China

The evolution of quantum systems under measurement is a central aspect of quantum mechanics. Decades of experimental progress have enabled the observation of the inner dynamics of a quantum measurement and the determination of the number of bits of information it reveals [1,2]. 
When a two level system -- a qubit -- is used as a probe of a larger system, it naturally leads to answering a single yes-no question about the system state followed by its corresponding quantum collapse. Each measurement thus reveals at most one bit of information. Is it possible to lift this constraint by not using the qubit as an ancilla?
Here we report an experiment where more than 3 bits of information about the photon number in a microwave resonator are simultaneously recorded by a superconducting qubit into 9 propagating modes of a transmission line. This recording occurs by exciting these 9 modes using multiplexed spectroscopy owing to the dependence of the qubit frequency on photon number [3,4]. We demonstrate the simultaneous acquisition of microwave signals emitted by the qubit into the nine propagating modes that each reveals information about a photon number from 0 to 8, enabled by the large bandwidth of present near quantum limited amplifiers
[5]. Further observing the multiplexed measurement back-action on the resonator allowed us to evidence an optimal qubit drive amplitude for information extraction. Our experiment unleashes the full potential of quantum meters by bringing the measurement process in the frequency domain. It extends the reach of simultaneous probing of a single qubit by multiple observers [6,7] to arbitrary systems such as our resonator. This concept can be applied to improve or renew quantum control based on measurement [8] with possible applications in quantum error correction and sensing.

1. Guerlin, C.et al. Progressive field-state collapse and quantum non-demolition photoncounting. Nature448, 889–93 (2007).
2. Murch, K. W., Weber, S. J., Macklin, C. &amp; Siddiqi, I. Observing single quantum trajectories of a superconducting quantum bit. Nature502, 211–214 (2013).
3. Minev, Z. K.et al. To catch and reverse a quantum jump mid-flight. Nature(2019).
4. Schuster, D. I.et al. Resolving photon number states in a superconducting circuit. Nature445,515–518 (2007).
5. Gely, M. F.et al. Observation and stabilization of photonic Fock states in a hot radio-frequency resonator. Science363, 1072–1075 (2019).
6. Hacohen-Gourgy, S.et al. Dynamics of simultaneously measured non-commuting observables. Nature538, 491 (2016).
7. Ficheux, Q., Jezouin, S., Leghtas, Z. &amp; Huard, B. Dynamics of a qubit while simultaneously monitoring its relaxation and dephasing. Nature Communications9, 1926 (2018).
8. Wiseman, H. M. &amp; Milburn, G. J. Quantum Measurement and Control (Cambridge UniversityPress, 2009).

Quantum 2020

Multiplexing photon number measurement of a resonator using a dispersively coupled qubit

photoncounter

fluorescence

qubit

multiplexing

The Institute of Physics and IOP Publishing, in partnership with the Chinese Physical Society (CPS) and the University of Science and Technology of China (USTC) are proud to have brought together the international quantum science community online for this inaugural conference.


Early career and leading researchers from universities, industry and government worldwide came together to share, learn and collaborate on the latest research and emerging areas of high interest across quantum science and technology. The broad themes covered by Quantum 2020 included:

Quantum communication;
Quantum computing and quantum simulation;
Quantum sensing, metrology and imaging;
Quantum information theory, software and algorithms;
Quantum foundations;
Quantum technology in industry.
The four-day event featured talks from world-renowned scientists, interactive poster sessions and two exciting panel discussions on the themes of Quantum Technology in Industry and Quantum Technology Programmes Around the World.


You will need to [register](https://www.iopconferences.org/iop/frontend/reg/tRegisterEmailNew.csp?pageID=978403&eventID=1552&tempPersonID=536224) to obtain a ticket. . If you have already registered you should have been sent an email from Underline (to the email address that you registered with) with the subject line ‘Welcome to Quantum 2020’. This email includes your ticket for the event. 

