Rydberg Nonlinear Quantum Optics
2017-10-12: Simon Ball passes PhD viva
Thesis title: "A coherent microwave interface for manipulation of single photons".
2017-07-14: Charles Adams gives invited talk at EGAS 49
Title: "Contactless photon-photon interactions".
2017-07-10: ICOLS 2017: Poster prize
Teodora Ilieva wins poster prize at ICOLS 2017.
2017-07-03: ICOLS 2017: Hot topics talk
PhD student Teodora Ilieva has been selected to present the latest results of the Rydberg Nonlinear Quantum Optics project on "Contactless Nonlinear Optics with Cold Rydberg Atoms" in a hot topics talk at ICOLS 2017.
2017-04-25: Hannes Busche passes PhD viva
Thesis title: "Contactless quantum non-linear optics with cold Rydberg atoms".
S. W. Ball, "A coherent microwave interface for manipulation of single optical photons", PhD Thesis, Durham University (2017).
H. Busche, "Contactless quantum non-linear optics with cold Rydberg atoms", PhD Thesis, Durham University (2017).
H. Busche, P. Huillery, S. W. Ball, T. V. Ilieva, M. P. A. Jones, and C. S. Adams, “Contactless non-linear optics mediated by long-range Rydberg interactions”, Nat. Phys. 13, 655–658 (2017).
H. Busche, S. W. Ball, and P. Huillery, “A high repetition rate experimental setup for quantum non-linear optics with cold Rydberg atoms”, Eur. Phys. J. Spec. Top. 225, 2839–2861 (2016).
O. Firstenberg, C. S. Adams, and S. Hofferberth, “Nonlinear quantum optics mediated by Rydberg interactions”, J. Phys. B: At. Mol. Opt. Phys. 49, 152003 (2016).
The lack of intrinsic interactions between optical photons combined with the ability to control the propagation of photons using optics makes them ideal carriers of information. At the same time, the lack of interactions makes processing of the encoded information at the level of individual quanta difficult. In conventional nonlinear optics, nonlinearities become apparent only at very high intensities. The Rydberg Nonlinear Quantum Optics project focusses on the creation of strong optical nonlinearities and effective interactions at the level of individual photons by interfacing optical photons with ultracold Rydberg atoms [1-3] confined in magneto-optical (MOT) and optical dipole traps, which exhibit strong dipolar interactions over distances of many micrometers.
To achieve a coherent mapping of the strong interactions between collective atomic Rydberg excitations  as well as the resulting effects such as dipole (or Rydberg) blockade , which permits only a single Rydberg excitation within a region of a few micrometers, onto optical photons, we are using quantum optical techniques such as electromagnetically induced transparency (EIT) and photon storage [6,7]. A brief introduction to the underlying concepts of Rydberg Nonlinear Quantum Optics can be found on our background pages.
We are interested in exploiting the mapping and the resulting nonlinearities for applications in quantum optics, quantum simulation, optical quantum information processing, and interfacing of optical photons with microwave fields. At the same time, we are also investigating the fundamental physics and collective behaviour of strongly interacting atomic dipoles ranging from the optical to the microwave regime.
Following the first experimental demonstration of a giant optical nonlinearity here at Durham  and the subsequent generation of highly non-classical states of light [9-11], Rydberg Quantum Nonlinear Optics is flourishing, and a variety of single photon devices have been realised . One of the most recent highlights of our work is the demonstration of a "contactless" interaction between photons  that are stored as collective Rydberg excitations in separate cold atom clouds and propagate in non-overlapping optical media. The interaction occurs over distances of more than 10 μm, well above the optical diffration limit. You can find out more on our results pages.
An overview of our experimental setup , which provides optical resolution on the order of 1 μm thanks to the incorporation of in-vacuo aspheric lenses and allows to perform experiments at effective repetition rates of about 150 kHz to acquire large datasets for the analysis of photon statistics, can be found here.
* indicates work by our group
* J. D. Pritchard, K. J. Weatherill, and C. S. Adams, “Nonlinear optics using cold Rydberg atoms”, Ann. Rev. Cold At. Mol. 1, 301–350 (2013).
* O. Firstenberg, C. S. Adams, and S. Hofferberth, “Nonlinear quantum optics mediated by Rydberg interactions”, J. Phys. B: At. Mol. Opt. Phys. 49, 152003 (2016).
 C. Murray and T. Pohl, “Quantum and Nonlinear Optics in Strongly Interacting Atomic Ensembles”, Adv. At. Mol. Opt. Phys. 65, 321–372 (2016).
 M. Saffman, T. G. Walker, and K. Mølmer, “Quantum information with Rydberg atoms”, Rev. Mod. Phys. 82, 2313–2363 (2010).
 M. D. Lukin, M. Fleischhauer, R. Côté, L. M. Duan, D. Jaksch, J. I. Cirac, and P. Zoller, “Dipole Blockade and Quantum Information Processing in Mesoscopic Atomic Ensembles”, Phys. Rev. Lett. 87, 037901 (2001).
