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Journal articleAlexander O, Ayuso D, Matthews M, et al., 2025,
Attosecond physics and technology
, Applied Physics Letters, Vol: 126, ISSN: 0003-6951In this Perspective, we provide an overview of the field of attosecond physics and technology for which the 2023 Nobel Prize in Physics was awarded to Pierre Agostini, Ferenc Krausz, and Anne L'Huillier. We look at the current state-of-the-art in attosecond timescale measurements and recent advances enabled by the light field driven technology inspired by the earlier work in the field. We end this Perspective with some comments on the exciting future directions now developing in the field.
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Journal articleKim M, 2025,
Experimental demonstration of generalized quantum fluctuation theorems in the presence of coherence
, Science Advances, ISSN: 2375-2548 -
Journal articleAthanasakis-Kaklamanakis M, Wilkins SG, Skripnikov LV, et al., 2025,
Electron correlation and relativistic effects in the excited states of radium monofluoride.
, Nat Commun, Vol: 16Highly accurate and precise electronic structure calculations of heavy radioactive atoms and their molecules are important for several research areas, including chemical, nuclear, and particle physics. Ab initio quantum chemistry can elucidate structural details in these systems that emerge from the interplay of relativistic and electron correlation effects, but the large number of electrons complicates the calculations, and the scarcity of experiments prevents insightful theory-experiment comparisons. Here we report the spectroscopy of the 14 lowest excited electronic states in the radioactive molecule radium monofluoride (RaF), which is proposed as a sensitive probe for searches of new physics. The observed excitation energies are compared with state-of-the-art relativistic Fock-space coupled cluster calculations, which achieve an agreement of ≥99.64% (within ~12 meV) with experiment for all states. Guided by theory, a firm assignment of the angular momentum and term symbol is made for 10 states and a tentative assignment for 4 states. The role of high-order electron correlation and quantum electrodynamics effects in the excitation energies is studied and found to be important for all states.
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Journal articleAllegre H, Broughton JJ, Klee T, et al., 2025,
Extension of high-harmonic generation cutoff in solids to 50 eV using MgO.
, Opt Lett, Vol: 50, Pages: 1492-1495High-harmonic generation (HHG) in solids driven by femtosecond lasers is a promising method for the compact production of coherent extreme ultraviolet (XUV) radiation but so far has been limited to photon energies below 40 eV. Here, we report the highest ever recorded photon energy for a harmonic in a solid sample, reaching 50 eV (31st harmonic) in 100-µm-thick MgO, using a 780 nm, 30 fs driving pulse. This is achieved through optimization of the spectrometer and detection efficiency, as well as an increase in emission efficiency enabled by a larger excitation area and the use of a multi-cycle pulse. We observe that the harmonic cutoff exhibits nontrivial behavior as a function of laser field strength, suggesting that an extension to our existing understanding of the generation process may be needed. This work demonstrates further the potential for compact XUV sources beyond 50 eV based on solid-state media.
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Journal articleCryer-Jenkins EA, Major KD, Clarke J, et al., 2025,
Enhanced Laser Cooling of a Mechanical Resonator via Zero-Photon Detection.
, Phys Rev Lett, Vol: 134Throughout quantum science and technology, measurement is used as a powerful resource for nonlinear operations and quantum state engineering. In particular, single-photon detection is commonly employed for quantum-information applications and tests of fundamental physics. By contrast, and perhaps counterintuitively, measurement of the absence of photons also provides useful information, and offers significant potential for a wide range of new experimental directions. Here, we propose and experimentally demonstrate cooling of a mechanical resonator below its laser-cooled mechanical occupation via zero-photon detection on the anti-Stokes scattered optical field and verify this cooling through heterodyne measurements. Our measurements are well captured by a stochastic master equation and the techniques introduced here open new avenues for cooling, quantum thermodynamics, quantum state engineering, and quantum measurement and control.
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Journal articleSun J, Vilchez-Estevez L, Vedral V, et al., 2025,
Probing spectral features of quantum many-body systems with quantum simulators.
