline towards this view.

How do we quantify system performance in quantum computers?

There is no simple agreed-upon formula that fully describes the power of a quantum processor. All that can be said is that the higher the qubit count, quality level, and connectivity, the better. The number of logical qubits is a function of the specific error correction algorithm that is used with the physical qubits. It is not directly related to the qubit quality. However, the lower the qubit quality, the more error correction you may want to put in.

Counting the number of qubits in a quantum computer to determine computational power is too simplistic to be functionally useful — differences in how individual qubits are connected, how the qubits themselves are designed, and environmental factors make this type of comparison inequitable.

Quantifying system performance plays a key role in assessing the progress toward achieving “quantum advantage” — when for certain practical use cases, we can definitively demonstrate a significant performance advantage over today’s classical computers. As a measure of quantum computational power, increases in quantum volume correlate with the ability to solve larger, more complex problems across a range of disciplines.

In case you are interested according to IBM, the quantum volume is the largest computational space a quantum computing device can explore. It is measured by calculating the number of physical qubits, connectivity between qubits, and time to decoherence, as well as the available hardware gate set, and number of operations that can be run in parallel. This discrete quantity scales exponentially with the number of qubits. A system that successfully searches a four-qubit space has quantum volume ²⁴ = 16.

Before I go you may also hear of ‘quantum ready’ — it means to be prepared to take full advantage of the quantum computing era as it arrives.

Why is it so important to verify the performance of quantum hardware?

Current capabilities of qubits allow us to do simulation to a reasonable enough level of accuracy. Quantum computation is rather more demanding in terms of error correction. It’s because precisely controlling a quantum computer is notoriously difficult. In a sense, merely looking at a quantum system unavoidably disturbs it, a manifestation of Heisenberg’s famous uncertainty principle. So if we want to use such a system to store and reliably process information, we need to keep that system nearly perfectly isolated from the outside world. At the same time, though, we want the qubits to interact with one another so we can process the information; we also need to control the system from the outside and eventually measure the qubits to learn the results of our computations. It is quite challenging to build a quantum system that satisfies all of these desiderata, and it has taken many years of progress in materials, fabrication, design, and control to get where we are now.

The apparent quantum supremacy achievement marks just the first of many steps necessary to develop practical quantum computers. The fragility of qubits makes it challenging to maintain specific quantum states over long periods when performing computational operations. That means it’s far from easy to cobble together large arrays involving the thousands or even millions of qubits that will likely be necessary for practical, general-purpose quantum computing.

Such huge qubit arrays will require error correction techniques that can detect and fix errors in the many individual qubits working together. A practical quantum computer will need to have full error correction and prove itself fault-tolerant — immune to the errors in logical operations and qubit measurements — to truly unleash the power of quantum computing.

If quantum supremacy was achieved, what would it mean for the QC community?

The next milestone would be to achieve quantum computational supremacy and useful quantum error-correction in the same system. Perhaps, firstly we may want to use a programmable QC, with qubit count in the range of 50 to 100 to do some useful quantum simulation (say, of a condensed-matter system like NV centers in diamond) much faster than any known classical method could do it. A second obvious milestone would be to demonstrate the use of quantum error-correction, to keep an encoded qubit alive for longer than the underlying physical qubits remain alive.

Is quantum annealing the same as quantum computing?

The difference between a quantum annealer and quantum computer is that a computer is a fully programmable device — one that you can program with an arbitrary sequence of nearest-neighbour 2-qubit gates, just by sending the appropriate signals from your classical computer whilst a quantum annealer is useful for a single type of calculation called “quadratic unconstrained binary optimization (QUBO)”. General-purpose quantum computers can be used for a wider variety of calculations.

Is the future of computation hybrid?

There are two approaches to hybrid; hybrid quantum computer and hybrid computer (classical computer combined with a quantum computer).

Hybrid quantum computers: Qubits themselves are incredibly powerful, yet delicate. They quickly lose their special quantum properties, typically within 100 microseconds (for state-of-the-art superconducting qubits), due in part to electromagnetic environment, vibrations, and temperature fluctuations. To make quantum computers more reliable and stable there is a need to harmoniously combine different technologies that complement each other.

Hybrid computer: What’s meant here is not just making control electronics more efficient and nearby (lowering latency) but optimizing algorithms such that the portions best suited for the quantum processor are run on it while other portions are optimized for classical systems. All quantum computers are blended systems in which classical computers play an essential role. Accenture has patents to that effect. Hybrid computing means utilizing the best of both the quantum and classical worlds, and lowering the barriers for companies of all sizes to get started using quantum computers.

Read on Part 4 here