 M. Fleischhauer, A. Imamoglu, and J. P. Marangos, “Electromagnetically induced transparency: Optics in coherent media”, Rev. Mod. Phys. 77, 633–673 (2005).
 M. Fleischhauer and M. D. Lukin, “Dark-State Polaritons in Electromagnetically Induced Transparency”, Phys. Rev. Lett. 84, 5094–5097 (2000).
* J. D. Pritchard, D. Maxwell, A. Gauguet, K. J. Weatherill, M. P. A. Jones, and C. S. Adams, “Cooperative Atom-Light Interaction in a Blockaded Rydberg Ensemble”, Phys. Rev. Lett. 105, 193603 (2010).
 Y. O. Dudin and A. Kuzmich, “Strongly Interacting Rydberg Excitations of a Cold Atomic Gas”, Science 336, 887–889 (2012).
 T. Peyronel, O. Firstenberg, Q.-Y. Liang, S. Hofferberth, A. V. Gorshkov, T. Pohl, M. D. Lukin, and V. VuletiÄ‡, “Quantum nonlinear optics with single photons enabled by strongly interacting atoms”, Nature 448, 57–60 (2012).
* D. Maxwell, D. J. Szwer, D. Paredes-Barato, H. Busche, J. D. Pritchard, A. Gauguet, K. J. Weatherill, M. P. A. Jones, and C. S. Adams, “Storage and Control of Optical Photons Using Rydberg Polaritons”, Phys. Rev. Lett. 110, 103001 (2013).
* H. Busche, P. Huillery, S. W. Ball, T. V. Ilieva, M. P. A. Jones, and C. S. Adams, “Contactless non-linear optics mediated by long-range Rydberg interactions”, Nat. Phys. 13, 655–658 (2017).
* H. Busche, S. W. Ball, and P. Huillery, “A high repetition rate experimental setup for quantum non-linear optics with cold Rydberg atoms”, Eur. Phys. J. Spec. Top. 225, 2839–2861 (2016).
We gratefully acknowledge funding from the following sources.
European Union (FP7 and Horizon 2020 programmes)
Engineering and Physical Research Council (EPSRC) grants
Defence Science and Technology Laboratory (DSTL)
Solid State Rydberg Systems
We are building an experiment to investigate excitons in cuprous oxide. Excitons are an electron and hole in a semiconductor that are bound by coloumb attraction. We are interested in high principle quantum number (n > 10) excited states or "Rydberg states" of these excitons. On the right is a spectrum showing the absorbance peaks caused by these excited states up to a principle quantum number of n = 11.
The size of the wavefunction of the exciton scales as n2 and so at the limit of the most recent measurements, the orbital is in the order of µm wide. These extended wavefunctions exhibit large dipole moments that shift the energy levels of nearby lattice sites. If this shift is larger than the linewidth of the laser used to excite the Rydberg excitons, then no more excitons can be generated within that region of the crystal. This leads to a "Rydberg blockade" effect, where only one Rydberg exciton can exist within a given volume. On the left is a diagram showing the energy level of a second instance of an exciton shifting as a function of its distance from the first instance. The distance RB is defined as the blockade radius. If R<RB then the system is in the blockaded regime where only one Rydberg exciton can exist for a given laser linewidth.
For high n Rydberg excitons the blockade radius is larger than the diffraction limited spot size of the excitation laser. Using a two photon excitation scheme, we plan to exploit the blockade effect to create a single photon emitter. Our hope is that Rydberg excitons in this solid state system, cooled to 3.1 K in a cryostat, should exhibit many of the characteristics of atomic Rydberg systems, which are widely studied here in Durham. This experiment is therefore an exciting new opportunity at the interface of gas-phase atomic spectroscopy and solid-state physics.Collaborators
Thermal Cs Rydberg
Strong Interactions in a High Number Density Rydberg Vapour
We use a 3-step excitation process to create Rydberg atoms in a Caesium vapour confined within a glass cell. By heating the cell, we can control the vapour pressure and the number density of excited atoms, and by measuring the fluorescence and transmission properties of the medium we can interrogate the strong interactions between the excited atoms.
The experiments are either pulsed (looking for transient effects) or continuous wave CW (exploring the steady states of the system). In the CW setup we find ‘intrinsic optical bistability’ where a hysteresis effect means that the medium has two distinct responses to the same stimulus. In the pulsed regime we measure ‘critical slowing down’ where the response time of the medium diverges as a first-order phase transition between the two bistable responses is approached. Although the optical bistability is a steady-state phenomenon, the system is far from equilibrium, as it is being driven and dissipating energy. Our experiment therefore provides an opportunity to study an unusual example of a non-equilibrium phase transition.