, Nat Commun, Vol: 16The efficient probing of spectral features is important for characterising and understanding the structure and dynamics of quantum materials. In this work, we establish a framework for probing the excitation spectrum of quantum many-body systems with quantum simulators. Our approach effectively realises a spectral detector by processing the dynamics of observables with time intervals drawn from a defined probability distribution, which only requires native time evolution governed by the Hamiltonian without ancilla. The critical element of our method is the engineered emergence of frequency resonance such that the excitation spectrum can be probed. We show that the time complexity for transition energy estimation has a logarithmic dependence on simulation accuracy and how such observation can be guaranteed in certain many-body systems. We discuss the noise robustness of our spectroscopic method and show that the total running time maintains polynomial dependence on accuracy in the presence of device noise. We further numerically test the error dependence and the scalability of our method for lattice models. We present simulation results for the spectral features of typical quantum systems, either gapped or gapless, including quantum spins, fermions and bosons. We demonstrate how excitation spectra of spin-lattice models can be probed experimentally with IBM quantum devices.
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Journal articleClarke J, Cryer-Jenkins EA, Gupta A, et al., 2025,
Theoretical framework for enhancing or enabling cooling of a mechanical resonator via the anti-Stokes or Stokes interaction and zero-photon detection
, Physical Review A, Vol: 111, ISSN: 2469-9926We develop a theoretical framework to describe how zero-photon detection may be utilized to enhance optomechanical laser cooling via the anti-Stokes interaction and, somewhat surprisingly, enable cooling via the Stokes interaction commonly associated with heating. Our description includes both pulsed and continuous measurements as well as optical detection efficiency and open-system dynamics. For both cases, we discuss how the cooling depends on the system parameters such as detection efficiency and optomechanical cooperativity, and we study the continuous-measurement-induced dynamics, contrasting with single-photon-detection events. For the Stokes case, we explore the interplay between cooling and heating via optomechanical parametric amplification, and we find the minimum efficiency required to cool a mechanical oscillator via zero-photon detection. This work serves as a companion article to our recent experiment [E. A. Cryer-Jenkins, Phys. Rev. Lett. 134, 073601 (2025)10.1103/PhysRevLett.134.073601], which demonstrated enhanced laser cooling of a mechanical oscillator via zero-photon detection on the anti-Stokes signal. The cooling techniques developed here can be applied to a wide range of areas including nonclassical state preparation, quantum thermodynamics, and avoiding the often unwanted heating effects of parametric amplification.
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Journal articleWang Y, Rodewald J, Lopez O, et al., 2025,
Wavelength modulation laser spectroscopy of N<inf>2</inf>O at 17 µm
, New Journal of Physics, Vol: 27Using a mid-infrared quantum cascade laser and wavelength modulation absorption spectroscopy, we measure the frequencies of ro-vibrational transitions of N2O in the 17 µm region with uncertainties below 5 MHz. These lines, corresponding to the bending mode of the molecule, can be used for calibration of spectrometers in this spectral region. We present a model for the lineshapes of absorption features in wavelength modulation spectroscopy that takes into account Doppler broadening, collisional broadening, saturation of the absorption, and lineshape distortion due to frequency and intensity modulation. Combining our data with previous measurements, we provide a set of spectroscopic parameters for several vibrational states of N2O. The lines measured here fall in the same spectral region as a mid-infrared frequency reference that we are currently developing using trapped, ultracold molecules. With such a frequency reference, the spectroscopic methods demonstrated here have the potential to improve frequency calibration in this part of the spectrum.
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Thesis dissertationGardner R, 2025,
Spectral purity of spontaneous parametric down conversion in a triply resonant cavity
Spectrally pure, highly squeezed light generated from cavity–enhanced spontaneous parametric downconversion is forseen to have uses in a wide range of applications, in optical quantum science andtechnology. This thesis demonstrates that type II spontaneous parametric down conversion in abirefringent, triply resonant cavity can be used to isolate squeezing in a single pair of broadband signaland idler field modes. To do so, the relationship between the cavity birefringence and finesses, andcorresponding conditions which maximise the spectral purity of the cavity output are determined. Thedesign and construction of a prototype cavity, which uses a KTP crystal, along with cavity finessesof approximately 100 for the pump field and 200 for the signal and idler fields is outlined in detail.Photon timing and photon statistics tests are performed on this protype, verifying its design andcontrol over the spectral purity of the cavity output. It is found that the resulting squeezing can bemade approximately spectrally pure with an effective mode number of 𝐾 = 1.25. Interference testsare also developed as a tool to characterise the spectral degeneracy of the signal and idler fields, andan interference visibility dip of 𝑉 = 88% is observed.
