A Hybrid Quantum Memory Enabled Network at Room Temperature

A Hybrid Quantum Memory Enabled Network at Room Temperature

Xiao-Ling Pang, Ai-Lin Yang, Jian-Peng Dou, Hang Li, Chao-Ni Zhang, Eilon Poem, Dylan J. Saunders, Hao Tang, Joshua Nunn, Ian A. Walmsley,    Xian-Min Jin xianmin.jin@sjtu.edu.cn State Key Laboratory of Advanced Optical Communication Systems and Networks, Department of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU, UK
July 24, 2019
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preprint: APS/123-QED

Quantum memory capable of storage and retrieval of flying photons on demand is crucial in developing quantum information technologies. In particular, to build efficient quantum computers and long-distance quantum communications, a broadband and room-temperature quantum memory associated with enabled quantum networks is of practical significance. Here, we present the first hybrid quantum memory enabled network by demonstrating two types of quantum memory building blocks and their interconnecting: an atomic-ensemble-based memory capable of generating and storing single atomic excitations which can then be converted to single photons, and an all-optical loop memory for mapping incoming photons in and out on demand, at room-temperature and with a broad acceptance bandwidth. Cascading these two types of quantum memories, we observe a well-preserved herald-single quantum cross-correlation, reaching a value of 22, and a violation of the Cauchy-Schwarz inequality up to 549 standard deviations. Such a network allows atomic excitations to be generated, stored, and converted to broadband photons, which are then transferred to the next node, stored, and faithfully retrieved, all at high speed and in a programmable fashion. The simultaneously demonstrated two type of quantum memories constitute a complete set for constructing a hybrid quantum network, representing a substantial step towards scalable quantum information processing.

A quantum network consisting of a large-scale distribution of quantum nodes and interconnecting channels remains an overarching goal of quantum information science. Such network could be used for quantum-enhanced technologies which hold promises to outperform classical systems in the fields of communication, computing and metrology Jeremy_nphoton_2009 (). Unfortunately, quantum channels suffer from exponential loss and high latency Xianmin_nphoton_2010 (); Gisin_nphoton_2007 (). In addition, the probabilistic generation of quantum states limits the scale of quantum systems locally Ladd_nature_2010 (); Walter_nphys_2012 ().A quantum memory capable of storing quantum states Lvovsky_nphoton_2009 () permits quantum communication over long distances with quantum repeaters Briegel_PRL_1998 (); DuanLM_nature_2001 (), and avoids the non-deterministic nature of quantum state generation by synchronizing stochastic events Nunn_PRL_2013 (); Paul_optica_2015 (); Eggleton_nCommun_2015 (); Paul_optica_2017 ().

Various works have been motivated on quantum memory theory and relevant physical implementations, such as optical delay lines and cavities Pittman_PRA_2002 (); Pittman_PRA_2002_cavity (); Leung_PRA_2006 (), electromagnetically induced transparency (EIT) Kuzmich_nature_2005 (); Eisaman_nature_2005 (); Xianmin_nphoton_2011 (), the DLCZ protocol DuanLM_nature_2001 (); Kuzmich_nature_2003 (); Chrapkiewicz_PRL_2017 (), photon-echo quantum memory Hosseini_nCommun_2011 (); Liao_PRL_2014 (), off-resonant Faraday interaction Julsgaard_nature_2004 (), and Raman memory Reim_PRL_2012 (); DingDS_nphoton_2015 (). In order to make a quantum memory device practical for scalable and high speed quantum information processing, the key requirements that have to be satisfied include broad acceptance bandwidth, high efficiency, long lifetime, low noise level, and preferably room temperature operation.

