NISQ Devices vs Fault Tolerant Quantum Computers

NISQ Devices vs Fault Tolerant Quantum Computers: Main Differences

Quantum computing is an exciting and rapidly advancing field that promises to revolutionize computing. There are two main approaches to building quantum computers, Noisy Intermediate Scale Quantum (NISQ) devices and fault-tolerant quantum computers. Here we explore the key differences between these two types of quantum computers.

What are NISQ devices?

NISQ devices refer to the early quantum computers that are currently being developed by companies like IBM, Google, Rigetti, and others. Some key characteristics of NISQ devices are:

Limited qubit count

  • NISQ devices typically have less than 100 qubits. For example, IBM recently announced a 127-qubit processor.
  • This limits the complexity of algorithms and applications that can run effectively.

Short coherence times

  • The quantum state stored in the qubits decoheres (is lost) rapidly over time.
  • NISQ device qubits may only maintain a coherent state for milliseconds.
  • This restricts the quantum circuit depth before errors accumulate.

High gate error rates

  • The operations manipulating the quantum state also introduce errors.
  • Gate fidelities are ~99.9% for single qubit gates and 90-95% for two qubit gates.
  • Error correction is very limited on current NISQ devices.

So in summary, NISQ devices have limited qubit numbers, qubit coherence times, and gate fidelities. However, they are currently accessible and demonstrate quantum effects on small proof of concept applications.

What are fault-tolerant quantum computers?

Fault tolerant quantum computers refer to the long term goal of building large-scale, fully error-corrected quantum computers. Key features include:

Large qubit arrays

  • Fault tolerant quantum computers will have thousands or even millions of logical qubits.
  • This will allow very complex quantum circuits and algorithms.

Logical qubits

  • Each logical qubit consists of multiple physical qubits for encoding and error correction.
  • This provides resilience against errors and decoherence.
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High gate fidelities

  • Fault tolerant quantum computers will achieve error rates of less than 0.01% per gate operation.
  • This allows very deep quantum circuits with negligible errors.

Full error correction

  • Multiple error correcting codes detect and fix qubit and gate errors.
  • This maintains the integrity of the quantum state for arbitrarily long times.

So in summary, fault tolerant quantum computers will be large, error-corrected, and reliable for running complex quantum algorithms far beyond the scale of NISQ devices. However, they are still a distant goal in quantum computing research and development.

Key Applications of NISQ devices vs fault tolerant quantum computers

NISQ devices and fault tolerant quantum computers will be suited for very different applications due to their different scales and error tolerance.

NISQ device applications

  • Quantum chemistry simulations
  • Small machine learning models
  • Small combinatorial optimization problems
  • Hybrid quantum classical algorithms
  • Prototype algorithms before scaling up

These applications are limited in scale and circuit depth, playing to NISQ strengths while mitigating limitations.

Fault tolerant applications

  • Shor’s algorithm for factoring large numbers
  • Large-scale machine learning
  • Optimizing complex systems
  • High precision quantum chemistry models
  • Running deep quantum circuits
  • General purpose “quantum advantage”

These complex algorithms require deep circuits across thousands+ logical qubits, only achievable on fault tolerant quantum hardware.

So real world quantum advantage aligns with realizing fault tolerant quantum computing. However, NISQ devices pave the way by developing quantum skills and applications to scale up on future fault tolerant hardware.

Hardware platforms

NISQ devices and fault tolerant quantum computers also leverage quite different hardware platforms and technologies.

NISQ hardware

  • Superconducting qubits: Most common qubit type in systems from Google, IBM, Rigetti etc. Based on controlling quantum states in superconducting electrical circuits.
  • Trapped ions: Qubits encoded in internal states of individually trapped and controlled ions. Used by IonQ and Honeywell.
  • Photonics: Qubits encoded in properties of photons transmitted in optical fiber or photonic chips. Used by PsiQuantum, Xanadu etc.