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Journal articleWang P, Kwon H, Luan C-Y, et al., 2025,
Author Correction: Snapshotting quantum dynamics at multiple time points.
, Nat Commun, Vol: 16 -
Journal articleWang J, Driver T, Franz PL, et al., 2025,
Probing Electronic Coherence between Core-Level Vacancies at Different Atomic Sites
, Physical Review X, Vol: 15The detailed understanding of electronic coherence in quantum systems requires measurements on the attosecond timescale. Attosecond x-ray pulses enable the study of electronic coherence in core-excited molecular systems. Here we report on the coherent motion of electrons in the 1,1-difluoroethylene ion following ionization of the K shell of the two nonequivalent carbon sites with a subfemtosecond x-ray pulse. Using the angular streaking technique to track the Auger-Meitner decay, we observe temporal modulations of the emission, indicating the electronic coherence of the core-excited ionic states, and extract a 6.5±0.8 fs average lifetime of the core-level vacancies. A quantum-mechanical model is employed to interpret the measurement, and we find the observed temporal modulations are independent of charge density oscillations. This work opens a new regime of coherent electronic motion, beyond charge migration, where electronic coherence manifests in the nonlocal quantum correlation between atomic sites while charge density oscillation is absent. Our results broaden the landscape of electronic coherence in molecular systems.
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Journal articleCryer-Jenkins EA, Leung AC, Rathee H, et al., 2025,
Brillouin-Mandelstam scattering in telecommunications optical fiber at millikelvin temperatures
, APL Photonics, Vol: 10Brillouin-Mandelstam scattering is a strong and readily accessible optical nonlinearity, enabling a wide array of applications and research directions. For instance, the three-wave mixing process has been employed to great success in narrow-linewidth lasers, sensing applications, microscopy, and signal processing. While most of these avenues focus on room temperature operation, there is now increasing interest in cryogenic operation owing to the scattering mechanism’s significant potential for applications and fundamental physics at low temperatures. Here, we measure the Brillouin scattering spectrum in standard single-mode telecommunication optical fibers at millikelvin temperatures using a closed-cycle dilution refrigerator and optical heterodyne detection. Our experiments are performed with a cryostat temperature from 50 mK to 27 K, extending previously reported measurements that utilized liquid helium-4 cryostats with temperatures greater than 1 K. At millikelvin temperatures, our experiment observes coherent acoustic interactions with microscopic defects in the amorphous material—two-level-systems (TLSs)—which has not been previously observed in optical fibers. The measured behavior of the linewidth with temperature is in agreement with the well-established models of ultrasonic attenuation in amorphous materials comprising a background intrinsic scattering, thermally activated scattering, and incoherent and coherent TLS interactions. This work provides a foundation for a wide range of applications and further research, including sensing applications, new approaches to investigate TLS physics, and Brillouin-scattering-based quantum science and technology.
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Journal articleGerry CC, Birrittella RJ, Alsing PM, et al., 2024,
Non-classicality and the effect of one photon.
, Philos Trans A Math Phys Eng Sci, Vol: 382The quantum interference effects of mixing the most non-classical states of light, number states, with the most classical-like of pure field states, the coherent state, are investigated. We demonstrate how the non-classicality of a single photon when mixed with a coherent field can transform the statistical properties of the output and further demonstrate that the entanglement of the output is independent of the coherent state amplitude.This article is part of the theme issue 'The quantum theory of light'.
