What is the longest time a qubit has survived with 0.9999 fidelity?

Question

I am pretty intrigued by the record time that a qubit has survived.

Answer 1

Well, for the longest coherence time ever, I'm finding this Science from 2013 entitled Room-Temperature Quantum Bit Storage Exceeding 39 Minutes Using Ionized Donors in Silicon-28, which indicates qubits that lasted for over 39 minutes; these, however, only had an 81% fidelity rate. (This is for qubits used in computation, not memory storage. For memory storage, see M. Stern's link.)

But you're looking for qubits with a high fidelity rate. In that case, I found a Nature Nanotechnology from 2014 entitled Storing quantum information for 30 seconds in a nanoelectronic device(alternate link to arXiv) which was coherent for 30 seconds - but had a greater than 99.99% fidelity rate, which is exactly what you're looking for. Most other papers I'm finding with a 99.99% fidelity rate or greater measure their coherence times in nano or microseconds.

I will keep looking.

Answer 2

Answer: Fidelity of 0.9999 at 1.08 seconds in 2013: http://science.sciencemag.org/content/342/6160/830.full?ijkey=uhZaDNPnwgTdA

More details: The $T_2$ was 180 minutes, or 3 hours.

What about the 81% that Heather mentioned?: The fidelity of 81% that Heather quotes, was actually referring to something else. In the same paper they wanted to show that they could change the temperature of the sample while still maintaining the spins in a coherent superposition. The sample was increased in temperature from 4.2K to 300K gradually over 6 minutes, held there for 2 minutes, then reduced back to 4.2K gradually over 4 minutes. After doing all that, the spins had impressively maintained a fidelity of 81% with respect to the starting state.

But that 12 minute experiment where they wanted to show that they can maintain coherence even when majorly disturbing the thermal equilibrium of the sample, was far less than the 3 hours $T_2$ they measured in an experiment where the coherence survived for 300 minutes (5 hours) with temperature kept constant at 1.2K.

What about the 2014 paper with 0.9999 fidelity?: This comes from Figure S2c in the Supplement, which is only up to 0.0002 seconds. If you want to get the fidelity at 30 seconds, or at 180 minutes, look at the $T_2$ times in Fig S1 of the supplement, and you will see that all of these are orders of magnitude smaller than what it was in the 2013 paper.

The authors admit this 3 times:
1) "Despite the record coherence times discussed above, our results do not match those obtained in bulk ensembles[6–8]" Reference 8 is the 2013 paper.
2) "This currently represents the record coherence for any single qubit in the solid state." Note they say "single" qubit and "solid state".
3) "which reach here a new record for solid-state single qubits with $T_2$ > 30 s in the $^{31}$P$^+$spin" Note the 30s is a T2!! This is much smaller than the $T_2$ = 180 minutes mentioned above.

Answer 3

At risk of causing a forever updated list of $T_2$ world records, I think the current record actually goes to ion trap qubits at over one hour. In fact, the limit here basically comes from how good your magnetic shielding is and how large your vacuum pumps are, so I would expect $T_2$ times on the span of days to be possible, as least for a setup optimized for breaking records (and not a whole lot of quantum computation)

https://www.nature.com/articles/s41467-020-20330-w