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arXiv:2007.12726 · Science Advances

Quantum cryptography with highly entangled photons from semiconductor quantum dots

Christian Schimpf, Marcus Reindl, Daniel Huber, Barbara Lehner, Saimon F. Covre da Silva, Santanu Manna, Michal Vyvlečka, Philip John Walther, Armando Rastelli

Published Apr 14, 2021• 150 citations
arXivHTMLPDF

Abstract

Semiconductor quantum dots are capable of emitting polarization entangled photon pairs with ultralow multipair emission probability even at maximum brightness. Using a quantum dot source with a fidelity as high as 0.987(8), we implement here quantum key distribution with an average quantum bit error rate as low as 1.9% over a time span of 13 hours. For a proof of principle, the key generation is performed with the BBM92 protocol between two buildings, connected by a 350-m-long fiber, resulting in an average raw (secure) key rate of 135 bits/s (86 bits/s) for a pumping rate of 80 MHz, without resorting to time- or frequency-filtering techniques. Our work demonstrates the viability of quantum dots as light sources for entanglement-based quantum key distribution and quantum networks. By increasing the excitation rate and embedding the dots in state-of-the-art photonic structures, key generation rates in the gigabits per second range are in principle at reach.

DOI
10.1126/sciadv.abe8905

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Semiconductor quantum dots are capable of emitting polarization entangled photon pairs with ultralow multipair emission probability even at maximum brightness. Using a quantum dot source with a fidelity as high as 0.987(8), we implement here quantum key distribution with an average quantum bit error rate as low as 1.9% over a time span of 13 hours. For a proof of principle, the key generation is performed with the BBM92 protocol between two buildings, connected by a 350-m-long fiber, resulting in an average raw (secure) key rate of 135 bits/s (86 bits/s) for a pumping rate of 80 MHz, without resorting to time- or frequency-filtering techniques. Our work demonstrates the viability of quantum dots as light sources for entanglement-based quantum key distribution and quantum networks. By increasing the excitation rate and embedding the dots in state-of-the-art photonic structures, key generation rates in the gigabits per second range are in principle at reach.
Entanglement-based quantum key distribution (QKD) promises enhanced robustness against eavesdropping and compatibility with future quantum networks. Among other sources, semiconductor quantum dots (QDs) can generate polarization-entangled photon pairs with near-unity entanglement fidelity and a multiphoton emission probability close to zero even at maximum brightness. These properties have been demonstrated under resonant two-photon excitation (TPE) and at operation temperatures below 10 K. However, source blinking is often reported under TPE conditions, limiting the maximum achievable photon rate. In addition, operation temperatures reachable with compact cryocoolers could facilitate the widespread deployment of QDs, e.g., in satellite-based quantum communication. We demonstrate blinking-free emission of highly entangled photon pairs from GaAs QDs embedded in a p-i-n diode. High fidelity entanglement persists at temperatures of at least 20 K, which we use to implement fiber-based QKD between two buildings with an average raw key rate of 55 bits / s and a qubit error rate of 8.4%. We are confident that by combining electrical control with already demonstrated photonic and strain engineering, QDs will keep approaching the ideal source of entangled photons for real world applications.
Abstract Quantum key distribution (QKD) enables the transmission of information that is secure against general attacks by eavesdroppers. The use of on-demand quantum light sources in QKD protocols is expected to help improve security and maximum tolerable loss. Semiconductor quantum dots (QDs) are a promising building block for quantum communication applications because of the deterministic emission of single photons with high brightness and low multiphoton contribution. Here we report on the first intercity QKD experiment using a bright deterministic single photon source. A BB84 protocol based on polarisation encoding is realised using the high-rate single photons in the telecommunication C-band emitted from a semiconductor QD embedded in a circular Bragg grating structure. Utilising the 79 km long link with 25.49 dB loss (equivalent to 130 km for the direct-connected optical fibre) between the German cities of Hannover and Braunschweig, a record-high secret key bits per pulse of 4.8 × 10−5 with an average quantum bit error ratio of ~ 0.65% are demonstrated. An asymptotic maximum tolerable loss of 28.11 dB is found, corresponding to a length of 144 km of standard telecommunication fibre. Deterministic semiconductor sources therefore challenge state-of-the-art QKD protocols and have the potential to excel in measurement device independent protocols and quantum repeater applications.