These technologies can currently achieve limited numbers of qubits with moderately high gate fidelities. They are also easier to fabricate and control experimentally.

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Fault tolerant hardware

  • Superconducting qubits: Superconducting qubits are still a candidate technology by adding in error correction codes.
  • Trapped ions: Individual ion control also allows detection and correction of errors at large scale.
  • Silicon spin qubits: Leverage extremely long quantum state coherence times of dopant atoms in silicon.
  • Topological qubits: Exotic quasi-particles that naturally resist external perturbation could enable very robust qubits.

These technologies offer routes to realizing millions of physical qubits and error rates low enough for fault tolerance. Each offers pros and cons around coherence, connectivity, controls and fabrication at scale.

Development timeline

Finally, NISQ devices are available now in the early 2020s, while large fault tolerant quantum computers could still be one or more decades away.

NISQ timeline

  • 2010s – Superconducting qubits first achieve basic gates and small circuits
  • 2020s – NISQ devices are online across academia and industry
    – Rapid scaling to ~1000 qubits throughout the decade
  • 2030s – NISQ systems maximize capability and form factors
    – Ubiquitous access to NISQ as cloud quantum computing service

Fault tolerant timeline

  • 2030s – Demonstration of logically encoded qubits
    – Error correction works but not yet to fault tolerance thresholds
  • 2040s – Systems cross fault tolerance thresholds
    – Factoring records possible with ~1000 logical qubits
  • 2050s – Large-scale fault tolerant systems built
    – Applications in materials science, machine learning, optimization etc.

There are significant technology challenges between today’s NISQ devices and a fully operational fault tolerant quantum computer. However, practical uses of NISQ along the development journey make it an exciting time in the quantum computing industry.


In conclusion, NISQ devices already demonstrate quantum functionality but over limited qubit numbers, circuit depths, and with minimal error correction. In contrast, fault tolerant quantum computers require encoding logical qubits across large qubit arrays sufficient to detect and dynamically correct errors. NISQ devices currently target applications in quantum chemistry, optimization and machine learning that work within a constrained circuit depth. Only fault tolerant systems can achieve the scale and complexity required for transformative quantum algorithms in decryption, materials science and more. Superconducting qubits are currently the most common platform among NISQ devices from IBM, Google and others, while fault tolerant systems may utilize exotic topological qubits or silicon spin qubits to achieve the necessary scale and error tolerance. While widespread fault tolerant quantum computing could still be decades away, NISQ devices offer early hands-on skill development and exploration of quantum advantages in the nearer term.

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Frequently Asked Questions

What is the qubit count range for NISQ devices vs fault tolerant quantum computers?

  • NISQ devices currently have less than 100 qubits but are rapidly increasing, likely to 1000s of qubits throughout the 2020s.
  • Fault tolerant quantum computers will require millions of physical qubits to realize single digit thousands of logical qubits.

How do logical qubits provide fault tolerance?

Logical qubits consist of multiple physical qubits utilizing quantum error correcting codes. This redundancy allows errors in a few physical qubits to be detected and corrected without compromising the logical qubit information.

What enables longer coherence times needed for fault tolerance?

Materials and designs providing very long electron spin or photon storage times enable physical qubit implementations less susceptible to decoherence. Leading candidates are silicon spin qubits or topological qubits.

What are the main technological obstacles to realizing fault tolerant quantum computing?

Extremely high physical qubit densities, reliable two-qubit interactions, and quantum gate fidelities consistently above 99.9% must all be achieved simultaneously on quantum hardware. This requires major materials science and physics breakthroughs to conclusively demonstrate.

When could fault tolerant quantum computers become available?

Most experts predict fault tolerant quantum computers will take at least 10-20 years to realize. Incremental progress through the 2030s could allow small fault tolerant proof of concepts before large commercially relevant systems are possible in the 2040s timeframe.

MK Usmaan