Publications by Patrick Ledingham


Raman quantum memory with built-in suppression of four-wave-mixing noise

Physical Review A American Physical Society 100 (2019) 033801

Thomas, T Hird, J Munns, B Brecht, D Saunders, J Nunn, IA Walmsley, PM Ledingham

Quantum memories are essential for large-scale quantum information networks. Along with high efficiency, storage lifetime, and optical bandwidth, it is critical that the memory adds negligible noise to the recalled signal. A common source of noise in optical quantum memories is spontaneous four-wave mixing. We develop and implement a technically simple scheme to suppress this noise mechanism by means of quantum interference. Using this scheme with a Raman memory in warm atomic vapor, we demonstrate over an order of magnitude improvement in noise performance. Furthermore we demonstrate a method to quantify the remaining noise contributions and present a route to enable further noise suppression. Our scheme opens the way to quantum demonstrations using a broadband memory, significantly advancing the search for scalable quantum photonic networks.


Optimal coherent filtering for single noisy photons

Physical Review Letters American Physical Society 123 (2019) 213604

S Gao, O Lazo-Arjona, B Brecht, KT Kaczmarek, J Nunn, P Ledingham, DJ Saunders, IA Walmsley

We introduce a filter using a noise-free quantum buffer with large optical bandwidth that can both filter temporal-spectral modes as well as interconvert them and change their frequency. We theoretically show that such quantum buffers optimally filter out temporal-spectral noise, producing identical single photons from many distinguishable noisy single-photon sources with the minimum required reduction in brightness. We then experimentally demonstrate a noise-free quantum buffer in a warm atomic system that is well matched to quantum dots. Based on these experiments, simulations show that our buffer can outperform all intensity (incoherent) filtering schemes for increasing indistinguishability.


Experimental demonstration of quantum effects in the operation of microscopic heat engines

Physical Review Letters American Physical Society 122 (2019) 110601

J Klatzow, J Becker, P Ledingham, C Weinzetl, K Kaczmarek, D Saunders, J Nunn, I Walmsley, R Uzdin, E Poem

The ability of the internal states of a working fluid to be in a coherent superposition is one of the basic properties of a quantum heat engine. It was recently predicted that in the regime of small engine action, this ability can enable a quantum heat engine to produce more power than any equivalent classical heat engine. It was also predicted that in the same regime, the presence of such internal coherence causes different types of quantum heat engines to become thermodynamically equivalent. Here, we use an ensemble of nitrogen vacancy centers in diamond for implementing two types of quantum heat engines, and experimentally observe both effects.


A noiseless quantum optical memory at room temperature

2017 Conference on Lasers and Electro-Optics Europe and European Quantum Electronics Conference (CLEO/Europe-EQEC) Institute of Electrical and Electronics Engineers (2017)

PM Ledingham, KT Kaczmarek, B Brecht, A Feizpour, GS Thekkadath, SE Thomas, JHD Munns, DJ Saunders, JAS Nunn, IA Walmsley

A quantum optical memory (QM) is a device that can store and release quantum states of light on demand. Such a device is capable of synchronising probabilistic events, for example, locally synchronising nondeterministic photon sources for the generation of multi-photon states, or successful quantum gate operations within a quantum computational architecture, as well as for globally synchronising the generation of entanglement over long distances within the context of a quantum repeater. Desirable attributes for a QM to be useful for these computational and communicational tasks include high end-to-end transmission (including storage and retrieval efficiency), large storage-time-bandwidth product, room temperature operation for scalability and, of utmost importance, noise free performance for true quantum operation.


Theory of noise suppression in Λ-type quantum memories by means of a cavity

Physical Review A American Physical Society 96 (2018)

JAS Nunn, J Munns, SE Thomas, KT Kaczmarek, C Qiu, A Feizpour, E Poem, B Brecht, DJ Saunders, PM Ledingham, DV Reddy, I Walmsley

Quantum memories, capable of storing single photons or other quantum states of light, to be retrieved on demand, offer a route to large-scale quantum information processing with light. A promising class of memories is based on far-off-resonant Raman absorption in ensembles of Λ-type atoms. However, at room temperature these systems exhibit unwanted four-wave mixing, which is prohibitive for applications at the single-photon level. Here, we show how this noise can be suppressed by placing the storage medium inside a moderate-finesse optical cavity, thereby removing the main roadblock hindering this approach to quantum memory.


