United Kingdom

The Hong-Ou-Mandel (HOM) effect is a fourth-degree interference originally intended to measure subpicosecond (or micrometre) time intervals between two photons. Lyons et al showed that there is sufficient precision in this method to measure the time delay with a HOM interferometer to the attosecond level (nanometre path length), giving a strong motivation to use this technique for thickness or refractive index estimation for transparent samples. Here, we propose a microscope using the HOM interference combined with a wavelength-entangled photon pair source allowing an interference fringe to be observed within the HOM interference dip. By varying the wavelength separation of the photons, the sensitivity of the microscope can be controlled. We propose to use temperature control of a periodically polled nonlinear crystal in a Sagnac architecture. By adaptively modifying the temperature of the nonlinear crystal in this arrangement, we aim to avoid ambiguity in parameter estimation.

Bristol Quantum Information Technologies Workshop

The two-colour Hong-Ou-Mandel microscope

experiment

metrology

imaging

**BQIT is a three day annual workshop** aimed at enabling leading UK and international academics and industrial partners to come together; to explore and discuss future ambitions and challenges in the field of **Quantum Information Technologies.** Since our conception in 2014 we have had 230 speakers and panellists who have presented their work and opinions on a range of topics, from quantum theory to innovation in industry.


### Welcome Message
The BQIT:21 team welcomes you to the eighth Bristol Quantum Information Technologies workshop! 

Last year was the first time in our workshop's history that we had gone fully virtual. The global circumstances that continue to impact us all professionally and personally have forced BQIT once again to be delivered as an all-virtual workshop for 2021. The access this creates to colleagues across the world who would otherwise be unable to join us, as well as being able to make the workshop free for all attendees due to the lower costs involved in delivering an online workshop, are just a couple of the extraordinary benefits of meeting online. This also means that we can welcome three times as many attendees to BQIT than previous years.   

We take this opportunity to thank each of our speakers and poster presenters for contributing their technical content, this year's sponsors, whose support this year enabled BQIT:21 to be free to all attendees, and to all of our attendees for joining us online this year. We recognise the extra energy required to stay connected and motivated during these difficult times, and are hugely encouraged to see so many of you willing to continue to connect with us on an online platform. 

It is our hope that you have a productive and enjoyable time while you engage and interact with BQIT:21, and to everyone that has helped take BQIT:21 online, thank you! 

**The BQIT board and organisation team **


![](https://assets.underline.io/uploads/markdown_image/1/image/a12e0c1ad3410144cb48557690e4ecaf.png)
### This year's workshop agenda is available here: [Virtual BQIT:21](https://drive.google.com/file/d/1QwsSM0ztnxC_lgUKf3TbO3GvnI44AQkY/view?usp=sharing)
For further information, please contact us at: **bqit-admin@bristol.ac.uk**

![](https://assets.underline.io/uploads/markdown_image/1/image/a12e0c1ad3410144cb48557690e4ecaf.png)

poster

Solid-state quantum emitters have great promise for quantum information applications such as, quantum computation and communication. Nitrogen-Vacancy (NV) centers in diamond are popular candidate due to its long-lived spin quantum state and well-defined optical transitions. Limitations of NV centers arise from the strongly temperature dependent dephasing effects caused by phonon interactions, which act to suppress the indistinguishability of photon emission essential for quantum interference. 
In this work we simulate the design of a cavity with an ultrasmall-mode-volume coupled to NV-center and show that we can extract highly efficient (99%) indistinguishable (>50%) photons at 200K. Upon filtering, we find an enhanced indistinguishability (73%) with efficiency (29%). We use a cavity-QED master equation approach to calculate efficiencies and indistinguishability of the photon emission at 200K and 300K. Typically, NV-centers are modelled as a two-level system with excess pure-dephasing. However, the NV-center consists of two orthogonally polarised dipoles which are non-degenerate in the presence of linear strain fields. To capture this polarisation splitting we model the NV-center as both a two- and three-level system and find that the inclusion of the non-degenerate excited states supresses indistinguishability and the results are optimized when the cavity orientation is aligned to one of these transition dipoles.


Efficient indistinguishable photons at 200K

We study the interplay between entanglement and chaos in the time evolution of Bohmian trajectories of a two qubit system, made of coherent states of the quantum harmonic oscillator. This simple quantum system has very rich dynamics from a Bohmian standpoint, since with a proper choice of the physical parameters one finds ordered (periodic or non-periodic) and chaotic trajectories. We provide strong numerical evidence that the chaotic trajectories of this system are ergodic regardless of the amount of the entanglement and study their impact on the dynamical approximation to Born's rule for an arbitrary initial configuration of the quantum particles.

Order and Chaos in Two Entangled Bohmian Qubits

Quantum conference key agreement (QCKA) establishes secure keys between multiple users from a single use of a GHZ state. In a network that provides photonic graph states as a resource between its nodes, this can be drastically more efficient than trying to establish a secure conference key using traditional 2-party primitives. 

Building on previous work where we demonstrated 4-party QCKA over a 50 km distance, we here move towards demonstrating this network advantage. Starting with a universal 6-qubit graph state, we will use network routing protocols to distill either 4-qubit GHZ states for QCKA or combinations of Bell pairs for two-party QKD. Then, a 4-user conference key can be established with half the resources.

Multi-party quantum cryptography in a graph-state network

Quantum measurements lie at the heart of experimental quantum science and quantum information processing. Traditionally, these measurements take the form of projecting a state intoa certain basis which tells us information about what the state is. Quantum state elimination (QSE) has recently garnered attention as an alternative approach where the outcomes tells uswhat the state is not. QSE plays a role in foundational physics about what is the nature ofa quantum state established by Pusey, Barrett and Rudolph (PBR). Recently, J. Crickmore, etal (Phys. Rev. Res., 2, 013256) proposed a new protocol for two-out-of-four state elimination extending QSE to a multiqubit setting. Specifically, the authors introduced a non-trivial set off our states that cannot be resolved by projective measurements on each qubit, instead requiring entangled measurement operators. These entangled measurements take the form of POVMs. States are encoded into pathmodes while the POVM is implemented interferometrically. We employ a beam displacer type interferometry for passive path mode stability. Specific detection events correspond to a POVM element which, along with a certain prepared state, results in two-out-of-four states being eliminated. Beyond being a proof of principle demonstration, useful quantum communication andcryptography schemes will be investigated.

Experimental realisation of 2 out of 4 quantum state elimination

Global quantum communications enable networked quantum information processing. Satellite QKD (SatQKD) overcome current optical fibre range limitations, but limited transmission times between satellites and ground stations constrains the amount of secret key due to finite-block size effects. We analyse these effects and their implications for system design and operation, for a trusted-node downlink employing an optimised BB84 with weak coherent pulse decoy states.

Finite Key Effects in Satellite-based Quantum Key Distribution

Introduction to the Angstrom Engineering Quantum Series Platform

Quantum Series Platform - Angstrom Engineering

Sponsor demo - IDQuantique

I present an overview of the LIGENTEC platform that provides new paths for quantum applications

LIGENTEC - Photonic Integrated Ciruits for quantum applications

Sponsor demo - University of Bristol

In this talk, I will present the main product of QuiX: a quantum photonic processor, i.e. a large-scale tunable multimode optical interferometer, for use in photonic quantum information processing applications. QuiX has recently demonstrated the world's largest photonic processor, and I will present results on the characterization, calibration and operation of this device.

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