However, for the last 20 years, it has been proven very challenging to meet all these requirements simultaneously. Considerable efforts have been made in pushing the bandwidth from the KHz and MHz regime to the GHz regime, and temperature from near absolute zero to room-temperature Kuzmich_nature_2003 (); Julsgaard_nature_2004 (); Kuzmich_nature_2005 (); Eisaman_nature_2005 (); Xianmin_nphoton_2011 (); Hosseini_nCommun_2011 (); Reim_PRL_2012 (); DingDS_nphoton_2015 (); Chrapkiewicz_PRL_2017 (). At room temperature, EIT and near off-resonance Raman perform well, but suffer from fluorescence noise which is impossible to be filtered out spectrally PanJW_PRA_2007 (). As for far off-resonance Raman memory, while it eliminates fluorescence noises, a new intrinsic noise, related to spontaneous Raman scattering, emerges Michelberger_NJP_2015 ().


Figure 1: A schematic diagram and experimental setup of a hybrid quantum memory enabled network. a. A quantum network consists of two different-functional nodes and their interconnecting. b. Write and read processes of FORD quantum memory. Solid lines represent three-level -type configuration of atoms, in which states and are hyperfine ground states of cesium atoms (), state is excited state; dash lines represent broad virtual energy levels induced by the write and read pulse. c. Setup of FORD quantum memory. WP: Wollaston prism. d. Time sequences of FORD quantum memory. e. Polarization switching in the mapping in-and-out processes shown with Bloch sphere. f. Setup of LOOP quantum memory. Two Pockels cells are controlled by write and read electrical signals from two channels of a field-programmable gate array module respectively. The time delay between write and read signals can be any positive integral multiples of one cycle (10.4 ns). A standard Hanbury Brown-Twiss interferometer with APD2 and APD3 can be plugged in to measure and . PC: Pockels cells, QWP: quarter wave plate, HWP: half wave plate. g. Time sequences of LOOP memory.

Recently, we have realized a broadband and room-temperature Far Off-Resonance Duan-Lukin-Cirac-Zoller (FORD) quantum memory, capable of operating with a high fidelity at the quantum regime Dou_arXiv_2017 (). Now we are continuously pursuing a quantum memory, which is broadband, room-temperature, and more importantly, compatible with the FORD quantum memory for further storing Stokes/anti-Stokes photons (i.e. mapping in) and retrieving them (i.e. mapping out) for controlled durations, without the addition of noise. Progress towards such a memory have been made by using excited states of atoms Kaczmarek_arXiv_2017 (); Finkelstein2018 () albeit with a limited lifetime. In another interesting route, it has been proven possible to trap and control photons on demand with the advances of ultra-low loss optical elementsChristine_science_2011 (); Paul_optica_2015 (); Paul_optica_2017 (), which can avoid the introduction of any additional noise Kuzmich_nature_2005 (); ChenZB_PRA_2007 (); Michelberger_NJP_2015 (); DingDS_nphoton_2015 (). Such quantum memory here is defined as LOOP memory: consisting of an all-optical storage loop and controllable polarization switches.

Here, we propose and experimentally demonstrate a broadband, room-temperature and hybrid quantum memory enabled network. The hybrid network combines a broadband FORD memory with a compatible all-optical LOOP memory: generation of non-classical photonic states with a built-in, controllable timing, in a warm atomic vapour, and trapping flying photons in an all-optical loop (see Figure 1a). After traversing the two quantum devices successively, the quantum states are observed to be in a very high fidelity as a Fock state. We obtain a violation of Cauchy-Schwarz inequality Clauser_PRD_1974 () up to 549 standard deviations. Furthermore, the measured cross-correlation of up to 22 well exceed 6, which ensures the violation of Bell’s inequality BaoXiaohui_nphoton_2012 () and therefore further quantum applications in practice.


Figure 2: Retrieval efficiency of LOOP memory. a. The retrieved distribution of anti-Stokes photons as a function of . b. The red solid line in the projection panel fits the measured retrieval efficiency with up to 0.999. Inserted figure shows that the pulse at ns with Gaussian fitting give the duration of the retrieved broadband pulse. Comparing with the duration of 1.77ns measured before LOOP memory, the pulse shape is well preserved.