A high-speed noise-free optical quantum memory

Physical Review A American Physical Society 97 (2018) 042316

K Kaczmarek, P Ledingham, B Brecht, S Thomas, G Thekkadath, O Lazo-Arjona, J Munns, E Poem, A Feizpour, D Saunders, J Nunn, I Walmsley

Optical quantum memories are devices that store and recall quantum light and are vital to the realisation of future photonic quantum networks. To date, much effort has been put into improving storage times and efficiencies of such devices to enable long-distance communications. However, less attention has been devoted to building quantum memories which add zero noise to the output. Even small additional noise can render the memory classical by destroying the fragile quantum signatures of the stored light. Therefore noise performance is a critical parameter for all quantum memories. Here we introduce an intrinsically noise-free quantum memory protocol based on two-photon off-resonant cascaded absorption (ORCA). We demonstrate successful storage of GHz-bandwidth heralded single photons in a warm atomic vapour with no added noise; confirmed by the unaltered photon number statistics upon recall. Our ORCA memory meets the stringent noise-requirements for quantum memories whilst combining high-speed and room-temperature operation with technical simplicity, and therefore is immediately applicable to low-latency quantum networks.


Two-way photonic interface for linking the Sr+ transition at 422 nm to the telecommunication C band

Physical Review Applied American Physical Society 10 (2018) 044012-

TA Wright, R Francis-Jones, CBE Gawith, J Becker, P Ledingham, PGR Smith, J Nunn, PJ Mosley, B Brecht, I Walmsley

We report a single-stage bi-directional interface capable of linking Sr+ trapped ion qubits in a long-distance quantum network. Our interface converts photons between the Sr+ emission wavelength at 422 nm and the telecoms C-band to enable low-loss transmission over optical fiber. We have achieved both up- and down-conversion at the single photon level with efficiencies of 9.4% and 1.1% respectively. Furthermore we demonstrate noise levels that are low enough to allow for genuine quantum operation in the future.


Temporal-mode selection with a Raman quantum memory

Frontiers in Optics 2017 Optical Society of America (2017)

J Munns, SE Thomas, KT Kaczmarek, PM Ledingham, DJ Saunders, JAS Nunn, B Brecht, IA Walmsley

Temporal modes (TMs) of pulsed single-photon states have been identified as appealing basis states for quantum information science. Recent work has seen progress towards TM-selective operations based on nonlinear optics. Here, we demonstrate for the first time a linear TM-selective device, namely a Raman quantum memory in warm atomic Caesium vapour. We achieve switching fidelities of 86.5% when operating the memory with ns-duration pulses. These results pave the way towards new quantum information applications, where TM-selection, TM-reshaping, and network synchronisation are achieved with one single device.


A noiseless quantum optical memory at room temperature

Optics InfoBase Conference Papers (2017)

KT Kaczmarek, PATRICK Ledingham, B Brecht, GS Thekkadath, O Lazo-Arjona, JOSEPH Munns, E Poem, A Feizpour, DJ Saunders, J Nunn, IA Walmsley

© OSA 2017. Quantum optical memories are devices that store quantum states of light, which can allow for the active synchronization of probabilistic events within large-scale quantum networks. Recent work on quantum memories have seen impressive quantum operation, albeit still suffering from noise on the output mode of the device. Here we demonstrate a noise-free quantum memory for light based on the off-resonant cascaded absorption of photons in a warm vapour of caesium atoms. The memory is characterized by measuring a noise floor of 8×10-6photons per pulse. We demonstrate genuine quantum operation by storing and recalling on-demand heralded single photons with a heralded second-order autocorrelation function of g(2)= 0:028±0:009.


