Fundamental Limits to Coherent Photon Generation with Solid-State Atomlike Transitions

Coherent generation of indistinguishable single photons is crucial for many quantum communication and processing protocols. Solid-state realizations of two-level atomic transitions or three-level spin-$\Lambda$ systems offer significant advantages over their atomic counterparts for this purpose, albeit decoherence can arise due to environmental couplings. One popular approach to mitigate dephasing is to operate in the weak excitation limit, where excited state population is minimal and coherently scattered photons dominate over incoherent emission. Here we probe the coherence of photons produced using two-level and spin-$\Lambda$ solid-state systems. We observe that the coupling of the atomic-like transitions to the vibronic transitions of the crystal lattice is independent of driving strength and detuning. We apply a polaron master equation to capture the non-Markovian dynamics of the ground state vibrational manifolds. These results provide insight into the fundamental limitations for photon coherence from solid-state quantum emitters, with the consequence that deterministic single-shot quantum protocols are impossible and inherently probabilistic approaches must be embraced.Comment: 16 pages [with supplementary information], 8 figure

Multiplexed Single Photons from Deterministically Positioned Nanowire Quantum Dots

Solid-state quantum emitters are excellent sources of on-demand indistinguishable or entangled photons and can host long-lived spin memories, crucial resources for photonic quantum information applications. However, their scalability remains an outstanding challenge. Here we present a scalable technique to multiplex streams of photons from multiple independent quantum dots, on-chip, into a fiber network for use off-chip. Multiplexing is achieved by incorporating a multi-core fiber into a confocal microscope and spatially matching the multiple foci, seven in this case, to quantum dots in an array of deterministically positioned nanowires. First, we report the coherent control of the emission of biexciton-exciton cascade from a single nanowire quantum dot under resonant two-photon excitation. Then, as a proof-of-principle demonstration, we perform parallel spectroscopy on the nanowire array to identify two nearly identical quantum dots at different positions which are subsequently tuned into resonance with an external magnetic field. Multiplexing of background-free single photons from these two quantum dots is then achieved. Our approach, applicable to all types of quantum emitters, can readily be scaled up to multiplex $>100$ quantum light sources, providing a breakthrough in hardware for photonic based quantum technologies. Immediate applications include quantum communication, quantum simulation, and quantum computation.Comment: 10 pages, 4 figure

Single-emitter quantum key distribution over 175 km of fiber with optimised finite key rates

Quantum key distribution with solid-state single-photon emitters is gaining traction due to their rapidly improving performance and compatibility with future quantum network architectures. In this work, we perform fibre-based quantum key distribution with a quantum dot frequency-converted to telecom wavelength, achieving count rates of 1.6 MHz with $g^{\left(2\right)}\left(0\right) = 3.6 \%$. We demonstrate positive key rates up to 175 km in the asymptotic regime. We then show that the community standard analysis for non-decoy state QKD drastically overestimates the acquisition time required to generate secure finite keys. Our improved analysis using the multiplicative Chernoff bound reduces the required number of received signals by a factor of $10^8$ over existing work, with the finite key rate approaching the asymptotic limit at all achievable distances for acquisition times of one hour. Over a practical distance of 100 km we achieve a finite key rate of 13 kbps after one minute of integration time. This result represents major progress towards the feasibility of long-distance single-emitter QKD networks.Comment: 9 pages, 3 figure

Single-emitter quantum key distribution over 175 km of fibre with optimised finite key rates

Quantum key distribution with solid-state single-photon emitters is gaining traction due to their rapidly improving performance and compatibility with future quantum networks. Here we emulate a quantum key distribution scheme with quantum-dot-generated single photons frequency-converted to 1550 nm, achieving count rates of 1.6 MHz with g20=3.6% and asymptotic positive key rates over 175 km of telecom fibre. We show that the commonly used finite-key analysis for non-decoy state QKD drastically overestimates secure key acquisition times due to overly loose bounds on statistical fluctuations. Using the tighter multiplicative Chernoff bound to constrain the estimated finite key parameters, we reduce the required number of received signals by a factor 108. The resulting finite key rate approaches the asymptotic limit at all achievable distances in acquisition times of one hour, and at 100 km we generate finite keys at 13 kbps for one minute of acquisition. This result is an important step towards long-distance single-emitter quantum networking

Coherent light-matter interaction in semiconductor quantum dots

Coherent light-matter interaction allows for population control of a single quantum emitter. Detection of the photons emitted immediately after the interaction is equivalent to reading out the state of the emitter. Frequency and time-domain measurements on these photons reveal the information about the coherence of the emitter, typically imprinted as the indistinguishability of the scattered photons. This thesis focuses on the optical spectroscopy of semiconductor quantum dots at cryogenic temperature (4 K) under coherent driving. By analyzing the coherence and the statistics of the scattered photons, the population inversion, the fundamental dephasing mechanisms, and the coherent coupling amongst emitters can be probed. First, the experimental data indicates that the coupling of the atom-like transitions to the vibronic transitions of the crystal lattice is independent of the driving strength, even for detuned excitation using the spin-Λ configuration. This imposes a fundamental limit to the coherence of the photons emitted from solid-state emitters. Next, the coherent dynamics of a two-level quantum emitter driven by a pair of symmetrically detuned phase-locked pulses is studied. The spectroscopic results of a solid-state two-level system show that coherent population control and a large amount of population inversion are possible using asymmetric dichromatic excitation, which is achieved by adjusting the relative weighting between the red- and blue-detuned pulses. Furthermore, this technique can be extended to multi-level systems like the biexciton-exciton cascade, such that a pair of suitably detuned laser pulses, each resonant to the biexciton-exciton or the exciton-ground state transition, can be used to achieve population inversion from the ground state to the excited (biexciton) state. In addition, coherent control of cooperative emission arising from two distant but indistinguishable solid-state emitters due to path erasure is demonstrated via the results from the photon correlations, measured with Hanbury Brown-Twiss and Hong-Ou-Mandel interferometers. Finally, applications of these single-photon emitters integrated in deterministically-positioned nanowires and micropillar cavities are discussed. The former allows for the demonstration of the parallel spectroscopy of up to 7 emitters using a multi-core-fiber-based confocal microscope. In the latter case, the coherence, indistinguishability as well as photon-number distribution of the scattered photons from a neutral exciton resonantly coupled to the cavity resonance are characterized, before they are converted to the telecommunication C-band via quantum frequency conversion. With these photons, the single-photon BB84 protocol is implemented and a secure key rate of ∼ 1 kHz after propagating through 150 km of optical fiber is observed. This constitutes a key step towards integration of this single-photon source for fiber-based quantum networking

Coherence in cooperative photon emission from indistinguishable quantum emitters

Photon-mediated interactions between atoms can arise via coupling to a common electromagnetic mode or by quantum interference. Here, we probe the role of coherence in cooperative emission arising from two distant but indistinguishable solid-state emitters because of path erasure. The primary signature of cooperative emission, the emergence of “bunching” at zero delay in an intensity correlation experiment, is used to characterize the indistinguishability of the emitters, their dephasing, and the degree of correlation in the joint system that can be coherently controlled. In a stark departure from a pair of uncorrelated emitters, in Hong-Ou-Mandel–type interference measurements, we observe photon statistics from a pair of indistinguishable emitters resembling that of a weak coherent state from an attenuated laser. Our experiments establish techniques to control and characterize cooperative behavior between matter qubits using the full quantum optics toolbox, a key step toward realizing large-scale quantum photonic networks