The schematic view of FORD protocol is shown in Figure 1b,1c and 1d. A strong write/read pulse is incident on an atomic ensemble, creating a Stokes photon via spontaneous Raman process, which heralds a successful creation of an atomic excitation with inherently unit efficiency DuanLM_nature_2001 (). After a programmable time delay , the correponding anti-Stokes photon is retrieved, where the retrieval efficiency in principle can also reach 100% Reim_PRL_2012 (). Broadband virtual energy levels can be addressed near the two-photon resonance by the strong and broadband write/read pulses, and are detuned by much more than usually applied in the standard DLCZ protocol. The strong write pulse with a detuning creates a broadband Stokes photon, while the atomic ensemble with a single created excitation becomes DuanLM_nature_2001 (); Nicolas_RMP_2011 ()

(1)

where is the number of atoms, ( ) the wave vector of write (Stokes) pulse, and the coordinate of th atom which is excited to state . After a programmable time delay , a read pulse with a detuning transforms the collective excitation state into a broadband anti-Stokes photon. The measured bandwidth is about 500 MHz, calculated by a convolution-based approach Dou_arXiv_2017 ().

Connected by an optical fiber, flying anti-Stokes photons are transmitted to the remote all-optical LOOP memory. As is shown in Figure 1e, 1f and 1g, a half wave plate together with a quarter wave plate rotates the polarization of the incoming anti-Stokes photons to be horizontally polarized. Once the anti-Stokes photon is mapped in the loop, two high-speed polarization switches (Pockels cells) are activated. The first Pockels cell switches the photon to be vertically polarized. The photon will be cycling counterclockwise until the second Pockels cell maps it out after a programmable delay by switching its polarization back. Then the retrieved anti-Stokes photon is collected by a fiber coupler, and with a 50:50 fiber beam splitter and avalanche photo diodes, a standard Hanbury Brown-Twiss experiment can be conducted to study photon statistics. Remarkably, the bandwidth of our all-optical LOOP memory is much higher than tens of THz, only limited by the response bandwidth of linear optical elements, which means the LOOP memory matches the bandwidth of the FORD memory excellently.

The retrieved anti-Stokes photons as a function of storage time are illustrated in Figure 2a and 2b. The retrieval efficiency of LOOP memory can be obtained by comparing the counts of the mapped-out anti-Stokes with the value before being mapped into the loop. The retrieval efficiency is up to 90% at a short storage time. The decrease of retrieval efficiency mainly results from photon loss produced by multiple reflections and dispersions on the optical elements in the loop. As is shown in Figure 2b, the red solid line implies that our current devices (two Pockels cells are being upgraded to a fast and low-loss one) lead to 10 cycles existence of photons in the loop. The inserted figure shows that the pulse shapes associated with their broadband nature of released anti-Stokes photons are preserved well after each memory.


Figure 3: Measured cross-correlation as a function of storage time. The cross-correlation (in blue) is fitted with form , where quadratic term comes from atomic motion, and linear term comes from background noise; The cross-correlation (in red) is fitted with form , where exponential term comes from the decrease of count rates. The data is obtained under the condition of a write/read beam waist of 58 m, and the detuning =4 GHz. Error bars are derived by Poisson distribution of avalanche photo diodes.

In order to evaluate the performance of preserving quantum correlation, we measure the second-order correlation function between the mapped out anti-Stokes photon and the Stokes photon. The results are presented in Figure 3 as a function of storage time and . The decrease of of FORD memory (in blue) mainly comes from atomic motion Zhao_nPhys_2009 () and background noise, while the slight drop of of LOOP memory (in red) comes from the decrease of count rate associated with signal-to-noise ratio. At the initial values of storage time, the nonclassicallity can be revealed clearly by a violation of Cauchy-Schwarz inequality Clauser_PRD_1974 ()

(2)

up to 549 standard deviations (the measured cross- and auto- correlations are , , ). The lifetime Zhao_nPhys_2009 (); Xianmin_nphoton_2011 () with cross-correlation dropping to implies the abilities of the memories to preserve quantum coherence and correlation. The lifetime of FORD quantum memory reaches 1.449 , while that of LOOP memory is also as high as 1.223 which suggests a noise-free nature of the LOOP memory even though it suffers from some photon loss. The measured before the LOOP memory is . The measured after LOOP memory, with a short storage time of 10.4 ns, is found up to , apparently there is no a notable degrade.


