Quantum Photonics

Deterministic generation of entangled photons

Device-independent cryptography achieves unbreakable security by exploiting quantum correlations (e.g. entanglement) between the sender and the receiver. The practical implementation of such protocols requires efficient and high-fidelity entangled photon sources. Solid state emitters are the way forward for efficient quantum light generation due to the possibility to integrate them into a nanophotonic structure. Recent demonstrations of high-fidelity (> 97%) polarization entanglement using the biexciton cascade in GaAs/AlGaAs quantum dots open exciting route for deterministic entanglement generation. The project will deal with several aspects of the nanophotonic interfacing of these quantum dots including: 1) Efficient preparation of the quantum dot, 2) Extraction of entangled photon pairs and state tomography, 3) Sources of noise limiting the entanglement fidelity, 4) near-future applications of the entangled photons for cryptography. For more information, contact Prof. Peter Lodahl (lodahl@nbi.ku.dk).

Coherent single photons: Interfacing with memories

The distribution of quantum states across nodes in a quantum network is the key requirement for secure communication and distributed computing. A typical quantum network will involve interfacing disparate qubits (matter, photon, spin, etc.) to ensure maximum efficiency. Photons are robust carriers of information and hence are critical for interconnection. The disparity between the frequency and bandwidth across the qubits pose challenges in the interconnection. Efficiently crafting the bandwidth of the photon is the key enabler for photonic interconnects. Quantum dot single photon sources typically generate photons with GHz bandwidth. Efficient interfacing of photons with quantum memories (holding time ~ 0.1 – 1 us) requires ~10 MHz bandwidth. A promising way forward to achieve such coherent single photons is through Raman processes in a Λ system. The project will deal with coherent photon generation using one of the Λ-systems of a trion in a InAs quantum dot.  Key aspects: 1) Branching ratio and limits on the bandwidth of photons, 2) Generation and characterization  3) Pulse shaping for bandwidth manipulation. For more information, contact Prof. Peter Lodahl (lodahl@nbi.ku.dk).

Nonlinear chiral quantum optics

Chiral quantum light-matter interactions are a recently discovered consequence of using nanophotonic platforms for quantum optical experiments.  In chiral scenarios, the light-matter interaction is strongly directional, meaning that the interaction of photons with a quantum dot strongly depends on which way they travel, allowing for the design of novel and non-reciprocal quantum devices. Within this project, we will explore the nonlinearity of such chiral quantum interactions, using a blend of numerical modelling and optical experiments. For more information, contact Prof. Peter Lodahl (lodahl@nbi.ku.dk).

Giant cooperativity in planar structures

Cooperativity is a fundamental property within quantum optics that captures the strength of the coupling between light (photons) and matter (quantum emitters).  Within this project, we wish to enhance this cooperativity using resonant, planar nanophotonic structures, thereby setting the stage for the creation of efficient and complex quantum architecture. For more information, contact Prof. Peter Lodahl (lodahl@nbi.ku.dk).

Multicolour quantum optics

Quantum emitters such as quantum dots are inherently nonlinear at the ultimate, low-energy limit; after all, they can only absorb a single photon at a time.  This inherently large nonlinearity means that the interaction of a single emitter with a single photon can be significantly tuned by a single, control photon.  Within this project, we will model this multicolour nonlinear interaction and search for signatures of our model in optical experiments. For more information, contact Prof. Peter Lodahl (lodahl@nbi.ku.dk).

Topological quantum photonics

Topological photonic edge states, which exist at the interface between especially engineered photonic crystals, have recently been shown to guide photons emitted by a single quantum dot.  This has generated considerable excitement within the quantum photonics community as these edge modes are protected from backscattering due to defects or fabrication imperfections and, additionally, allow for directional and non-reciprocal light-matter interactions.  Within this project, we will study quantum light-matter interactions with topological edge-states using numerical modelling techniques. For more information, contact Prof. Peter Lodahl (lodahl@nbi.ku.dk).

Scalable chip-to-fiber coupling

The project aims at building an off-chip fiber delay based on optical fibers. Off-chip fiber delays are key ingredients for performing advanced quantum protocols and emitter de-multiplexing. Based on our state-of-the-art chip-to-fiber couplers, we are planning to couple multiple fibers to nanophotonic waveguides using matrix arrays. The project involves building/adapting a room-temperature setup for aligning and bonding chips to matrix arrays, perform numerical simulations of the optimal power transfer to fibers, design and (optionally) fabricate the chip and measure it in the lab. For more information, contact Asst. Prof. Leonardo Midolo (midolo@nbi.ku.dk).

Resonance fluorescence and Purcell enhancement

The goal is to design a device that enables a scheme for resonantly driving an emitter inside a cavity. There are different strategies that involve combining a tunable cavity and a waveguide-based resonant excitation scheme. The project involves numerical simulations of the device to improve the extinction between the pump laser and the emitted photons. The device is then characterized in the lab. Optionally, the student has the possibility to get training to the cleanroom and fabricate the sample. For more information, contact Asst. Prof. Leonardo Midolo (midolo@nbi.ku.dk).

Electro-optical routing of single-photons

Design a waveguide-based electro-optic switch operating at >10 MHz for fast optical routing of photons. The router is key to implement protocols for de-multiplexing single-photons. Based on our recent results in electro-optic routing technology, we plan to build an integrated Mach-Zehnder interferometer with embedded phase shifters. The project involves the design, fabrication, and characterization of the device. An interest in acquiring nanofabrication skills is preferred.  For more information, contact Asst. Prof. Leonardo Midolo (midolo@nbi.ku.dk).

