Quantum error correction can preserve quantum information in the presence of local errors, but correlated errors are fatal. For superconducting qubits, high-energy particle impacts from background radioactivity produce energetic phonons that travel throughout the substrate and create excitations above the superconducting ground state, known as quasiparticles, which can poison all qubits on the chip. We use normal metal reservoirs on the chip back side to downconvert phonons to low energies where they can no longer poison qubits. We introduce a pump-probe scheme involving controlled injection of pair-breaking phonons into the qubit chips. We examine quasiparticle poisoning on chips with and without back-side metallization and demonstrate a reduction in the flux of pair-breaking phonons by over a factor of 20. We use a Ramsey interferometer scheme to simultaneously monitor quasiparticle parity on three qubits for each chip and observe a two-order of magnitude reduction in correlated poisoning due to background radiation.

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AbstmctAn overview of the NA!lTONAL ELECTRIC CODE* requirements for enclosures for arcing devices in Class I locations is presented, as well as the theory of explosionproof protection and the standards applicable to enclosures constructed with that type of protection. Three factors which bear on the satisfactory field performance of such *pent are summized: installation, environment and mamtenance. Proper maintenance procedures applicable to explosionproof enclosures are suggested and the results of improper maintenance are demonstrated through highlights of actual laboratory tests.