Research
Quantum repeaters and quantum networks
Quantum networks will offer novel functionalities which cannot be realised on the classical Internet alone. To achieve this task we need to find reliable ways of transferring quantum information over long distances and generating long distance entnaglement. Unfortunately transmitting quantum states over long distances is much more challenging than transmission of classical signals. Our classical Internet relies on fibre-optical infrustructure. While optical signals can get lost or absorbed in the fibre, standard amplification techniques can overcome this problem for classical communication. For quantum communication the situation is more tricky as the no cloning theorem forbids direct signal amplification. Therefore we need to develop more advanced techniques of overcoming losses in the fibre for quantum communication. In my work I design various types of quantum repeaters that aim to overcome this challenge.
Quantum Repeaters based on bosonic error correcting codes
One type of quantum repeaters utilises quantum error correcting codes. The specific class of codes that can be particularly useful for optical channels are bosonic codes which encode information in the quadratures of the light field. An example of such a code which can work very well for correcting loss errors is the Gottesman-Kitaev-Preskill (GKP) code, encoding information in a grid-like structure in the phase space of optical quadratures. We look for novel ways of utilising this encoding in quantum communication, in particular in the context of concatinating it with a discrete variable code on the second level.
One-way quantum repeater based on concatenation of the GKP code with a small discrete variable code on the higher level. We also have two repeater types to correct errors on two levels. If the simpler GKP repeater fails to correct the GKP error (red GKP qubit), that error can later be corrected on the higher level in the more powerful multi-qubit repeater. See npj Quantum Inf 7, 102 (2021) for more details.
Two-way all-photonic quantum repeater based on the GKP code. Due to the extra analog information provided by the GKP code the generated entangled links in step 1 can be ranked according to their expected quality. That information allows to make better decisions about which links to connect at the repeater nodes. The architecture is all-photonic so all the qubits stored at the repeaters are stored in optically simulated quantum memories consisting of the photonic qubits stored in the local fibre and encoded in concatenated GKP and small discrete variable codes.
Near-term quantum repeater and remote entanglement generation experiments with NV centres
We design and anylyse performance of various proof of principle quantum repeater and remote entanglement generation schemes that could be implemented near-term on the platform of NV-centres in diamond.
Our proposals for near-term NV-centers based proof-of-principle quantum repeater schemes. See Phys. Rev. A 99, 052330 (2019), Quantum Sci. Technol. 3, 034002 (2018) and Quantum Sci. Technol. 2, 034002 (2017) for more details.
Entanglement distillation
Noisy operations in quantum repeaters decrease the quality of the generated entanglement. Fortunately entanglement is like alcohol, since it is distillable and we can distill high quality entanglement from weaker entanglement by performing only local operations on the entangled particles and then using classical communication to exchange certain measurement outcomes. In our work we look for optimal distillation schemes and investigate the fundamental limits on performance of practical entanglement distillation.
In entanglement distillation Alice and Bob can probabilistically transform large number of weakly entangled states into a smaller number of more strongly entangled copies. They can achieve that using only local operations and classical communication. See Phys. Rev. A 97, 062333 (2018) for more details on our work on optimizing practical entanglement distillation. Credit for the measurement devices and Alice and Bob figures: Dmytro Vasylyev.
Quantum Key Distribution
The technologically most mature application of quantum networks is quantum key distribution (QKD). Specifically, quantum networks allow for distribution of symmetric cryptographic keys with unconditional security. These keys can then be used for secure message transfer using the one-time pad encryption scheme. The information-theoretic security of quantum key distribution relies on two fundamental features of quantum mechanics: the uncertainty principle and monogamy of entanglement. In our work we investigate practical QKD protocols and postprocessing schemes for the generated QKD data under experimentally realistic noise models.
In entanglement-based QKD Alice and Bob generate secret-key by measuring entangled particles. Monogamy of entanglement guarantees that if they can certify to be entangled with each other, then no eavesdropper can be correlated with their key. In our work we compare and analyse security of various QKD post-processing schemes under experimentally relevant noise in the communication channel, see Phys. Rev. A 101, 062321 (2020) for more details on our recent work. FIgure credit: Gláucia Murta with Alice's and Bob's design by Daniel Cavalcanti.