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Nature Communications

Picosecond pulse shaping of single photons using quantum dots

Brennan C. Pursley, Sam Carter, Michael K. Yakes, A. S. Bracker, D. Gammon

Published Jan 3, 2018• 55 citations
Publisher

Abstract

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.

DOI
10.1038/s41467-017-02552-7

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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.
Semiconductor quantum dots have been featured in a number of proposals for quantum computing because of the advantages afforded by confinement on the nanoscale, such as well separated, discrete energy levels and ease of optical manipulation. In addition, they can leverage established semiconductor fabrication techniques and, like the quantum dots used in this work, can be designed to match the telecommunication band, improving the scalability and potential integration of the platform into existing technologies. This thesis work applied optical pulse engineering to manipulate exciton qubits in self-assembled InAs quantum dots. Optimal quantum control theory was used to design pulse shapes that implement high fidelity single-qubit and two-qubit operations, with constraints on the numerical optimization that ensure that the pulses can be accurately implemented using a commercial 4f pulse shaper. In the case of single-qubit operations, the results showed that two uncoupled qubits can be manipulated in parallel using a single phase-shaped laser pulse, provided their optoelectronic properties are sufficiently different. It was demonstrated that targeted differences in inversion and phase between the qubits can be achieved on an ultrafast timescale with high fidelity. In the case of two-qubit operations, the controlled-rotation gate was optimized using amplitude-only and phase-only pulse shaping schemes. The shaped pulses for both schemes were shown to produce higher fidelity operations. The ability to implement robust state inversion on short timescales is particularly useful for technologies such as ultrafast optical switches, single-photon sources, and entangled-photon sources. This work has demonstrated state inversion in a single InAs quantum dot via adiabatic rapid passage using linearly chirped laser pulses. The achieved gate times were an order of magnitude shorter than previous demonstrations. Theoretical predictions of the dependence of the inversion efficiency of the exciton on the sign of the pulse chirp were also verified experimentally, allowing for the identification of phonon-mediated dephasing as the dominant source of decoherence.
Control of single photon wave-packets is an important resource for developing hybrid quantum systems which are composed of different physical systems interacting via photons. Here, we extend this control to triggered photons emitted by a quantum dot, temporally shaping single photon wave-packets on timescales fast compared to their radiative decay by electro-optic modulation. In particular, telecommunications-band single photons resulting from the recombination of an exciton in a quantum dot with exponentially decaying wave-packets are synchronously modulated to create Gaussian-shaped single photon wave-packets. We explore other pulse shapes and investigate the feasibility of this technique for increasing the indistinguishability of quantum dot generated single photons.
InAs quantum dots (QDs) can be used as optically coupled quantum storage devices for quantum information applications. The QD can be charged with a single electron, where the spin state (up or down) provides a long lived quantum bit (qubit). The QD's optically excited states are used to initialize, manipulate, and read out the electron spin state with laser pulses. However, most practical quantum information applications require many interacting qubits, forming a quantum network. Since QDs are based on semiconductor technology, and are compatible with standard nano-fabrication processing, there is promise that they can provide a solid state platform where a scalable quantum information architecture is realizable. We focus on scaling the QD system to multiple qubits using intermediate spin-photon entangled states. In this work, experimental and theoretical techniques are developed to study the QD-light matter interaction at the single photon level. Resonance fluorescence from a single QD is experimentally realized, and the single photon nature of the scattered radiation is verified through intensity correlation experiments. Transient fluorescence measurements on resonantly excited QDs are performed using time correlated single photon counting techniques to measure the excited state lifetime. High speed electro-optic modulators are used to time gate narrow bandwidth lasers, so that a QD can be driven under step-wise excitation, allowing for the direct observation of time-dependent optical Rabi oscillations. From these measurements, we are able to extract a decoherence rate which is consistent with the lifetime limit, indicating that pure dephasing is negligible in this system. These techniques are applied to the QD spin system to demonstrate a spin-photon entangled state, by performing correlation measurements on the spin and photon state in two bases. A lower bound on the entanglement fidelity of 0.59(4) is achieved, which exceeds the classical limit of 0.5 by more than two standard deviations. The entanglement fidelity is limited primarily by the finite timing resolution of available single photon detectors. Taking this into account, we achieve 84% of the apparatus limited fidelity. These spin-entangled photons can be used to mediate entanglement between distant QD spins, providing the basis of an optically coupled QD spin network.
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.
Long-distance transmission of quantum information between various systems, such as single-photon sources, quantum memories or matter-based quantum information processing units is hampered by disparate spectral-temporal properties of photons that can be efficiently emitted or absorbed by such systems. Quantum frequency conversion, i.e. low noise nonlinear optical frequency conversion, is an established method to address photon central frequency distinguishability. However, it does not address the issue of single-photon spectral bandwidth and timescale variations, which can range from single picoseconds to hundreds of nanoseconds.
A deterministic "on demand" source of single photons is a basic building block for linear quantum computation \cite{linear}, quantum cryptography \cite{crypto}, quantum teleportation \cite{teleport}, and quantum networks \cite{network}. In all these applications, quantum interference between two single-photon pulses on a symmetric beam splitter has been exploited \cite{review}, which imposes stringent requirement for the implemented single photons to be indistinguishable in all degrees of freedom, including their frequencies, spectral widths, pulse shapes, and polarizations. To generate single photons one requires a pumping mechanism to excite a "two-level emitter" and an efficient channeling of the subsequently emitted photons. The efficiency of the source can be enhanced by coupling the emitter to the waveguide \cite{manga} or a microcavity mode \cite{cklaw}. However, hitherto the solid-state single photon sources realized by using a quantum dot coupled with a microcavity rely on incoherent pumping of excitons, which leads to problems with timing jitter \cite{kiraz}, leading to a trade off between efficiency and indistinguishability. Here we introduce a means to realize a highly efficient solid state source of indistinguishable single photons using cavity-assisted adiabatic Raman passage in a single quantum dot - cavity system. We demonstrate pulse-triggered single photons with 100% efficiency and >90>90>90% indistinguishability using currently available experimental parameters.