If you have a ticket and are experiencing problems in accessing the event, please contact us. 

**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://s3.eu-west-1.amazonaws.com/underline.prod/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**

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Bristol Quantum Information Technologies Workshop

The evolution of quantum systems under measurement is a central aspect of quantum mechanics. Decades of experimental progress have enabled the observation of the inner dynamics of a quantum measurement and the determination of the number of bits of information it reveals [1,2]. 
When a two level system -- a qubit -- is used as a probe of a larger system, it naturally leads to answering a single yes-no question about the system state followed by its corresponding quantum collapse. Each measurement thus reveals at most one bit of information. Is it possible to lift this constraint by not using the qubit as an ancilla?
Here we report an experiment where more than 3 bits of information about the photon number in a microwave resonator are simultaneously recorded by a superconducting qubit into 9 propagating modes of a transmission line. This recording occurs by exciting these 9 modes using multiplexed spectroscopy owing to the dependence of the qubit frequency on photon number [3,4]. We demonstrate the simultaneous acquisition of microwave signals emitted by the qubit into the nine propagating modes that each reveals information about a photon number from 0 to 8, enabled by the large bandwidth of present near quantum limited amplifiers
[5]. Further observing the multiplexed measurement back-action on the resonator allowed us to evidence an optimal qubit drive amplitude for information extraction. Our experiment unleashes the full potential of quantum meters by bringing the measurement process in the frequency domain. It extends the reach of simultaneous probing of a single qubit by multiple observers [6,7] to arbitrary systems such as our resonator. This concept can be applied to improve or renew quantum control based on measurement [8] with possible applications in quantum error correction and sensing.

1. Guerlin, C.et al. Progressive field-state collapse and quantum non-demolition photoncounting. Nature448, 889–93 (2007).
2. Murch, K. W., Weber, S. J., Macklin, C. & Siddiqi, I. Observing single quantum trajectories of a superconducting quantum bit. Nature502, 211–214 (2013).
3. Minev, Z. K.et al. To catch and reverse a quantum jump mid-flight. Nature(2019).
4. Schuster, D. I.et al. Resolving photon number states in a superconducting circuit. Nature445,515–518 (2007).
5. Gely, M. F.et al. Observation and stabilization of photonic Fock states in a hot radio-frequency resonator. Science363, 1072–1075 (2019).
6. Hacohen-Gourgy, S.et al. Dynamics of simultaneously measured non-commuting observables. Nature538, 491 (2016).
7. Ficheux, Q., Jezouin, S., Leghtas, Z. & Huard, B. Dynamics of a qubit while simultaneously monitoring its relaxation and dephasing. Nature Communications9, 1926 (2018).
8. Wiseman, H. M. & Milburn, G. J. Quantum Measurement and Control (Cambridge UniversityPress, 2009).

poster

Quantum key distribution is the first application of quantum physics which has entered the commercial market. The security of QKD protocols relies on the quantum nature of the physical system used. In this work, we present a QKD protocol, security of which depends upon the contextuality of mono-party system. It has been shown[1] that Bell CHSH non-locality inequality[2] probes contextuality in single four-level system. We use this contextuality inequality in the proposed protocol as a security check. The advantage of the proposed protocols as compared to Ekert is that it does not involve entangled states, but provides the same level of security. Thus, the cost of generation of entangled states is saved. The experimental key generation rate of proposed protocol will also be higher than the one with entangled states because one can use weak coherent pulses instead of limited brightness of entangled photon pair sources[3].With advancement in generation and manipulation of higher dimensional OAM states, our protocols can be experimentally realized. Thus, we also propose the setup for experimental implementation of the protocol using orbital angular momentum states. Our work is generalizable to conference QKD as w

Contextuality based Quantum key distribution(QKD)

The frequency stabilisation or 'locking' of narrow linewidth lasers to atomic transitions is a common requirement in atomic and molecular physics experiments. There are many well-established techniques for locking a laser on, or close to resonance including saturated absorption spectroscopy (SAS), polarisation spectroscopy, dichroic atomic vapour level lock (DAVLL), etc. However, it can be desirable to lock away from resonance which is more difficult. Here we demonstrate a simple, tunable frequency reference based upon SAS and frequency modulation spectroscopy using the Rb-85 D1 line in a strong magnetic field as a reference for a Cs Rydberg transition.

