A quantum dot as a source of time-bin entangled multi-photon states

Abstract

A quantum computer has the potential to revolutionize multiple industries by enabling a drastic speed-up relative to classical computers for certain quantum algorithms and simulations. Linear optical quantum computing is an approach that uses photons as qubits, which are known for suffering little from decoherence. A source of multiple entangled and indistinguishable photons would be a significant step in the development of an optical quantum computer. Consequently, multiple proposals for the generation of such a stream of photons have recently been put forward. Here we introduce an alternative scheme based on a semiconductor quantum dot (QD) embedded in an optical microcavity in a magnetic field. A single charge carrier trapped in the dot has an associated spin that can be controlled by ultrashort optical pulses. Photons are sequentially generated by resonant scattering from the QD, while the charge spin is used to determine the encoding of the photons into time-bins. In this way a multi-photon entangled state can be gradually built up. With a simple optical pulse sequence we demonstrate a proof of principle experiment of our proposal by showing that the time-bin of a single photon is dependent on the measured state of the trapped charge spin.

Realizing a deterministic source of multipartite-entangled photonic qubits

“…As interesting future directions, one could explore applications of the presented source for one-way quantum computing with cluster states 6 , 26 , 27 , Heisenberg-limited metrology, or teleportation with GHZ states 28 – 30 , or photon loss resilient quantum communication with W states 31 , 32 . In addition, this versatile source of quantum many-body states of electromagnetic radiation, able to perform generic gates of the CNOT and SWAP families, could be used to access a larger variety of quantum many-body states in the MPS family, e.g., as ground states of variational quantum algorithms 33 , 34 .…”

A coherent spin-photon interface with waveguide induced cycling transitions

“…High-fidelity spin initialization is the essential starting point for applications of spin-photon interfaces. By analysing the steady state fluorescence at the end of the spin pumping histograms we estimate lower bounds of the spin initialization fidelitities F (XM ) s = ↓| ρ |↓ = 99.1% and F (XP ) s = ⇑| ρ |⇑ = 98.6%, which are the highest values so far reported in photonic nanostructures [12,18,19]. In contrast to cross polarization experiments, our laser polarization control allows us to avoid driving the x-transitions which otherwise reduce the initialization fidelity through re-pumping.…”

“…These maximally entangled states have applications in measurement-based quantum computing [8] and quantum repeaters [9,10] and so far one-dimensional cluster states containing 3 qubits have been generated with QDs [11]. A recent protocol based on time-bin encoded photonic qubits has been put forward allowing scaling up to tens of high-fidelity entangled photons for experimentally relevant parameters [12,13]. Crucially, this protocol requires an optical transition with high cyclicity, i.e.…”

“…Such an enhancement has recently been demonstrated in cavity systems including single rare-earth ion spins [22]. QDs coupled to photonic crystal cavities [18,19] and micropillar cavities [12,20] also exhibit selective enhancement, although the cyclicity has not been explicitly demonstrated.…”

A Source of Indistinguishable Time-Bin Entangled Photons from a Waveguide-Embedded Quantum Dot

“…The protocol is analyzed in Refs. [12,13] and the first experimental steps were implemented with a micropillar QD source [15] while spin-photon Bell-state generation was realized with nitrogen-vacancy centers [16,17]. Contrary to the Lindner-Rudolph protocol, we apply strong magnetic fields which permit the use of spin-echo pulses and render the protocol insensitive to spin-dephasing [12].…”