Figure 4: Joint cross-correlation of the hybrid quantum memory enabled network as a function of storage times. The surface is fitted with a multiplication of the two decay functions. Inserted figure facilitates a better viewing of the measured points.

In a large-scale hybrid quantum memory network, as shown in Figure 1a, photons must be able to be generated and hopped individually between large numbers of quantum memory nodes with a programmable way to achieve certain quantum enhanced tasks. Here we demonstrate the ability to coordinate the storage times in quantum memories by setting an arbitrary (). Based on experimental data presented in Figure 4, we build a simple model to give the residual quantum correlation by multiplying the two obtained decay functions. At an arbitrarily chosen point shown in the middle of Figure 4, we measure the value of to be , and find it agrees with the simulation value of 14.4735 very well. In a large area, including the point (480,122.4), the cross-correlations in such hybrid quantum memory enabled network all exceeds the key boundary of 6 to violate Bell’s inequality BaoXiaohui_nphoton_2012 ().

In summary, we have proposed and experimentally demonstrated a broadband, room-temperature and hybrid quantum memory enabled network. The demonstrated building block consists of two different-functional quantum nodes: an atomic-ensemble-based memory capable of generating and storing quantum states, and an all-optical memory for mapping incoming photons in and out, at room-temperature and broadband regime. Such two different-function quantum nodes constitute a complete and controllable quantum system, and they can operate independently, which means we are able to address one without disturbing the other. Our hybrid quantum memory enabled network may provide an elegant solution for scalable quantum information processing Ladd_nature_2010 (); Walter_nphys_2012 (); Lvovsky_nphoton_2009 (); Briegel_PRL_1998 (), which requires capabilities of on-demand creating, storing and distributing quantum states among large numbers of nodes through interconnecting channelsKimble_nature_2008 ().

Methods

Experimental details of FORD quantum Memory: We employ cesium atoms Cs to achieve large optical depth due to its higher saturated vapor pressure comparing with other alkali atoms. The 75-mm-long cesium cell with 10 Torr Ne buffer gas is placed into a magnetic shielding, and heated to 61C to obtain an optical depth about 1000. A beam of horizontal polarized write/read (W/R) laser creates Stokes (S) and anti-Stokes (AS) photons with programable time lag. In our FORD memory, write and read pulses are set collinear for maximizing the spin wave lifetime of excitation Zhao_nPhys_2009 (). It has been proven that Stokes (anti-Stokes) photons are collinear with write (read) pulses DuanLM_PRA_2002 (), but with orthogonal polarization Nunn_Ph.D_2008 (), so we can utilize a Wollaston prism to separate the retrieved photons from the strong addressing light. Meanwhile, since Stokes and anti-Stokes phtons are also coaxial under the phase-matching condition, six home-built cavities arranged in a double pass configuration are employed to split Stokes and anti-Stokes photons. The transmission rate of each cavity reaches around while the extinction rate of each cavity against noise is up to 500:1.

Acknowledgments

The authors thank Jian-Wei Pan for helpful discussions. This work was supported by National Key R&D Program of China (2017YFA0303700); National Natural Science Foundation of China (NSFC) (11374211, 61734005, 11690033); Shanghai Municipal Education Commission (SMEC)(16SG09, 2017-01-07-00-02-E00049); Science and Technology Commission of Shanghai Municipality (STCSM) (15QA1402200, 16JC1400405); Open fund from HPCL (201511-01). X.-M.J. acknowledges support from the National Young 1000 Talents Plan.

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