Zeeman Shifted Atomic Reference for efficient laser stabilisation and Rydberg excitation

We discuss here three use case studies of domain wall encoding presented in [1]. One is the flight gate scheduling problem, another is the network routing problem and the third is the 3-coloring problem. The data for the flight gate problem is retrieved from database of a medium size German airport where the scale of complexity has been reduced to fit into current capacity of quantum processor. The network graph structure is emulated using a realistic topology generator for WMN (NPART) and the encodings were conducted in NS3. And the 3-coloring problem instances are generated using the random graph tool in Python. We can show the algorithms that employ domain wall encoding can successfully return solutions that conform to the formatting and domain wall encoding demonstrates higher embedding efficiency over the traditional one hot encoding method. We used X-Hot ratio to investigate into the disconformity issue of one hot scheme and notice that all the bad solution variables are in 0-Hot format. In all cases examined here, we find that the domain wall encoding almost always yields superior performance to one hot.
[1] Chancellor N 2019 Quantum Sci. Technol. 4 045004 https://doi.org/10.1088/2058-9565/ab33c2


Domain Wall Encoding Use Case Studies

A nanomechanical oscillator can be used as a sensitive probe of a small linearized mechanical force. We propose a basic short-pulsed quantum optomechanical scheme using a coherent light mode in the cavity and weak light-matter interactions. By applying homodyne detection to only the interacting light mode, we obtained a method which combines high-precision multiparameter estimation with robustness. We demonstrate the feasibility of our scheme using the parameters of state-of-the-art experimental setups, and thereby open the way to new nanomechanical sensors.

High-precision multiparameter estimation of mechanical force by quantum optomechanics

Topological quantum computing promises a hardware-level solution for the problem of decoherence. In particular, quantum gates based on the braiding of Majorana zero modes in superconductors are expected to be robust against control imperfections. Here, we combine earlier ideas for initialisation, control and readout to propose a setup where the robustness of the braiding-based gate could be demonstrated. By numerical experiments, we demonstrate how quasistatic disorder degrades the quality of braiding-based gates, and how that effect is reduced by increasing the system size. Our numerical experiments shed light on various practical aspects of future experiments aiming at demonstrating topologically protected quantum dynamics.

Robust topological quantum gates in numerical experiments

We theoretically investigate [1] the error mechanisms that degrade charge, spin and Majorana qubit readout based on gate reflectometry [2]. We consider the effects of charge noise and overdrive effects. We find that charge noise leads to broadening of the distribution of the parametric capacitance of the device, and hence adds uncertainty to the measured reflectance, in addition to amplifier noise. Hence charge noise leads to a reduced readout fidelity. In reflectometry, it is tempting to increase the probe strength to maximize signal-to-noise ratio and hence the readout fidelity. However, we find that in the case of Majorana qubit readout, contrast to the charge and spin qubit, there is an optimal finite probe strength leading to a maximal readout fidelity. Moreover, we show that applying a strong probe pulse causes coherent transitions between quantum states, which leads to further reduction of the readout fidelity in the form of resonant dips.
References
[1] V.Derakhshan.Maman,M.F.Gonzalez-Zalba,andA.Pályi,“Chargenoiseandoverdrive errors in reflectometry-based charge, spin and Majorana qubit readout,” arXiv: 2006.12391, 1–13, Jun. 2020.
[2] S. Plugge, et.al., , “Majorana box qubits,” New J. Phys., vol. 19, no. 1, p. 012001, Jan. 2017.