Nonequilibrium Phase Transition in a Dilute Rydberg Ensemble
C. Carr, R. Ritter, C. G. Wade, C. S. Adams, and K. J. Weatherill
Phys. Rev. Lett. 111, 113901 (2013)
We have demonstrated a nonequilibrium phase transition in a dilute thermal atomic gas. The phase transition, between states of low and high Rydberg occupancy, is induced by resonant dipole-dipole interactions between Rydberg atoms. The gas can be considered as dilute as the atoms are separated by distances much greater than the wavelength of the optical transitions used to excite them. In the frequency domain, we observe a mean-field shift of the Rydberg state which results in intrinsic optical bistability above a critical Rydberg number density. In the time domain, we observe critical slowing down where the recovery time to system perturbations diverges with critical exponent α=-0.53±0.10. The atomic emission spectrum of the phase with high Rydberg occupancy provides evidence for a superradiant cascade.
Two-Electron Rydberg systems
Laser cooling and laser spectroscopy are powerful techniques for controlling the motion and quantum state of individual atoms. In this project we aim to extend this level of control to the interactions between the atoms. By exciting laser-cooled strontium atoms to a very high lying electronic energy state - known as a Rydberg state, we can switch on strong, long-range van der Waals interactions between the atoms, which completely dominate the behaviour of the atom cloud. Uniquely, our experiment uses strontium atoms, which also have a second valence electron that can be used to probe and manipulate the Rydberg gas. So far, we have shown that this can provide information on collisions that can transfer atoms from one Rydberg state to another, with very high temporal resolution. We have also extended these techniques to provide spatial information, enabling us to probe spatial correlations in this strongly interacting system. We have most recently coupled the excited state of a strontium narrow-line MOT to a high-lying Rydberg state, known as Rydberg dressing, to create a many-body system with long-range interactions and active cooling.
Current Research Directions:
A promising approach to control both interacting and dissipative properties of a Rydberg gas is to off-resonantly couple the atomic ground state to a Rydberg state, allowing to admix some interacting Rydberg character to a long lived state. We are currently using a novel dressing scheme where we dress the upper state of the spin forbidden 5s2 1S0 → 5s5p 3P1 narrow-line MOT transition to a high lying Rydberg state. Unlike previous Rydberg-dressing experiments, this upper state is dissipative which facilitates active cooling rather than just conservative trapping of a Rydberg-dressed ensemble. We allow atoms to acquire the special properties of Rydberg states, i.e. sensitivity to DC electric field and long-range dipolar interactions, whilst still being laser-cooled on the closed MOT transition. Active cooling of a Rydberg dressed gas remains a relatively unexplored area and one could expect interesting physics to arise from the presence of cooling and mechanical effects of the interactions.
We have recently developed a Monte-Carlo simulation to gain further insights into narrow-line MOT's, both in the presence and absence of the dressing laser. This model is able to quantitatively reproduce the spatial, temporal and thermal dynamics of narrow-line MOT's. The figure below is from our latest arXiv submission and shows excellent agreement between the experimental (top row) and theoretical (bottom row) absorption images for a variety of dressing laser detunings.
Narrow line Rydberg spectroscopy
Most Rydberg spectroscopy in strontium has been performed on singlet Rydberg states that are relatively accessible with blue wavelengths (461nm and 413nm) and a broad intermediate transition (30.2MHz). We are currently exploring previously unobserved triplet Rydberg states accessible via the 7.5kHz spin-forbidden 5s5p 3P1 state, using 689nm and 319nm lasers, both frequency stabilised to better than 40kHz. These states offer a wide range of properties, with isotropic, anisotropic, attractive and repulsive Rydberg interactions.
Using a femtosecond optical frequency comb to measure energy levels to better than 50kHz absolute precision, observing coupling strength using Autler Townes spectroscopy, measuring the polarizability of the Rydberg states, and using autoionisation spectroscopy of the Rydberg states as a measure of both quantum defects and blackbody transfer rates, we can perform very detailed high resolution spectroscopy of a range of Rydberg states.
Spin squeezing in Rydberg lattice clocks
Atomic clocks based on an extremely narrow optical transition in strontium are currently leading the field in stability and accuracy, enabling frequency measurements at the 10-18 level. In an ongoing collaboration with the group of Thomas Pohlat the Max Planck Institute for Complex Systems in Dresden, we are investigating the use of Rydberg states to improve the performance of the clock even further.
This work was supported by the EU STREP HAIRS, a network of researchers from Durham, Nottingham, Tubingen, Dresden and Palaiseau with the aim of using these techniques to create a hybrid quantum system of atoms and superconducting circuits.
The figure on the right is from our publication where we propose to create very strongly squeezed states in a lattice clock using Rydberg dressing, with implications for quantum-enhanced frequency metrology.