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arXiv:1804.00713 · Physical Review X

Controllable Photonic Time-Bin Qubits from a Quantum Dot

J. P. Lee, L. M. Wells, B. Villa, Sokratis Kalliakos, R. M. Stevenson, D. J. P. Ellis, I. Farrer, D. A. Ritchie, A. J. Bennett, A. J. Shields

Published Jun 27, 2018• 17 citations
arXivHTMLPDF

Abstract

Photonic time-bin qubits are well suited to transmission via optical fibers and waveguide circuits.The states take the form 1 ffiffiffiffi ffi ð2Þ p ðαj0iþe iϕ βj1iÞ, with j0i and j1i referring to the early and late time bin, respectively.By controlling the phase of a laser driving a spin-flip Raman transition in a single-holecharged InAs quantum dot, we demonstrate complete control over the phase, ϕ.We show that this photon generation process can be performed deterministically, with only a moderate loss in coherence.Finally, we encode different qubits in different energies of the Raman scattered light, paving the way for wavelengthdivision multiplexing at the single-photon level.

DOI
10.1103/physrevx.8.021078

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Photonic time-bin qubits are well suited to transmission via optical fibers and waveguide circuits.The states take the form 1 ffiffiffiffi ffi ð2Þ p ðαj0iþe iϕ βj1iÞ, with j0i and j1i referring to the early and late time bin, respectively.By controlling the phase of a laser driving a spin-flip Raman transition in a single-holecharged InAs quantum dot, we demonstrate complete control over the phase, ϕ.We show that this photon generation process can be performed deterministically, with only a moderate loss in coherence.Finally, we encode different qubits in different energies of the Raman scattered light, paving the way for wavelengthdivision multiplexing at the single-photon level.
Photonic time-bin qubits are well suited to transmission via optical fibers and waveguide circuits. The states take the form 1(2)(α|0⟩+eiϕβ|1⟩), with |0⟩ and |1⟩ referring to the early and late time bin, respectively. By controlling the phase of a laser driving a spin-flip Raman transition in a single-hole-charged InAs quantum dot, we demonstrate complete control over the phase, ϕ. We show that this photon generation process can be performed deterministically, with only a moderate loss in coherence. Finally, we encode different qubits in different energies of the Raman scattered light, paving the way for wavelength-division multiplexing at the single-photon level.
Temporal encodings of quantum information are prevalent in applications because of their suitability for long-distance quantum communication and their compatibility with optical fiber communication networks. Perhaps the simplest temporal encoding is time-bin encoding, i. e. in superpositions of two (or more) temporally separated optical pulses. Early attempts at generating time-bin entanglement from single quantum emitters was not able to avoid the problem of re-excitation or was converted probabilistically from polarization entangled photon pairs from a quantum dot. Direct generation requires a metastable level to carry the coherence and avoid double pair emission into the desired time bins. In order to use dark exciton states as metastable states we have worked on their efficient creation and coherent control in the presence of in-plane magnetic fields. Much of this is based on our recent work on advanced excitation schemes using chirped pulses. With chirped pulses we are now able to deterministically populate a dark exciton state and to transfer this population to the biexciton, which can then emit a photon pair.
The strong confinement of semiconductor excitons in a quantum dot gives rise to atomlike behavior. The full benefit of such a structure is best observed in resonant excitation where the excited state can be deterministically populated and coherently manipulated. Because of the large refractive index and device geometry it remains challenging to observe resonantly excited emission that is free from laser scattering in III/V self-assembled quantum dots. Here we exploit the biexciton binding energy to create an extremely clean single photon source via two-photon resonant excitation of an InAs/GaAs quantum dot. We observe complete suppression of the excitation laser and multiphoton emissions. Additionally, we perform full coherent control of the ground-biexciton state qubit and observe an extended coherence time using an all-optical echo technique. The deterministic coherent photon pair creation makes this system suitable for the generation of time-bin entanglement and experiments on the interaction of photons from dissimilar sources.
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.
Quantum dots (QDs) are an excellent single-photon source that can be combined with a spin quantum memory. Many quantum technologies require increased control over the characteristics of emitted photons. A powerful approach is to trigger coherent Raman photons from QDs with a Λ energy-level system, such as the spin singlet-triplet system in two coupled QDs. The temporal and spectral behavior of single Raman photons can be varied simply by modifying the excitation source. Here, we demonstrate control of the single-photon pulse shape in a solid-state system on a timescale much shorter than the radiative lifetime, in addition to control of the frequency and bandwidth. We achieve a photon pulse width of 80 ps-an order of magnitude shorter than the exciton lifetime. Possible applications include time-bin encoding of quantum information, matching photons from different sources, and efficient single-photon transfer in a quantum network.
Time-bin qubits, where information is encoded in a single photon at different times, have been widely used in optical-fiber- and waveguide-based quantum communications. With the recent developments in distributed quantum computation, it is logical to ask whether time-bin encoded qubits may be useful in that context. We have recently realized a time-bin qubit controlled-phase (C-phase) gate using a $2\ifmmode\times\else\texttimes\fi{}2$ optical switch based on a lithium-niobate waveguide, with which we demonstrated the generation of an entangled state. However, the experiment was performed with only a pair of input states and thus the functionality of the C-phase gate was not fully verified. In this research, we use quantum process tomography to establish a process fidelity of $97.1\mathrm{%}$. Furthermore, we demonstrate the controlled-not gate operation with a process fidelity greater than $94\mathrm{%}$. This study confirms that typical two-qubit logic gates used in quantum computational circuits can be implemented with time-bin qubits and thus it is a significant step forward for the realization of distributed quantum computation based on time-bin qubits.