Establishing secure communication is an important requirement in existing and future networks. Although current solutions rely on classical approaches, quantum mechanics offers the possibility of moving from computational to information-theoretic security. Quantum key distribution (QKD) has been extensively studied as a means of achieving such security. Recently, epitaxial quantum dots started to be studied and employed in quantum cryptography, because of their rapid development that made them one of the brightest sources of quantum light to date. In particular, they are known to be excellent sources of polarization-entangled photon pairs, especially droplet-etched dots operating at $\sim$780 nm. Here, we employ such a quantum dot to generate entangled photon pairs under 1 GHz excitation rate. Using quantum frequency conversion, we interface the emitted photons with the telecommunication wavelength range while preserving their polarization entanglement. This enables quantum key distribution over a 1.59 km field-deployed fibre link across the University of Stuttgart - campus Vaihingen, using the BBM92 protocol: we achieve a secure key rate of 206 Hz over 19 h of stable operation, resulting in a total secure key length of 14.2 Mbit. This key enables the safe encryption and decryption of a colour image with further implementation of error correction and privacy amplification. These results demonstrate important steps towards hybrid quantum networks by combining high-quality entangled-photon sources operated at high excitation rates, efficient frequency conversion module, and picosecond-level time synchronization in a realistic deployed network.
Quantum key distribution (QKD) enables the transmission of information that is secure against general attacks by eavesdroppers. The use of on-demand quantum light sources in QKD protocols is expected to help improve security and maximum tolerable loss. Semiconductor quantum dots (QDs) are a promising building block for quantum communication applications because of the deterministic emission of single photons with high brightness and low multiphoton contribution. Here we report on the first intercity QKD experiment using a bright deterministic single photon source. A BB84 protocol based on polarisation encoding is realised using the high-rate single photons in the telecommunication C-band emitted from a semiconductor QD embedded in a circular Bragg grating structure. Utilising the 79 km long link with 25.49 dB loss (equivalent to 130 km for the direct-connected optical fibre) between the German cities of Hannover and Braunschweig, a record-high secret key bits per pulse of 4.8 × 10−5 with an average quantum bit error ratio of ~ 0.65% are demonstrated. An asymptotic maximum tolerable loss of 28.11 dB is found, corresponding to a length of 144 km of standard telecommunication fibre. Deterministic semiconductor sources therefore challenge state-of-the-art QKD protocols and have the potential to excel in measurement device independent protocols and quantum repeater applications.
An on-demand source of bright entangled photon pairs is desirable for quantum key distribution (QKD) and quantum repeaters. The leading candidate to generate entangled photon pairs is based on spontaneous parametric down-conversion (SPDC) in a non-linear crystal. However, a fundamental trade-off exists between entanglement fidelity and efficiency in SPDC sources due to multiphoton emission at high brightness, which limits the pair extraction efficiency to 0.1% when operating at near-unity fidelity. Quantum dots in photonic nanostructures can in principle overcome this trade-off; however, the quantum dots that have achieved an en- entanglement fidelity on par with an SPDC source (99%) have poor pair extraction efficiency of 0.01%. Here, we show a measured peak concurrence of 95.3% ± 0.5% and pair extraction efficiency of 0.65% from an InAsP quantum dot in an InP photonic nanowire waveguide. Additionally, we show that an oscillating two-photon Bell state generated by a semiconductor quantum dot can establish a secure key for peer-to-peer QKD while using all generated photon pairs. Using our time-resolved QKD scheme alleviates the need to remove the exciton fine structure splitting.
Entangled photon pairs are a ubiquitous resource in quantum technologies, used in quantum key distribution and quantum networking as well as fundamental tests of non-locality. For scalable quantum networks, pairs that are indistinguishable in all unentangled degrees of freedom are essential, as they enable high-fidelity entanglement swapping across network nodes. To date the most-studied sources of "swappable" entangled photon pairs have been based on spontaneous parametric down-conversion (SPDC) in non-linear crystals. However, the probabilistic nature and unavoidable trade-off between brightness and unwanted multi-photon emission limits their performance in lossy channels. Here, we demonstrate a high-fidelity source of "swappable" entangled photon pairs using a semiconductor quantum dot (QD) coupled to a tunable microcavity. By actively modulating the QD emission between orthogonal polarisation states, delaying one path in a low-loss Herriott cell, and recombining the two on a balanced beam splitter, we generate entangled photon pairs with a fidelity of 96.1±0.596.1\pm0.596.1±0.5 %. We identify and mitigate fidelity-limiting factors, achieving a maximum fidelity of 98.1±0.598.1\pm0.598.1±0.5 % through time-resolved post-selection. The scheme suppresses residual multi-photon events concentrated near the excitation pulse and has only a modest impact on the rate. Furthermore, the photons are mutually indistinguishable, enabling efficient entanglement swapping. Our results establish semiconductor QDs as a viable platform for quantum network-compatible swappable entangled photon pair generation, with feasible entanglement generation rates exceeding 0.5 Gpairs/s.