Quantum correlations between single telecom photons and a multimode on-demand solid-state quantum memory

Physical Review X American Physical Society 7 (2017) 021028

A Seri, A Lenhard, D Rieländer, M Gündoğan, P Ledingham, M Mazzera, H De Riedmatten

Quantum correlations between long-lived quantum memories and telecom photons that can propagate with low loss in optical fibers are an essential resource for the realization of large-scale quantum information networks. Significant progress has been realized in this direction with atomic and solid-state systems. Here, we demonstrate quantum correlations between a telecom photon and a multimode ondemand solid state quantum memory. This is achieved by mapping a correlated single photon onto a spin collective excitation in a Pr3+:Y2SiO5 crystal for a controllable time. The stored single photons are generated by cavity-enhanced spontaneous parametric down-conversion and heralded by their partner photons at telecom wavelength. These results represent the first demonstration of a multimode on-demand solid state quantum memory for external quantum states of light. They provide an important resource for quantum repeaters and pave the way for the implementation of quantum information networks with distant solid state quantum nodes.


A noise-free quantum memory for broadband light at room temperature

Quantum Information and Measurement (QIM) 2017 Optical Society of America (2017)

KT Kaczmarek, PM Ledingham, B Brecht, A Feizpour, GS Thekkadath, SE Thomas, JHD Munns, DJ Saunders, IA Walmsley, JAS Nunn

We have developed a novel protocol for broadband, noise-free light-matter interactions using off-resonant two-photon absorption. We have successfully stored and retrieved 1.5 GHz bandwidth heralded single photons in warm cesium vapour, measuring a g(2)h= 0:39±0:05.


QLad: A noise-free quantum memory for broadband light at room temperature

Conference on Lasers and Electro-Optics (CLEO 2017) Optical Society of America (2017)

KT Kaczmarek, PM Ledingham, B Brecht, A Feizpour, GS Thekkadath, SE Thomas, JHD Munns, DJ Saunders, IA Walmsley, JAS Nunn

We implement a low-noise, broadband quantum memory for light via off-resonant two-photon absorption in warm atomic vapour. We store heralded single photons and verify that the retrieved fields are anti-bunched.


High efficiency Raman memory by suppressing radiation trapping

New Journal of Physics IOP Publishing 19 (2017) 063034-063034

S Thomas, JHD Munns, KT Kaczmarek, C Qiu, B Brecht, A Feizpour, PM Ledingham, IA Walmsley, J Nunn, DJ Saunders

Raman interactions in alkali vapours are used in applications such as atomic clocks, optical signal processing, generation of squeezed light and Raman quantum memories for temporal multiplexing. To achieve a strong interaction the alkali ensemble needs both a large optical depth and a high level of spin-polarisation.Weimplement a technique known as quenching using a molecular buffer gas which allows near-perfect spin-polarisation of over 99.5% in caesium vapour at high optical depths of up to ∼2 × 10 5 ; a factor of 4 higher than can be achievedwithout quenching.We use this systemto explore efficient light storage with high gain in a GHz bandwidth Raman memory.


Cavity-enhanced room-temperature broadband Raman memory

Physical Review Letters American Physical Society 116 (2016) 090501-

D Saunders, J Munns, T Champion, E al.

Broadband quantum memories hold great promise as multiplexing elements in future photonic quantum information protocols. Alkali-vapor Raman memories combine high-bandwidth storage, on-demand readout, and operation at room temperature without collisional fluorescence noise. However, previous implementations have required large control pulse energies and have suffered from four-wave-mixing noise. Here, we present a Raman memory where the storage interaction is enhanced by a low-finesse birefringent cavity tuned into simultaneous resonance with the signal and control fields, dramatically reducing the energy required to drive the memory. By engineering antiresonance for the anti-Stokes field, we also suppress the four-wave-mixing noise and report the lowest unconditional noise floor yet achieved in a Raman-type warm vapor memory, (15±2)×10^{-3} photons per pulse, with a total efficiency of (9.5±0.5)%.