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Journal articleWhite A, Popa S, Mellado Munoz J, et al., 2024,
Slow molecular beams from a cryogenic buffer gas source
, Physical Review Research, Vol: 6, ISSN: 2643-1564We study the properties of a cryogenic buffer gas source that uses a low temperature two-stage buffer gas cell to produce very slow beams of ytterbium monofluoride molecules. The molecules are produced by laser ablation inside the cell and extracted into a beam by a flow of cold helium. We measure the flux and velocity distribution of the beam as a function of ablation energy, helium flow rate, cell temperature, and the size of the gap between the first and second stages of the cell. We also compare the velocity distributions from one-stage and two-stage cells. The one-stage cell emits a beam with a speed of about 82 m s¯¹ and a translational temperature of 0.63 K. The slowest beams are obtained using the two-stage cell at the lowest achievable cell temperature of 1.8 K. This beam has a peak velocity of 56 m s¯¹ and a flux of 9×10⁹ ground state molecules per steradian per pulse, with a substantial fraction at speeds below 40 m s¯¹. These slow molecules can be decelerated further by radiation pressure slowing and then captured in a magneto-optical trap.
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Journal articleOrozco Ruiz M, Le NH, Mintert F, 2024,
Quantum control without quantum states
, PRX Quantum, ISSN: 2691-3399We show that combining ideas from the fields of quantum invariants and of optimal control can be used to design optimal quantum control solutions without explicit reference to quantum states. We describe how control problems for state preparation and the realization of propagators can be formulated in this approach, and we provide explicit control solutions for a spin chain with an extended Ising Hamiltonian. The states considered for state-preparation protocols include eigenstates of Hamiltonians with more than pairwise interactions, and these Hamiltonians are also used for the definition of target propagators. The cost of describing suitable time-evolving operators grows only quadratically with the system size, allowing us to construct explicit control solutions for up to 50 spins. While sub-exponential scaling is obtained only in special cases, we provide several examples that demonstrate favourable scaling beyond the extended Ising model.
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Journal articleFerte A, Austin D, Johnson AS, et al., 2024,
Signature of Attochemical Quantum Interference upon Ionization and Excitation of an Electronic Wave Packet in Fluorobenzene
, PHYSICAL REVIEW LETTERS, Vol: 133, ISSN: 0031-9007 -
Journal articleVylegzhanin A, Nic Chormaic S, Brown DJ, 2024,
Rydberg electromagnetically induced transparency based laser lock to Zeeman sublevels with 0.6 GHz scanning range.
, Rev Sci Instrum, Vol: 95We propose a technique for frequency locking a laser to the Zeeman sublevel transitions between the 5P3/2 intermediate and 32D5/2 Rydberg states in 87Rb. This method allows for continuous frequency tuning over 0.6 GHz by varying an applied external magnetic field. In the presence of the applied field, the electromagnetically induced transparency (EIT) spectrum of an atomic vapor splits via the Zeeman effect according to the strength of the magnetic field and the polarization of the pump and probe lasers. We show that the 480 nm pump laser, responsible for transitions between the Zeeman sublevels of the intermediate state and the Rydberg state, can be locked to the Zeeman-split EIT peaks. The short-term frequency stability of the laser lock is 0.15 MHz, and the long-term stability is within 0.5 MHz. The linewidth of the laser lock is ∼0.8 and ∼1.8 MHz in the presence and absence of the external magnetic field, respectively. In addition, we show that in the absence of an applied magnetic field and adequate shielding, the frequency shift of the lock point has a peak-to-peak variation of 1.6 MHz depending on the polarization of the pump field, while when locked to Zeeman sublevels, this variation is reduced to 0.6 MHz. The proposed technique is useful for research involving Rydberg atoms, where large continuous tuning of the laser frequency with stable locking is required.
