One-dimensional quantum emitters and photons for quantum technologies: 1D QED

Lead Research Organisation: University of Bristol
Department Name: Electrical and Electronic Engineering


Quantum technologies exploit the intrinsic quantum nature of particles such as photons and electrons. It has been known for some time that the ability to control the exact state of these particles, and to precisely control how they interact, will lead to unprecedented breakthroughs in a variety of technological applications.

One of the immediate goals of quantum technologies is to exploit the fact that quantum particles can never be copied whilst retaining all of their information. Because the particle cannot be cloned, one may encode a cryptographic key in this way, as an eavesdropper would reveal their presence by changing the photon state as it is measured. Practical cryptographic "quantum key distribution", however, has limited information transfer rate by the fact that one needs to ensure that only one photon is transmitted per bit. To make sure that exactly one photon is generated, a "single photon source" from a quantum emitter such as an atom is required, or in our case, an "artificial atom", a quantum dot. We will fabricate single photon sources that output photons with very high efficiency into a fibre at a useful telecommunications wavelength (1300nm).

The long-term goal of quantum technologies is to create a "universal quantum computer". This would use quantum particles as "quantum bits" that show the property of "superposition" (the ability to prepare a particle two states at once) and "entanglement" (sharing the superposition between several particles). Manipulating quantum bit interactions leads to a way of performing calculations with a complexity that speeds up exponentially with number of quantum bits. Preparing states for a quantum computer that will perform any calculation (a "universal" computer), however, is very challenging.

Nevertheless, if one has a particular complex problem to solve, one may turn to quantum simulation instead. In this case, a calculation may be pre-programmed. It is known that by using photonic circuits (essentially a photon circuit consisting of the equivalent of mirrors and beamsplitters) one may perform a quantum simulation. A network of many channels is set up, and single photons input into chosen channels. However, an important requirement is that, again, controlled single photons must be available. The requirements are more stringent than for quantum communication. A second requirement is that each photon must be absolutely identical in bandwidth, wavelength and polarization - this is known as "indistinguishability". Indistinguishable photons input onto a beamsplitter undergo quantum interference that acts as a logic gate.
Truly indistinguishable single photons are extremely difficult to create. Nevertheless, a great deal of progress has been made in precisely controlling single photons using single atoms trapped in an optical cavity. However, atoms emit photons slowly, and collecting all photons is difficult. The rate at which single photons can be generated is presently still too low and the experimental setup involved very large, and unsuitable for anywhere except a laboratory.

However, quantum dots have very similar properties to atoms. These emit light far faster than atoms (at a rate of 1 billion photons per second) and may also be incorporated into semiconductor "cavities". In this proposal, I will show that one may collect the light extremely efficiently using similar optical fibre technology to that used in telecommunication networks. By doing this, I will provide single photon sources to quantum communication networks and quantum simulation devices. This will lead to absolutely secure communications, and the ability to calculate properties of novel materials or complex molecules to help design new drugs, and factorize large prime numbers used in cryptography.

Planned Impact

In this proposal I will create (i) high quality indistinguishable (>99%) single photon sources (SPS) on demand (efficiency >90%) (ii) microsecond quantum memories and (iii) photonic cluster states, all with outputs to a single mode fibre. These are based on self-assembled quantum dots and will be easily interfaced with other quantum technology devices, such as quantum communication networks and quantum optical integrated circuits. Impact will be mainly on immediate and future quantum technologies, but will also impact fundamental physical science in the mid to long term.

In quantum key distribution and other quantum communication applications, both single photon sources and quantum memories are important. At present, a QKD system uses a strongly attenuated laser with decoy states that give approximately 0.3 photons per pulse, and significant post-processing is required. A single photon source of efficiency >70% produces significant increase in efficiency, and therefore effectively increases the feasible distance between repeater stations by ~50km. A quantum memory relay would allow concatenation of links of ~100km, which if loss resistant would allow quantum states to be transported over distances of >1000km. Thus this single photon source could be the route to enabling practical commercial QKD systems to become widespread.

Non-universal quantum computing is a class of computational techniques that address specific classically intractable tasks with qubits. Examples include quantum simulation, a technique which simulates classically intractable systems such as in quantum chemistry (where even the full quantum mechanical modelling of even 10 atom molecules is intractable classically) or solid-state physics, or for specific prime number factorisation tasks. Boson sampling involves inputting non-interacting bosons (in this case photons) into a linear-optic interferometer circuit (relatively easy to fabricate and at room temperature with multiple fibre input and output) that might, for example require input of n photons into an m channel device. Subsequent interference in the device performs a specific calculation. All these devices require indistinguishable on-demand single photons. With a 20 photon input (feasible using QDs and fibre output with a simple lossless 100MHz switch (eg EOM)) classically intractable problems are achievable. While development of protocols to map specific problems is still underway, the overwhelming barrier to the realisation of these devices are efficient and indistinguishable single photon sources.

