On a realistic interpretation of spontaneous decay processes

Sun, Yuan

As a fundamental phenomenon in quantum systems, spontaneous emission constitutes an inevitable source of error, which ultimately degrades the fidelity of quantum logic gates. A successful quantum logic gate needs to operate on the condition that no decay event, such as spontaneous emission, occurs. Such successes can be ensured by post-selection based on syndrome extraction according to the theory of quantum error correction or quantum error mitigation. In this case, the wave function of qubits remains a pure state but is subject to additional influences from spontaneous emission, even without actual decay events. Therefore, such a process must be appropriately described by a modified version of Schrödinger equation for the dynamics conditioned on no-decay. Calculations reveal that this effect must be seriously taken into consideration for the design of high-fidelity quantum logic gates. With respect to realistic experimental conditions, even if the coherence is well preserved, improving the fidelity of manipulating physical qubits requires careful consideration of the subtle influences of decay processes such as spontaneous emission. Specifically, the gate and readout processes in the atomic qubit platform are discussed.

N. A. Enaki

Kern, O, Alber, G

The stabilizing properties of one-error correcting jump codes are explored under realistic non-ideal conditions. For this purpose the quantum algorithm of the tent-map is decomposed into a universal set of Hamiltonian quantum gates which ensure perfect correction of spontaneous decay processes under ideal circumstances even if they occur during a gate operation. An entanglement gate is presented which is capable of entangling any two logical qubits of different one-error correcting code spaces. With the help of this gate simultaneous spontaneous decay processes affecting physical qubits of different code spaces can be corrected and decoherence can be suppressed significantly.

H. Ekstein, A. J. F. Siegert

John N. Dodd

Xihua Yang, Mingfei Cheng, Xiaodong Zeng +2 more

In contrast to the general argument that the spontaneous decay is intrinsically incoherent in nature and harmful to entanglement, here, we show that nearly perfect entanglement between two bright pump fields can be realized via spontaneous decay-induced destructive quantum interference in a closed double \ensuremath{\Lambda}-type four-level system with the energy separation of the excited doublet comparable to their decay rates. The high degree of bipartite entanglement results from the cancellation of spontaneous emission and subsequent elimination of the associated noise due to the destructive quantum interference between the two spontaneous emission pathways from the upper doublet to each of the two lower levels when the two strong pump fields are tuned to the particular frequencies where quantum interference takes place. This scheme is particularly suitable for the generation of entanglement between two bright light fields, and may find potential applications in realistic quantum information processing.