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Journal articleHanif F, Das D, Halliwell J, et al., 2024,
Testing Whether Gravity Acts as a Quantum Entity When Measured
, PHYSICAL REVIEW LETTERS, Vol: 133, ISSN: 0031-9007 -
Journal articleWang P, Kwon H, Luan C-Y, et al., 2024,
Snapshotting quantum dynamics at multiple time points
, NATURE COMMUNICATIONS, Vol: 15 -
Journal articlePitchford A, Rakhubovsky AA, Mukherjee R, et al., 2024,
Bayesian optimization of non-classical optomechanical correlations
, QUANTUM SCIENCE AND TECHNOLOGY, Vol: 9, ISSN: 2058-9565 -
Journal articleSchofield RC, Fu M, Clarke E, et al., 2024,
Bose–Einstein condensation of light in a semiconductor quantum well microcavity
, Nature Photonics, Vol: 18, ISSN: 1749-4885When particles with integer spin accumulate at low temperature and high density, they undergo Bose–Einstein condensation (BEC). Atoms, magnons, solid-state excitons, surface plasmon polaritons and excitons coupled to light exhibit BEC, which results in high coherence due to massive occupation of the respective system’s ground state. Surprisingly, photons were shown to exhibit BEC recently in organic-dye-flled optical microcavities, which—owing to the photon’s low mass—occurs at room temperature. Here we demonstrate that photons within an inorganic semiconductor microcavity also thermalize and undergo BEC. Although semiconductor lasers are understood to operate out of thermal equilibrium, we identify a region of good thermalization in our system where we can clearly distinguish laser action from BEC. Semiconductor microcavities are a robust system for exploring the physics and applications of quantum statistical photon condensates. In practical terms, photon BECs ofer their critical behaviour at lower thresholds than lasers. Our study shows two further advantages: the lack of dark electronic states in inorganic semiconductors allows these BECs to be sustained continuously; and quantum wells ofer stronger photon–photon scattering. We measure an unoptimized interaction parameter ( g̃ ≳ 10–3), which is large enough to access the rich physics of interactions within BECs, such as superfuid light.
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Journal articleRuberti M, Averbukh V, Mintert F, et al., 2024,
Bell test of quantum entanglement in attosecond photoionization
, Physical Review X, Vol: 14, ISSN: 2160-3308Attosecond physics enables the study of ultrafast coherent electron dynamics in matter upon photoexcitation and photoionization, revealing spectacular effects such as hole migration and coherentAuger dynamics in molecules. In the photoionization scenario, there has been a strong focus onprobing the physical manifestations of internal quantum coherence within the individual parent ionand photoelectron systems. However, quantum correlations between these two subsystems emergingfrom attosecond photoionization events have thus far remained much more elusive. In this work, wedesign theoretically and model numerically a direct probe of quantum entanglement in attosecondphotoionization in the form of a Bell test. We simulate from first principles a Bell test protocolfor the case of noble gas atoms photoionized by ultrashort, circularly polarized infrared laser pulsesin the strong-field regime predicting robust violation of the Bell inequality. This theoretical resultpaves the way for the direct observation of entanglement in the context of ultrafast photoionizationof many-electron systems. Our work provides a novel perspective on attosecond physics directedtoward the detection of quantum correlations between systems born during attosecond photoionization and unraveling the signatures of entanglement in ultrafast coherent molecular dynamics,including in the chemical decomposition pathways of molecular ions.
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Journal articleYu S, Jia Z, Zhang A, et al., 2024,
Shedding Light on the Future: Exploring Quantum Neural Networks through Optics
, ADVANCED QUANTUM TECHNOLOGIES -
Journal articleLee JP, Avni T, Alexander O, et al., 2024,
Few-femtosecond soft X-ray transient absorption spectroscopy with tuneable DUV-Vis pump pulses
, Optica, Vol: 11, Pages: 1320-1323, ISSN: 2334-2536Achieving few-femtosecond resolution for a pump-probe experiment is crucial to measuring the fastest electron dynamics and for creating superpositions of valence states in quantum systems. However, traditional UV-Vis pump pulses cannot achieve few-fs durations and usually operate at fixed wavelengths. Here, we present, to our knowledge, an unprecedented temporal resolution and pump tuneability for UV-Vis-pumped soft X-ray transient absorption spectroscopy. We have combined few-fs deep-UV to visible tuneable pump pulses from resonant dispersive wave emission in hollow capillary fiber with attosecond soft X-ray probe pulses from high harmonic generation. We achieve sub-5-fs time resolution, sub-fs interferometric stability, and continuous tuneability of the pump pulses from 230 to 700 nm. We demonstrate that the pump can initiate an ultrafast photochemical reaction and that the dynamics at different atomic sites can be resolved simultaneously. These capabilities will allow studies of the fastest electronic dynamics in a large range of photochemical, photobiological and photovoltaic reactions.
