48 Fully-connected Near-perfect Qubits As the Sweet Spot Goal for Near-term Quantum Computing

In a nutshell

Quantum computing desperately needs to show some realistic results addressing more than toy-like problems within the next two to three years if a Quantum Winter is to be averted

What’s wrong with current quantum computers?


Essence of the proposal

Finally enable the use of quantum phase estimation to support quantum computational chemistry and finally enable at least a hint of actual quantum advantage

Quantum Fourier transform (QFT) as the critical algorithmic building block

Little data with a big solution space — the sweet spot for quantum computing

Near-term: The next two to three years

Quantum parallelism — simultaneously evaluate many alternative solutions in parallel

Quantum computational leverage — the degree of quantum parallelism — how many alternative solutions are simultaneously evaluated in parallel

Quantum advantage in the range of 1,000 to 1,000,000 X

Sorry, but dramatic quantum advantage will remain out of reach

Quantum advantage depends on the algorithm

Raw quantum computational leverage must be discounted by shot count to get net quantum advantage

Start with an upgraded 27-qubit quantum computer

Maybe even a 36-qubit stepping stone

Need near-perfect qubits with 3.25 to 4 nines of qubit fidelity

Lack of fine granularity of phase and probability amplitude limit quantum Fourier transform to 20 to 32 qubits

20 bits of granularity for phase will support a 20-qubit quantum Fourier transform for quantum computational leverage of one million to one

Quantum phase estimation (QPE) will finally be enabled

Quantum amplitude estimation (QAE) will finally be enabled

Why 48 qubits and not 56, 64, 72, 80, 96, 128, 160, or 256?

More than 48 qubits doesn’t help if qubit fidelity and connectivity aren’t there and if quantum Fourier transform precision is less than 24 qubits

Lack of fine granularity of phase may be the ultimate limit which makes 48 qubits the largest configuration which can effectively use a quantum Fourier transform

48 qubits is likely about as high as we can go with full state vector simulation

Fantasizing about a 72-qubit quantum computer

Need for a quantum state bus or dynamically-routable resonators for enhanced connectivity for transmon qubits

Quantum state bus or dynamically-routable resonators

Is extensive classical IP a severe impediment to pursuing a quantum state bus or dynamically-routable resonators?

Greater total circuit size and maximum circuit depth

Where are all of the 40-qubit algorithms?

Where are all of the scalable algorithms?

Quantum Volume (QV) may be limited

Other technical risks

Will this even be feasible?

When? Two to three years, or so, seems like a solid goal

Earliest availability?

Will this be enough? For who? Maybe… it will vary

Will this be enough for quantum computational chemistry with quantum phase estimation? Maybe… it will vary

What about the impact on other quantum application categories? Impact will vary

How best to prepare for this 48-qubit future? Focus on simulation of 24 to 40-qubit quantum algorithms, and scalable algorithms

Ramp up efforts for more powerful simulators — shoot for 48 qubits

Of course we want to get way beyond this ASAP, but we’re not even close to this yet

A credible and palpable goal to aim at

A clear path to avert a potential Quantum Winter

What’s next after this proposal has been fully implemented? Unclear and uncharted territory

No clear roadmap to outline the path for quantum algorithms that can effectively exploit more than about 48 qubits for real quantum computers

Quantum error correction (QEC)? Not yet required

This proposal doesn’t rely on the distant fantasy of full quantum error correction


Qubit counts for IBM

What about Shor’s factoring algorithm?

My original proposal for this topic

Summary and conclusions



Freelance Consultant

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