Charge noise and overdrive errors in dispersive readout of charge and Majorana qubits

Precise probing of quantum systems is one of the keys to progress in diverse quantum technologies, including quantum sensing and metrology, quantum information processing and quantum many-body manipulations. Controlled quantum spins are sensitive probes of the environment and a powerful tool for characterizing highly-complex quantum systems on a molecular-scale. We explore the ability of a qubit-probe to characterize unknown parameters of its environment. By resorting to quantum estimation theory, we analytically find the ultimate bound on the precision of estimating key parameters of a broad class of ubiquitous environmental noises which the qubit may probe. By optimizing the dynamical control on the probe under realistic constraints one may attain the maximal accuracy bound on the estimation of these parameters by the least number of measurements possible [1-5]. Applications of this protocol to quantum sensing are illustrated for quantum qubit-probes serving as thermometers [3] and sensors for imaging or monitoring biological processes [4-5]
[1] A. Zwick, G. A. Álvarez, G. Kurizki, Phys. Rev. Applied 5: 014007, 2016.
[2] A. Zwick, G. A. Álvarez, G. Kurizki, Phys. Rev. A 94: 042122, 2016.
[3] V. Mukherjee, A. Zwick, A. Ghosh, X. Chen, G. Kurizki, Commun Phys 2: 162, 2019.
[4] A. Zwick, D. Suter, G. Kurizki, G. A. Álvarez, Phys. Rev. Applied 14, 024088, 2020.
[5] M. Capiglioni, A. Zwick, P. Jiménez and G. A. Álvarez, arXiv:2006.02035, 2020.

Operating quantum sensors at their ultimate precision limit by optimization

Based on the rapid developments of quantum technology, effectively manipulating single-photon quantum state transport along the frequency domain plays an important role in the field of quantum communications. Here, we propose an atom waveguide system, which can realize controllable light-atom interaction in the single-photon level. The system is composed of a 1D-waveguide coupling to a V-type three-level atom ultrastrongly driven by an external field. Under the ultrastrong driving, the rotating-wave approximation is no longer valid between the transition of the two atomic excited states, which induces the atomic virtual transitions and quantum interferences of the output photon. We perform both theoretical calculations and simulations to study the reflection and transmission spectra of the single-photon quantum transport. By properly choosing the frequency of the incident photon and the Rabi frequency of the driven field, one can generate different frequency-correlated single-photon states and achieve unidirectional single-photon frequency conversions [ACS Photonics. 7, 2010 (2020)]. Our work demonstrates the feasibility for all-optically manipulating single-photon quantum state in an atom-waveguide experimental platform, which may promote the fundamental research on the photon-photon interaction and development of the quantum information processing.

Multifunctional frequency manipulations on single photon in an atom-waveguide system

Limiting the energy of incoming/outgoing optical signals is a central task in quantum cryptography systems. We proposed and demonstrated an optical power limiter based on the thermo-optical defocusing effect of an acrylic prism. Its potential for constructing a centralized quantum secure network is highlighted.

Securing practical quantum cryptography with optical power limiters

we experimentally demonstrate a 1.25 GHz chip-based MDI QKD where both clients possess a low-cost silicon photonic transmitter chip. The transmitter can be further integrated with the laser based on wire-bounding or hybrid integration. We perform a complete demonstration of polarization-encoding MDI-QKD and distill finite-key rates higher than the previous experiment. We also characterize the side channels observed in our high-speed chip-based MDI QKD system. This work paves the way for low-cost, wafer-scale manufactured MDI-QKD system, and represents a key step towards building QKD network with untrusted relays.

Multiplexing photon number measurement of a resonator using a dispersively coupled qubit

Next from Quantum 2020

Contextuality based Quantum key distribution(QKD)

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Next generation instruments for quantum control and readout from Zurich Instruments

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