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Journal articleGemmell NR, Ma Y, Pearce E, et al., 2024,
Coupling undetected sensing modes by quantum erasure
, APL Quantum, Vol: 1<jats:p>Imaging with undetected photons (IUP) enables the possibility of sensing changes in the phase and the transmission of a beam of light that need never be detected. This has led to the possibility of infrared sensing with visible silicon camera technology, for example. Relying on the interference of two identical pairs of photons, IUP was initially achieved using unidirectional paths through two nonlinear crystal pair sources. More recently, folded arrangements using bidirectional paths through a single-crystal have become common for their simplicity. Here, we theoretically model and experimentally implement a novel setup involving three interference paths through a single nonlinear crystal. This establishes two independent IUP sensing modes in addition to a third linear interference mode. We achieve this using a polarization state quantum eraser approach, with excellent agreement between experiment and theory. This system provides a new route to control and optimize IUP interference in a single-crystal folded arrangement by using controllable quantum erasure to balance the interferometer, opening the door to new implementations and applications for IUP.</jats:p>
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Journal articleAlexander O, Egun F, Rego L, et al., 2024,
Attosecond impulsive stimulated x-ray Raman scattering in liquid water
, Science Advances, Vol: 10, ISSN: 2375-2548We report the measurement of impulsive stimulated x-ray Raman scattering in neutral liquid water. An attosecond pulse drives the excitations of an electronic wavepacket in water molecules. The process comprises two steps: a transition to core-excited states near the oxygen atoms accompanied by transition to valence-excited states. Thus, the wavepacket is impulsively created at a specific atomic site within a few hundred attoseconds through a nonlinear interaction between the water and the x-ray pulse. We observe this nonlinear signature in an intensity-dependent Stokes Raman sideband at 526 eV. Our measurements are supported by our state-of-the-art calculations based on the polarization response of water dimers in bulk solvation and propagation of attosecond x-ray pulses at liquid density.
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Journal articleDriver T, Mountney M, Wang J, et al., 2024,
Attosecond delays in X-ray molecular ionization
, NATURE, Vol: 632, ISSN: 0028-0836 -
Journal articleBressanini G, Genoni MG, Kim MS, et al., 2024,
Multi-parameter quantum estimation of single- and two-mode pure Gaussian states
, Journal of Physics A: Mathematical and Theoretical, Vol: 57, ISSN: 1751-8113We discuss the ultimate precision bounds on the multiparameter estimation of single- and two-mode pure Gaussian states. By leveraging on previous approaches that focused on the estimation of a complex displacement only, we derive the Holevo Cramér–Rao bound (HCRB) for both displacement and squeezing parameter characterizing single and two-mode squeezed states. In the single-mode scenario, we obtain an analytical bound and find that it degrades monotonically as the squeezing increases. Furthermore, we prove that heterodyne detection is nearly optimal in the large squeezing limit, but in general the optimal measurement must include non-Gaussian resources. On the other hand, in the two-mode setting, the HCRB improves as the squeezing parameter grows and we show that it can be attained using double-homodyne detection.
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Journal articleHaug T, Kim MS, 2024,
Generalization of quantum machine learning models using quantum Fisher information metric
, Physical Review Letters, Vol: 133, ISSN: 0031-9007Generalization is the ability of machine learning models to make accurate predictions on new data by learning from training data. However, understanding generalization of quantum machine learning models has been a major challenge. Here, we introduce the data quantum Fisher information metric (DQFIM). It describes the capacity of variational quantum algorithms depending on variational ansatz, training data, and their symmetries. We apply the DQFIM to quantify circuit parameters and training data needed to successfully train and generalize. Using the dynamical Lie algebra, we explain how to generalize using a low number of training states. Counterintuitively, breaking symmetries of the training data can help to improve generalization. Finally, we find that out-of-distribution generalization, where training and testing data are drawn from different data distributions, can be better than using the same distribution. Our work provides a useful framework to explore the power of quantum machine learning models.
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Journal articleLatacz BM, Fleck M, Jaeger JI, et al., 2024,
Orders of Magnitude Improved Cyclotron-Mode Cooling for Nondestructive Spin Quantum Transition Spectroscopy with Single Trapped Antiprotons
, PHYSICAL REVIEW LETTERS, Vol: 133, ISSN: 0031-9007
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