Abstract A practical way to link separate nodes in quantum networks is to send photons over the standard telecom fibre network. This requires sub-Poissonian photon sources in the wavelength band around 1550 nm, with photon coherence times sufficient to enable the many interference-based technologies at the heart of quantum networks. Here, we show that droplet epitaxy InAs/InP quantum dots emitting in the telecom C-band can provide photons with coherence times exceeding 1 ns under low power non-resonant excitation, and demonstrate that these coherence times enable near-optimal interference with a C-band polarisation-encoded laser qubit, with visibilities only limited by the quantum dot multiphoton emission. Using entangled photons, we further show teleportation of such qubits in six different bases with average postselected fidelity reaching 88.3 ± 4.0%. Beyond direct applications in long-distance quantum communication, the high degree of coherence in these quantum dots is promising for future spin-based telecom quantum network applications.
Abstract A practical way to link separate nodes in quantum networks is to send photons over the standard telecom fibre network. This requires sub-Poissonian photon sources in the wavelength band around 1550 nm, with photon coherence times sufficient to enable the many interference-based technologies at the heart of quantum networks. Here, we show that droplet epitaxy InAs/InP quantum dots emitting in the telecom C-band can provide photons with coherence times exceeding 1 ns under low power non-resonant excitation, and demonstrate that these coherence times enable near-optimal interference with a C-band polarisation-encoded laser qubit, with visibilities only limited by the quantum dot multiphoton emission. Using entangled photons, we further show teleportation of such qubits in six different bases with average postselected fidelity reaching 88.3 ± 4.0%. Beyond direct applications in long-distance quantum communication, the high degree of coherence in these quantum dots is promising for future spin-based telecom quantum network applications.
A practical way to link separate nodes in quantum networks is to send photons over the standard telecom fibre network. This requires sub-Poissonian photon sources in the telecom wavelength band around 1550 nm, where the photon coherence time has to be sufficient to enable the many interference-based technologies at the heart of quantum networks. Here, we show that droplet epitaxy InAs/InP quantum dots emitting in the telecom C-band can provide photons with coherence times exceeding 1 ns even under non-resonant excitation, more than a factor two longer than values reported for shorter wavelength quantum dots under similar conditions. We demonstrate that these coherence times enable near-optimal interference with a C-band laser qubit, with visibilities only limited by the quantum dot multiphoton emission. Using entangled photons, we further show teleportation of such qubits in six different bases with average fidelity reaching 88.3$\pm$4%. Beyond direct applications in long-distance quantum communication, the high degree of coherence in these quantum dots is promising for future spin based telecom quantum network applications.
Quantum networks, relying on the distribution of quantum entanglement between remote locations, have the potential to transform quantum computation and secure long-distance quantum communication. However, a fundamental ingredient for fibre-based implementations of such networks, namely entanglement between a single spin and a photon directly emitted at telecom wavelengths, has been unattainable so far. Here, we use a negatively charged exciton in an InAs/InP quantum dot to implement an optically active spin qubit taking advantage of the lowest-loss transmission window, the telecom C-band. We investigate the coherent interactions of the spin-qubit system under resonant excitation, demonstrating high fidelity spin initialisation and coherent control using picosecond pulses. We further use these tools to measure the coherence of a single, undisturbed electron spin in our system. Finally, we demonstrate spin-photon entanglement in a solid-state system with entanglement fidelity F = 80.07 ± 2.9%, more than 10 standard deviations above the classical limit.
Quantum networks have the potential to transform secure communication via quantum key distribution and enable novel concepts in distributed quantum computing and sensing. Coherent quantum light generation at telecom wavelengths is fundamental for fibre-based network implementations, but Fourier-limited emission and subnatural linewidth photons have so far only been reported from systems operating in the visible to near-infrared wavelength range. Here, we use InAs/InP quantum dots to demonstrate photons with coherence times much longer than the Fourier limit at telecom wavelength via elastic scattering of excitation laser photons. Further, we show that even the inelastically scattered photons have coherence times within the error bars of the Fourier limit. Finally, we make direct use of the minimal attenuation in fibre for these photons by measuring two-photon interference after 25 km of fibre, demonstrating finite interference visibility for photons emitted about 100,000 excitation cycles apart.
Semiconductor quantum dots (QDs) are well established as sub-Poissonian sources of entangled photon pairs. To improve the utility of a QD light source, it would be advantageous to extend their emission further into the near infrared, into the low absorption wavelength windows utilised in long-haul optical telecommunication. Initial experiments succeeded in interfering O-band (1260—1360 nm) photons from an InAs/GaAs QD with dissimilar photons from a laser, an important mechanism for quantum teleportation. Interference visibilities as high as 60 ± 6 % were recorded, surpassing the 50 % threshold imposed by classical electrodynamics. Later, polarisation-entanglement of a similar QD was observed, with pairs of telecom-wavelength photons from the radiative cascade of the biexciton state exhibiting fidelities of 92.0 ± 0.2 % to the Bell state. Subsequently, an O-band telecom-wavelength quantum relay was realised. Again using an InAs/GaAs QD device, this represents the first implementation of a sub-Poissonian telecom-wavelength quantum relay, to the best knowledge of the author. The relay proved capable of implementing the famous four-state BB84 protocol, with a mean teleportation fidelity as high as 94.5 ± 2.2 %, which would contribute 0.385 secure bits per teleported qubit. After characterisation by way of quantum process tomography, the performance of the relay was also evaluated to be capable of implementing a six-state QKD protocol. In an effort to further extend the emitted light from a QD into the telecom C-band (1530—1565 nm), alternative material systems were investigated. InAs QDs on a substrate of InP were shown to emit much more readily in the fibre-telecom O- and C-bands than their InAs/GaAs counterparts, largely due to the reduced lattice mismatch between the QD and substrate for InAs/InP (~3 %) compared to InAs/GaAs (~7 %). Additionally, to minimize the fine structure splitting (FSS) of the exciton level, which deteriorates the observed polarisation-entanglement, a new mode of dot growth was investigated. Known as droplet epitaxy (D-E), QDs grown in this mode showed a fourfold reduction in the FSS compared to dots grown in the Stranski-Krastanow mode. This improvement would allow observation of polarisation-entanglement in the telecom C-band. In subsequent work performed by colleagues at the Toshiba Cambridge Research Labs, these D-E QDs were embedded in a p-i-n doped optical cavity, processed with electrical contacts, and found to emit entangled pairs of photons under electrical excitation. The work of this thesis provides considerable technological advances to the field of entangled-light sources, that in the near future may allow for deterministic quantum repeaters operating at megahertz rates, and in the further future could facilitate the distribution of coherent multipartite states across a distributed quantum network.
Semiconductor quantum dots are prime candidates for quantum network applications such as quantum relays, but their typical emission wavelength, polarization qubit encoding scheme and low operating frequency are incompatible with existing technologies. Our work shows that InAs/InP quantum dots driven with GHz-clocked pulses, in combination with qubit transcoding interferometers, can bridge these gaps. The demonstrated teleportation of time-bin qubits in the telecom C band even when repetition rates exceed the inverse lifetime of the dot shows the potential for integrating such devices with long-distance quantum network technologies.