Quantum Fault Tolerance: Beyond Requirements and Desiderata

1. Beyond the DiVincenzo Criteria:Requirements and Desiderata forFault-Tolerance Daniel Gottesman

2. The DiVincenzo Criteria • A scalable physical system with well-characterized qubits. • The ability to initialize the state of the qubits to a simple fiducial state, such as . • Long relevant decoherence times, much longer than the gate operation time. • A “universal” set of quantum gates. • A qubit-specific measurement capability. • The ability to interconvert stationary and flying qubits. • The ability to faithfully transmit flying qubits between specified locations.

3. Requirements for Fault-Tolerance • Low gate error rates. • Ability to perform operations in parallel. • A way of remaining in, or returning to, the computational Hilbert space. • A source of fresh initialized qubits during the computation. • Benign error scaling: error rates that do not increase as the computer gets larger, and no large-scale correlated errors.

4. Additional Desiderata • Ability to perform gates between distant qubits. • Fast and reliable measurement and classical computation. • Little or no error correlation (unless the registers are linked by a gate). • Very low error rates. • High parallelism • An ample supply of extra qubits. • Even lower error rates.

5. Concatenated Codes Threshold for fault-tolerance proven using concatenated error-correcting codes. Error correction is performed more frequently at lower levels of concatenation. One qubit is encoded as n, which are encoded as n2, … Effective error rate

6. Parallel Operations Fault-tolerant gates are easily parallelized. Error correction operations should be applied in parallel, so we can correct all errors before decoherence sets in. Threshold calculations assume full parallelism.

7. Erasure Errors For instance: loss of atoms Losing one is not too serious, but losing all is fatal. Erasures are a problem for: • Quantum cellular automata • Encoded universality

8. Fresh Ancilla States We need a constant source of fresh blank qubits to perform error correction. Thermodynamically, noise introduces entropy into the system. Error correction pumps entropy into cold ancilla states. Data Used ancillas become noisy. Ancillas warm up while they wait. Ancilla

9. Fresh Ancilla States Used ancillas can be replaced by new ancillas, but we must ensure ancillas do not wait too long: otherwise, there is an exponential loss of purity. In particular: • It is not sufficient to initialize all qubits at the start of computation. For instance, this is a problem for liquid-state NMR.

10. Large-Scale Error Rates The error rate for a given qubit should not increase when we add more qubits to the computer. For instance: • Long-range crosstalk (such as 1/r2 Coulomb coupling) (Short-range crosstalk is OK, since it stops increasing after neighbors are added.)

11. Correlated Errors Small-scale correlations are acceptable: We can choose an error-correcting code which corrects multiple errors. Large-scale correlations are fatal: A large fraction of the computer fails with reasonable probability. Note: This type of error is rare in most systems.

12. Error Threshold The value of the error threshold depends on many factors. With current error-correction circuitry and all other desiderata: • Provable threshold for combined gate and storage errors of about 10-4. • Actual threshold: perhaps 10-3. • With better circuits: maybe 10-2? Without desiderata, threshold decreases.

13. The Meaning of Error Rates Cited error rates are error probabilities; that is, the probability of projecting onto the correct state after one step. E.g.: Rotation by angle q has error probability q2. • Gate errors: errors caused by an imperfect gate. • Storage errors: errors that occur even when no gate is performed. Error rates are for a particular universal gate set.

14. Long-Range Gates Most calculated thresholds assume we can perform gates between qubits at arbitrary distances. (For instance, this might be possible if we can link to quantum communication lines.) If not, we need better error rates to get a threshold, since we use additional gates to move data around during error correction.

15. Long-Range Gates Threshold still exists with only local gates: We must arrange computer so error correction can be done with mostly local interactions. Optimal arrangements are not well-studied, but: • Storage threshold 10-4 with local gates (using topological codes). • Most frequent gates are between nearby qubits, so medium-range interactions may be sufficient.

16. Fast Classical Processing Fast measurement and classical processing is very useful for error correction to compute the actual type and location of errors. We can implement the classical circuit with quantum gates if necessary, but this adds overhead: the classical circuit must be made classically fault-tolerant. Threshold unknown in this case.

17. Correlated Errors Redux Small-scale correlations are not fatal, but are still better avoided. We assume correlated errors can occur when a gate interacts two qubits. Any other source of multiple-qubit errors is an additional error rate not included in the threshold calculations. The worst case is correlated errors within a block of the code, but the system can be designed so that such qubits are well separated.

18. Not Dangerous: Coherent Errors Coherent errors can add error amplitudes, not error probabilities. However, this is only in the worst case; random coherent errors will instead add like probabilities. Rotation by q: Prob. Rotation by 2q: Prob. Threshold calculations assume incoherent errors, so proof requires squaring threshold when coherent errors are dominant. However, EC circuits mix coherent errors between qubits, preventing worst case (unproven).

19. Not Helpful: Restricted Error Model Error rates assume all kinds of error are possible. However, restricting the types of possible error (or likely error) does not help very much: • Performing gates on a state tends to mix different types of error. • Difficult to design error-correcting codes and fault-tolerant protocols for other errors. Note: other approaches may help here.

20. Top Ten Reasons Your Quantum Computer Doesn’t Work Lowest contractor bid: $19.99 (large gate errors). Computer refuses to start without morning cup of coffee (no initialization). Built from pieces of crashed UFO (not scalable). It’s been in the fridge for longer than the moldy bread (no fresh qubits). The dog ate my computer (correlated errors).

21. Top Ten Reasons Your QuantumComputer Doesn’t Work Built with ideal qubit system: neutrinos (no universal gates). Gate queuing designed by Disney (no parallel operations). Qubit union has mob ties (erasure errors). Operated by Florida elections committee (unreliable measurement). Unionized qubits insist on long breaks (short decoherence time).