In situ characterization of an optically thick atom-filled cavity

Physical Review A American Physical Society 93 (2016)

J Munns, C Qiu, P Ledingham, I Walmsley, J Nunn, DJ Saunders

<p style="text-align:justify;"> A means for precise experimental characterization of the dielectric susceptibility of an atomic gas inside an optical cavity is important for the design and operation of quantum light-matter interfaces, particularly in the context of quantum information processing. Here we present a numerically optimized theoretical model to predict the spectral response of an atom-filled cavity, accounting for both homogeneous and inhomogeneous broadening at high optical densities. We investigate the regime where the two broadening mechanisms are of similar magnitude, which makes the use of common approximations invalid. Our model agrees with an experimental implementation with warm caesium vapor in a ring cavity. From the cavity response, we are able to extract important experimental parameters, for instance the ground-state populations, total number density, and the magnitudes of both homogeneous and inhomogeneous broadening. </p>


Spectral-hole memory for light at the single-photon level

Physical Review A American Physical Society 93 (2016)

K Kutluer, MF Pascual-Winter, J Dajczgewand, P Ledingham, M Mazzera, T Chanelière, H De Riedmatten

We demonstrate a solid-state spin-wave optical memory based on stopped light in a spectral hole. A long-lived narrow spectral hole is created by optical pumping in the inhomogeneous absorption profile of a Pr3+:Y2SiO5 crystal. Optical pulses sent through the spectral hole experience a strong reduction of their group velocity and are spatially compressed in the crystal. A short Raman pulse transfers the optical excitation to the spin state before the light pulse exits the crystal, effectively stopping the light. After a controllable delay, a second Raman pulse is sent, which leads to the emission of the stored photons. We reach storage and retrieval efficiencies for bright pulses of up to 39% in a 5-mm-long crystal. We also show that our device works at the single-photon level by storing and retrieving 3-μs-long weak coherent pulses with efficiencies up to 31%, demonstrating the most efficient spin-wave solid-state optical memory at the single-photon level so far. We reach an unconditional noise level of (9±1)×10-3 photons per pulse in a detection window of 4μs, leading to a signal-to-noise ratio of 33±4 for an average input photon number of 1, making our device promising for long-lived storage of nonclassical light.


Ultrahigh and persistent optical depths of alkali vapours for quantum memories in hollow-core photonic crystal fibers

2016 CONFERENCE ON LASERS AND ELECTRO-OPTICS (CLEO) (2016)

KT Kaczmarek, DJ Saunders, A Feizpour, PM Ledingham, B Brecht, E Poem, IA Walmsley, J Nunn, IEEE


A Cavity-Enhanced Room-Temperature Broadband Raman Memory

2016 CONFERENCE ON LASERS AND ELECTRO-OPTICS (CLEO) (2016)

PM Ledingham, JHD Munns, SE Thomas, TFM Champion, C Qiu, KT Kaczmarek, A Feizpour, E Poem, IA Walmsley, J Nunn, DJ Saunders, IEEE


Bad cavities for good memories: Suppression of four-wave mixing in Raman memories

Proceedings of Frontiers in Optics 2015, FIO 2015 (2015)

J Munns, TFM Champion, C Qiu, PM Ledingham, DJ Saunders, IA Walmsley, J Nunn

© 2015 Optical Society of America. Quantum memories enable the synchronisation of photonic operations. Raman memories are a promising platform, but are susceptible to four-wave mixing noise. We present a demonstration of a cavity-enhanced Raman memory, showing suppression of four-wave mixing.


Solid State Spin-Wave Quantum Memory for Time-Bin Qubits.

Physical review letters 114 (2015) 230501-

M Gündoğan, PM Ledingham, K Kutluer, M Mazzera, H de Riedmatten

We demonstrate the first solid-state spin-wave optical quantum memory with on-demand read-out. Using the full atomic frequency comb scheme in a Pr(3+):Y2SiO5 crystal, we store weak coherent pulses at the single-photon level with a signal-to-noise ratio >10. Narrow-band spectral filtering based on spectral hole burning in a second Pr(3+):Y2SiO5 crystal is used to filter out the excess noise created by control pulses to reach an unconditional noise level of (2.0±0.3)×10(-3) photons per pulse. We also report spin-wave storage of photonic time-bin qubits with conditional fidelities higher than achievable by a measure and prepare strategy, demonstrating that the spin-wave memory operates in the quantum regime. This makes our device the first demonstration of a quantum memory for time-bin qubits, with on-demand read-out of the stored quantum information. These results represent an important step for the use of solid-state quantum memories in scalable quantum networks.

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