Chippen over dem alle
The chip over them all

• Link to article on Weekendavisen (2023)

Peter Lodahl interviewed: De skal løse de store problemer: Kvantecomputere trodser enhver logik
Peter Lodahl interviewed: They should solve all the big problems: The quantum computer defies all logic

• Link to article on Herning Folkeblad (2023)

Kvantestartup er klar med sin første revolutionerende chip
Quantum startup is ready with their first revolutionary chip. 

• Link to article on Børsen (2023)

Danske forskere krypterede samtale med kvanteteknologi og en hacker gik i fælden
Danish researchers encrypted conversation with quantum technology and a hacker got caught
Articles on the quantum encrypted channel between DTU and NBI.

• Link to article on DR.dk (2022)
• Link to article on Politiken (2022)
• Link to article on Børsen (2022)

Nyt firma vil masseproducere qubit til optisk kvantecomputer
New company will mass produce qubits for optical quantum computers

• Link to article from Ingeniøren (2021)

Kvantecomputeren - det nye rumkapløb
The quantum computer - the new space race

• Link to story on Danish national television DR.dk (2021)

Dansk chip kan danne grundlag for optiske kvantesimulatorer
Danish chip will form the foundation for optical quantum simulators

• Link to article from Ingeniøren (2020)

Kvante-gennembrud: Dansk opfindelse kan give computere superkræfter
Quantum breakthrough: Danish invention can give computers superpowers

• Link to article on Videnskab.dk (2020)

Megabevilling til kvantesimulatorer viser, at grundforskning lønner sig.
Large grant for quantumsimulators shows that fundamental research pays off.
Article (in Danish) on our new center Solid-state quantum simulators for biochemistry.

• Link to article on uniavisen.dk (2020).

"Den interne proces var ekstrem. Vild og blodig."
"The internal process was extreme. Wild and bloody."
Profile (in Danish) of recent recognitions and scientific accomplishments by Prof. Peter Lodahl and his Quantum Photonics research group.

• Link to article on uniavisen.dk (2017).

Danske forskere baner vejen for fremtidens computer.
Danish researchers pave the way for future computer.
Article (in Danish) on our recent effors towards implementing quantum technology.

• Link to article on information.dk (2017).

Building the Quantum Internet.
Interesting article on how a solid-state material based quantum internet could be realized:

• Link to Association for Computing Machinery (ACM) article (2017).

Chiral quantum optics, Nature (2017).
Highlighted in more than 25 news stories in scientific and general online magazine. Here are some examples:

• First Step Towards Photonic Quantum Network.
  
• Kvantenetværket der kender forskel på højre og venstre (further down the page).
  
• Development of Photonic Quantum Network

Deterministic photon-emitter coupling in chiral photonic circuits, Nature Nanotechnology (2015).

• Quantum networks: Back and forth are not equal distances, Phys.org (July 2015).
• Physicists have forced light to move in one direction inside the chip, Ria.ru (July 2015).
• Kvantemekanisk gennembrud: Dansk chip kan styre lys-partikler, DR (July 2015).

Nonuniversal intensity correlations in a two-dimensional Anderson-localizing random medium, Physical Review Letters 109, 253902 (2012).

• The paths of photons are random - but coordinated (December 2012).
• How photons mutually coordinate their paths, Novus light Technologies (January 2013).
• Photons sense each other, Photonics.com (January 2013).
• Light Labyrinth, Optics&Photonics News (March 2013).

Optical cavity cooling of mechanical modes of a semiconductor nanomembrane, Nature Physics 8, 168-172 (2012).

• Heating cools a semiconductor, physicsworld.com, (January 2012).

Strongly Modified Plasmon-Matter Interaction with Mesoscopic Quantum Emitters, Nature Physics 7, 215-218 (2011).

• Enhanced Coupling of Mesoscopic Quantum Dots to Plasmons, 2Physics.com, (January 2011).

Quantum Interference and Entanglement Induced by Multiple Scattering of Light, Phys. Rev. Lett. 105, 090501 (2010).

• Kvantecomputeren får potens af danske fysikere, Videnskab.dk, (August 2010).

Cavity quantum electrodynamics with Anderson-localized modes, Science 327, 1352 (2010).

• Quantum Devices to Thrive on Disorder, Optics & Photonics focus, (March 2011).
• Uperfekte chips kan bane vejen for ny kvanteteknologi, Aktuel Naturvidenskab nr. 6, 2010.
• For quantum computer, add a dash of disorder, ScienceNews, (March 2010).
• Shoddy construction beats precision in quantum world, NewScientist, (March. 2010).
• Imperfect Chips Enhance Quantum Technology, photonics.com, (March 2010).
• Quantum Computing Thrives on Chaos, Wired Science, (March 2010).
• Imperfections are perfect, Alpha Galileo, (March 2010).
• DTU-forskere: Uorden er vejen til chips i kvantecomputere (in Danish), Ingeniøren, (March 2010). 

Observation of Spatial Quantum Correlations Induced by Multiple Scattering of Nonclassical Light, Phys. Rev. Lett. 102, 193901 (2009).

• Tåget kvantefysik giver lys idé (in Danish), videnskab.dk, (Dec. 2009).

Podcast

• Christiansen & Andersen, Lyset

Popular papers

1) P. Lodahl, At kontrollere udsendelsen af lys (in Danish), Aktuel Naturvidenskab 4, 15-17 (2004).

2) P. Lodahl and W. L. Vos, Controlling spontaneous emission with photonic crystals, Photonics Spectra 39 (3): 64+ MAR (2005).

3) P. Lodahl, Lys i fotoniske krystaller og optiske nanobokse (in Danish), Naturens Verden 1, p. 29-35 (2006).

4) P.T. Kristensen, P. Lodahl, and J. Mørk, To decay or not to decay – or both! Quantum mechanics of spontaneous emission, DOPS Nyt 1, p. 4-8 (2008).