The Quantum Investor's Notebook logo The Quantum Investor's Notebook
← Back

Thursday, June 18, 2026

The Physical Qubit: The Fragile Raw Material

A logical qubitA reliable 'qubit' built by bundling many error-prone physical qubits together with error correction. These are the units that actually matter for useful computing. is the finished product. The physical qubitAn actual piece of qubit hardware. On its own it is fragile and makes frequent errors. is the raw material: one real, fragile piece of quantum hardware, and the thing every quantum computer is ultimately made of.

What a physical qubit actually is

The thing a physical qubit is, is an actual two-level quantum system built into hardware, a real object you could in principle point at.1 It might be a superconducting circuit chilled to near absolute zeroThe coldest temperature physically possible (about -273.15 Celsius), where atomic motion almost stops., a single trapped ionA qubit made from a single electrically charged atom held in place by electromagnetic fields and controlled with lasers. pinned in place by fields and lasers, or the spin of one electron. Whatever the material, the job is the same: hold one qubitThe basic unit of a quantum computer. Like a 'bit' in a normal computer, but instead of being only 0 or 1 it can be 0, 1, or a blend of both at once.'s worth of quantum information, a superpositionThe quantum ability of a qubit to be in a combination of 0 and 1 simultaneously, which gives quantum computers their power. of 0 and 1, long enough to compute with. The trouble is that long enough turns out to be very short.

The job is to hold one qubit's worth of information long enough to compute. The trouble is that long enough is very short.

Not every quantum system can do that job. A usable physical qubit has to clear a short checklist, the DiVincenzo criteriaThe short checklist a physical system must meet to work as a qubit: it must scale, be controllable by a few operations, and let errors be removed.: you must be able to make many of them, set them to a known starting state, keep them coherent for longer than your operations take, run a universal set of gates on them, and read them out reliably.1 Most candidate systems fail at least one, which is why only a handful of platforms, or modalities, are seriously in the race.

A single physical qubit holding a superposition of 0 and 1 is struck by noise from its environment and decoheres into a plain 0 or 1. Two notes: T1 is energy leaking out, and T2 is the qubit's phase scrambling, which happens faster.
Figure 1: A single physical qubit, left to itself. Noise from the environment scrambles its state (decoherence): its energy drains on the T1 timescale and its phase scatters even faster on T2. This fragility is why one physical qubit is never enough.

Real hardware, real numbers

So how good are today's physical qubitsAn actual piece of qubit hardware. On its own it is fragile and makes frequent errors.? It depends entirely on the platform, and the spread is enormous.2 Trapped ionsA qubit made from a single electrically charged atom held in place by electromagnetic fields and controlled with lasers. are the steady ones, holding their state for seconds. Superconducting qubitsA qubit made from tiny electrical circuits chilled to near absolute zero, where they lose all electrical resistance. are far faster to operate but twitchier, historically keeping their state for only microseconds, though that figure has climbed sharply since.3 At the accurate end, the best trapped-ion operations now clear 99.9 percent fidelity.4

Every platform is a trade between speed, stability, and how hard it is to build many of them.

Two numbers carry most of the story. The coherence timeHow long a qubit holds its delicate quantum state before noise scrambles it. Longer coherence means more time to compute. is how long a qubitThe basic unit of a quantum computer. Like a 'bit' in a normal computer, but instead of being only 0 or 1 it can be 0, 1, or a blend of both at once. keeps its state before noise scrambles it; the gate fidelityA score (for example 99.9%) for how accurately a quantum gate does what it is supposed to do. Higher means fewer errors. is how accurately you can operate on it. Both are measured, both are climbing, and neither is yet good enough on its own.

Why one is never enough

The reason is decoherenceThe loss of a qubit's quantum information as it interacts with the outside world. The main reason qubits are fragile.. Left to itself, a physical qubit leaks its information into the surroundings: its energy drains away on a timescale called T1, and its delicate phase scrambles even faster, on a timescale called T2.2 That second one is the real enemy, because it costs no energy and so happens all the time. No amount of careful engineering has yet made a single physical qubit steady enough to carry a long calculation by itself.

I will be honest about where that leaves us. The physical qubit is the foundation, and on its own it is not good enough. Its error rates sit above what fault toleranceThe milestone where a quantum computer can run long calculations correctly despite ongoing errors. It is the field's holy grail. demands, so the only known path is to spend many physical qubits building one reliable logical qubitA reliable 'qubit' built by bundling many error-prone physical qubits together with error correction. These are the units that actually matter for useful computing. through quantum error correctionTechniques that combine many shaky physical qubits into fewer reliable ones, so a long calculation stays correct.. Improving the raw material and bundling it into the finished product are the same race run from both ends: every gain in the physical qubit lowers the price of the logical one.

Sources

  1. DiVincenzo, D. P. “The Physical Implementation of Quantum Computation.” Fortschritte der Physik 48, 771–783 (2000). DOI: 10.1002/1521-3978(200009)48:9/11<771::AID-PROP771>3.0.CO;2-E. Preprint: arXiv:quant-ph/0002077.
  2. Ladd, T. D., Jelezko, F., Laflamme, R., Nakamura, Y., Monroe, C. & O’Brien, J. L. “Quantum Computers.” Nature 464, 45–53 (2010). DOI: 10.1038/nature08812. Preprint: arXiv:1009.2267.
  3. Kjaergaard, M., Schwartz, M. E., Braumüller, J., Krantz, P., Wang, J. I.-J., Gustavsson, S. & Oliver, W. D. “Superconducting Qubits: Current State of Play.” Annual Review of Condensed Matter Physics 11, 369–395 (2020). DOI: 10.1146/annurev-conmatphys-031119-050605. Preprint: arXiv:1905.13641.
  4. Ballance, C. J., Harty, T. P., Linke, N. M., Sepiol, M. A. & Lucas, D. M. “High-Fidelity Quantum Logic Gates Using Trapped-Ion Hyperfine Qubits.” Physical Review Letters 117, 060504 (2016). DOI: 10.1103/PhysRevLett.117.060504. Preprint: arXiv:1512.04600.