Dephasing mechanisms of diamond-based nuclear-spin memories for quantum networks

Abstract

We probe dephasing mechanisms within a quantum network node consisting of a single nitrogenvacancy centre electron spin that is hyperfine coupled to surrounding 13 C nuclear-spin quantum memories. Previous studies have analysed memory dephasing caused by the stochastic electron-spin reset process, which is a component of optical internode entangling protocols. Here, we find, by using dynamical decoupling techniques and exploiting phase matching conditions in the electron-nuclear dynamics, that control infidelities and quasi-static noise are the major contributors to memory dephasing induced by the entangling sequence. These insights enable us to demonstrate a 19-fold improved memory performance which is still not limited by the electron reinitialization process. We further perform pump-probe studies to investigate the spin-flip channels during the optical electron spin reset. We find that spin-flips occur via decay from the meta-stable singlet states with a branching ratio of 8(1):1:1, in contrast with previous work. These results allow us to formulate straightforward improvements to diamond-based quantum networks and similar architectures. arXiv:1802.05996v1 [quant-ph]

A link layer protocol for quantum networks

“…Noise due to generation: One may wonder why one does not continuously trigger entanglement generation locally whenever the node wants a pair, or why one does not continuously produce pairs and then this entanglement is either discarded or otherwise made directly available. In the NV system, triggering entanglement generation causes the memory qubits to degrade faster [58,82]. As such we would like to achieve agreement between nodes to avoid triggering unless entanglement it is indeed desired.…”

“…Between every entanglement attempt, the electron spin (communication qubit) needs to be reset. The dominant source of noise on the nuclear spins (memory qubits) during the entanglement attempts is due to this re-initialization of the electron spin, as described in [58]. We model the noise on the nuclear spins as a fixed amount of dephasing noise…”

“…see equation (2) in [58]. If the length of the Bloch vector in the equatorial plane of the state in the nuclear spin is before the entanglement attempts r X Y , then after N attempts the length will be…”

Spin measurements of NV centers coupled to a photonic crystal cavity

“…Such a spin-mixing was both theoretically predicted [9,14,46] and experimentally confirmed. [8] If, however, the only spin-mixing process was due to ISC, the modeled population would end up in the m s = 0 state for sufficient long-lasting laser excitation and the modeled fluorescence would thus be maximized. Instead, the spin-dependent fluorescence curves in our experiment ( Fig.…”

“…[5] Furthermore, the electron spin may be coherently manipulated by microwave signals [6] and purely optically initialized as well as read-out. [7] Spin initialization and read-out are enabled by a spin-selective intersystem crossing (ISC) towards the singlet system: the long lifetime in the singlet system facilitates a spin-dependent fluorescence and a preferred decay towards the m s = 0 ground state, [8,9] allowing for fast spin initialization. Spin polarizations of about 80 % at room temperature [10,11] and over 99 % at liquid helium temperature [5] may be reached, as well as spin-dependent fluorescence contrasts of up to 30 % for an optical spin read-out under non-resonant laser excitation.…”