Neil Sinclair

Conditional detection of pure quantum states of light after storage in a Tm-doped waveguide.

We demonstrate the conditional detection of time-bin qubits after storage in and retrieval from a photon-echo-based waveguide quantum memory. Each qubit is encoded into one member of a photon pair produced via spontaneous parametric down-conversion, and the conditioning is achieved by the detection of the other member of the pair. By performing projection measurements with the stored and retrieved photons onto different bases, we obtain an average storage fidelity of 0.885±0.020, which exceeds the relevant classical bounds and shows the suitability of our integrated light-matter interface for future applications of quantum information processing.

An integrated processor for photonic quantum states using a broadband light-matter interface

Faithful storage and coherent manipulation of quantum optical pulses are key for long distance quantum communications and quantum computing. Combining these functions in a light?matter interface that can be integrated on-chip with other photonic quantum technologies, e.g. sources of entangled photons, is an important step towards these applications. To date there have only been a few demonstrations of coherent pulse manipulation utilizing optical storage devices compatible with quantum states, and that only in atomic gas media (making integration difficult) and with limited capabilities. Here we describe how a broadband waveguide quantum memory based on the atomic frequency comb (AFC) protocol can be used as a programmable processor for essentially arbitrary spectral and temporal manipulations of individual quantum optical pulses. Using weak coherent optical pulses at the few photon level, we experimentally demonstrate sequencing, time-to-frequency multiplexing and demultiplexing, splitting, interfering, temporal and spectral filtering, compressing and stretching as well as selective delaying. Our integrated light?matter interface offers high-rate, robust and easily configurable manipulation of quantum optical pulses and brings fully practical optical quantum devices one step closer to reality. Furthermore, as the AFC protocol is suitable for storage of intense light pulses, our processor may also find applications in classical communications.

Two-photon interference of weak coherent laser pulses recalled from separate solid-state quantum memories

Quantum memories allowing reversible transfer of quantum states between light and matter are central to quantum repeaters, quantum networks and linear optics quantum computing. Significant progress regarding the faithful transfer of quantum information has been reported in recent years. However, none of these demonstrations confirm that the re-emitted photons remain suitable for two-photon interference measurements, such as C-NOT gates and Bell-state measurements, which constitute another key ingredient for all aforementioned applications. Here, using pairs of laser pulses at the single-photon level, we demonstrate two-photon interference and Bell-state measurements after either none, one or both pulses have been reversibly mapped to separate thulium-doped lithium niobate waveguides. As the interference is always near the theoretical maximum, we conclude that our solid-state quantum memories, in addition to faithfully mapping quantum information, also preserve the entire photonic wavefunction. Hence, our memories are generally suitable for future applications of quantum information processing that require two-photon interference.

Proposal and proof-of-principle demonstration of non-destructive detection of photonic qubits using a Tm:LiNbO 3 waveguide

Non-destructive detection of photonic qubits is an enabling technology for quantum information processing and quantum communication. For practical applications, such as quantum repeaters and networks, it is desirable to implement such detection in a way that allows some form of multiplexing as well as easy integration with other components such as solid-state quantum memories. Here, we propose an approach to non-destructive photonic qubit detection that promises to have all the mentioned features. Mediated by an impurity-doped crystal, a signal photon in an arbitrary time-bin qubit state modulates the phase of an intense probe pulse that is stored during the interaction. Using a thulium-doped waveguide in LiNbO3, we perform a proof-of-principle experiment with macroscopic signal pulses, demonstrating the expected cross-phase modulation as well as the ability to preserve the coherence between temporal modes. Our findings open the path to a new key component of quantum photonics based on rare-earth-ion-doped crystals.Non-destructive detection of photonic qubits is an enabling technology for quantum information processing and quantum communication [1–10]. For practical applications such as quantum repeaters [11] and networks [12, 13], it is desirable to implement such detection in a way that allows some form of multiplexing as well as easy integration with other components such as solid-state quantum memories [14, 15]. Here we propose an approach to non-destructive photonic qubit detection that promises to have all the mentioned features. Mediated by an impurity-doped crystal, a signal photon in an arbitrary time-bin qubit state [16] modulates the phase of an intense probe pulse that is stored during the interaction. Using a thulium-doped waveguide in LiNbO3, we perform a proof-of-principle experiment with macroscopic signal pulses, demonstrating the expected cross-phase modulation as well as the ability to preserve the coherence between temporal modes. Our findings open the path to a new key component of quantum photonics based on rare-earthion doped crystals. The ability to detect photonic qubits non-destructively is highly desirable for photonic quantum information processing and quantum communication. For instance, it makes it possible to use precious resource states (say entangled photon pairs for quantum teleportation) only when the input photons are actually there. This is all the more essential in situations of significant loss, such as for quantum repeaters [11, 17]. Non-destructive detection of photons [6] and heralded storage of photonic qubits [8] (which, when combined with readout, is equivalent to non-destructive detection) have recently been realized in sophisticated quantum electrodynamics experiments that combine single-atom control and high-finesse cavities, and work at one specific atomic transition frequency. For practical applications it is important to have a simple and robust implementation of the same functionality but with added flexibility. In particular, it should allow for multiplexing, and be compatible with existing quantum information processing and communication components. Here we propose a detection scheme that has all of these characteristics. The basic principle, illustrated in Fig. 1, is based on cross-phase modulation between a weak signal and a strong probe pulse mediated by a rare-earth ion doped crystal — a technology platform whose suitability for quantum photonics has already been demonstrated [15, 18–24]. For single-photon sensitivity, -2.5 -2 -1.5 -1 -0.5 0 0.5 1 A bs or pt io n [a rb . u ni ts ]