In optics and photonics technologies the ability to achieve >70dB polarization extinction whilst retaining low losses would be a useful resource for many quantum and classical technologies, for example on-chip four-wave-mixing.

In the longer term, single photon sources may have extensive academic impact, for example in quantum simulations of molecules used in biochemistry and medicine, or in simulating complex phenomena etc. The extension of this technique to different wavelengths (eg to visible using colloidal QDs) may allow a direct delivery of well-defined packets of energy to single absorbers, allowing nanoscientists and biochemists to measure photochemical changes at the single molecule level.

Finally, efficient QD photon collection would allow a step-change in the capabilities of present QD-based basic Physics research. For instance, the ability to perform single shot measurements would allow sensitive quantum non-demolition techniques to be used to probe the electron spin dynamics. In certain circumstances, this would make, for instance, the probing of the nuclear spin state of the QD highly sensitive. In the long term, this may lead to an ability to use the millisecond coherence time nuclear spins as a quantum memory.
Description Key findings:
-The approach of using high Q-factor cavities to couple QDs to photons is unnecessary: well-designed low Q-factor structures are better: they are more fabrication tolerant, have high efficiencies and show no problems with polarization degeneracy.
-We have a greater understanding of how spins and nanostructures have an interplay that goes beyond a simple atomic-like cavity QED approach.
-We attempted and failed to affix single mode fibres to our nanostructures with efficient outcoupling. Different approaches should be taken.
-We have demonstrated the world's brightest QD single photon source (69% in our recent PRL from 2022) with the added advantage that we have full access to polarization states, allowing entangled photon emission and spin-photon interfaces
Exploitation Route Our work has demonstrated that by and large the community should probably abandon high Q-factor structures for QDs unless there is a specific reason, eg a high Purcell factor is required. This will make fabrication of QD devices using scalable manufacturing methods more achievable.
Sectors Aerospace

Defence and Marine

Digital/Communication/Information Technologies (including Software)


including Industrial Biotechology

Security and Diplomacy

Description FCDO-SRF
Geographic Reach Multiple continents/international 
Policy Influence Type Contribution to a national consultation/review
Description Horizon 2020 Quantum Flagship
Amount € 2,809,812 (EUR)
Organisation European Union 
Sector Public
Country European Union (EU)
Title Supporting Data for "Optimal simultaneous measurements of incompatible observables of a single photon" 
Description EPSRC fellowship project Data will include electromagnetic simulations, raw spectroscopic data, processed data and details of processing code. 
Type Of Material Database/Collection of data 
Year Produced 2019 
Provided To Others? Yes  
Description COST 
Form Of Engagement Activity A formal working group, expert panel or dialogue
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Policymakers/politicians
Results and Impact As a result of my activities in quantum technologies I was asked to be UK representative on COST Action "Nanoscale Quantum Optics". Here I became gender balance advisor and tackled questions regarding under representation of women in the quantum technology area.

I gave information sessions about implicit bias and other issues at scientific meetings, to the general audience of all attendees - men and women scientists. I also ran discussion sessions and ran two surveys to monitor attitudes towards gender imbalance to monitor changes in opinion. The survey showed that after the intervention many more men reported being engaged in activities to counteract underrepresentation of women in science.

The outcome was that I was asked to present at Europe's foremost science policy conference, ESOF 2018. I was also asked to present in Feb 2020 to policy makers in Brussels (from COST and from Horizon 2020) about the activities I had undertaken.
Year(s) Of Engagement Activity 2016,2017,2018,2019
Description Co-founder of quantum technology "QPHOT" session at ICTON2016 onwards 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Industry/Business
Results and Impact After a successful organisation of a workshop related to quantum spin effects at ICTON2015, I, along with Dr. Daryl Beggs at the University of Bristol have co-founded a new session for ICTON on the topic of quantum photonic technologies. The aim is to engage those in the fields of classical photonics and telecommunications, including many attendees from Industry, with quantum technologies. ICTON (International Conference on Transparent Optical Networks) is primarily focused on classical photonic devices and telecommunications technologies.
Year(s) Of Engagement Activity 2016
Description HTLGI festival 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact I was a panel member on a debate on "The Quantum Hoax" at Hay-on-Wye at the "How the Light Gets in Festival". The debate was recorded and posted on YouTube by IAI
Year(s) Of Engagement Activity 2023