Measuring and analyzing excitation-induced decoherence in rare-earth-doped optical materials

A method is introduced for quantitatively analyzing photon echo decay measurements to characterize excitation-induced decoherence resulting from the phenomenon of instantaneous spectral diffusion. Detailed analysis is presented that allows fundamental material properties to be extracted that predict and describe excitation-induced decoherence for a broad range of measurements, applications and experimental conditions. Motivated by the need for a method that enables systematic studies of ultra-low decoherence systems and direct comparison of properties between optical materials, this approach employs simple techniques and analytical expressions that avoid the need for difficult to measure and often unknown material parameters or numerical simulations. This measurement and analysis approach is demonstrated for the 3 H6 to 3 H4 optical transition of three thulium-doped crystals, Tm 3+ :YAG, Tm 3+ :LiNbO3 and Tm 3+ :YGG, that are currently employed in quantum information and classical signal processing demonstrations where minimizing decoherence is essential to achieve high efficiencies and large signal bandwidths. These new results reveal more than two orders of magnitude variation in sensitivity to excitation-induced decoherence among the materials studied and establish that the Tm 3+ :YGG system offers the longest optical coherence lifetimes and the lowest levels of excitation-induced decoherence yet observed for any known thulium-doped material.

Tm 3+ Tm 3+ : Y3Ga5O12 materials for spectrally multiplexed quantum memories.

We investigate the relevant spectroscopic properties of the 795 nm (3)H(6)↔(3)H(4) transition in 1% Tm(3+):Y(3)Ga(5)O(12) at temperatures as low as 1.2 K for optical quantum memories based on persistent spectral tailoring of narrow absorption features. Our measurements reveal that this transition has uniform coherence properties over a 56 GHz bandwidth, and a simple hyperfine structure split by ± 44 MHz/T with lifetimes of up to hours. Furthermore, we find a (3)F(4) population lifetime of 64 ms-one of the longest lifetimes observed for an electronic level in a solid--and an exceptionally long coherence lifetime of 490 μs--the longest ever observed for optical transitions of Tm(3+) ions in a crystal. Our results suggest that this material allows realizing broadband quantum memories that enable spectrally multiplexed quantum repeaters.

Entanglement between more than two hundred macroscopic atomic ensembles in a solid

There are both fundamental and practical motivations for studying whether quantum entanglement can exist in macroscopic systems. However, multiparty entanglement is generally fragile and difficult to quantify. Dicke states are multiparty entangled states where a single excitation is delocalized over many systems. Building on previous work on quantum memories for photons, we create a Dicke state in a solid by storing a single photon in a crystal that contains many large atomic ensembles with distinct resonance frequencies. The photon is re-emitted at a well-defined time due to an interference effect analogous to multi-slit diffraction. We derive a lower bound for the number of entangled ensembles based on the contrast of the interference and the single-photon character of the input, and we experimentally demonstrate entanglement between over two hundred ensembles, each containing a billion atoms. We also illustrate the fact that each individual ensemble contains further entanglement.Multipartite entanglement is of both fundamental and practical interest, but is notoriously difficult to witness and characterise. Here, Zarkeshian et al. demonstrate multipartite entanglement in an atomic frequency comb storing a single photon in a Dicke state spread over a macroscopic ensemble.

Optical decoherence studies ofTm3+:Y3Ga5O12

Decoherence of the 795 nm

Quantum memory for quantum repeaters

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Frequency multiplexed quantum memories with read-out on demand for quantum repeaters

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