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High Resolution On-Chip Thin-Film Lithium Niobate Single-Photon Buffer

Cagin Ekici,1 Yonghe Yu,1 Jeremy C. Adcock,1 Alif Laila Muthali,1 Heyun Tan,2
Hao Li,2 Leif Katsuo Oxenløwe,1 Xinlun Cai,2 and Yunhong Ding1,*

1 Center for Silicon Photonics for Optical Communication (SPOC), Department of
Electrical and Photonics Engineering, Technical University of Denmark, Lyngby,
Denmark

2State Key Laboratory of Optoelectronic Materials and Technologies, School of
Electronics and Information Technology, Sun Yat-sen University, Guangzhou
510275, China

<EMAIL_ADDRESS>
###### Abstract

We experimentally demonstrate a room-temperature, voltage controlled, short-
term quantum photonics memory on a lithium niobate chip. Our chip is capable
of resolving 100 ps time steps with 0.74 dB loss per round-trip.

##  1 Introduction

Short-term quantum photonics memories or single-photon buffers are essential
for quantum technologies, since they provide a synchronization scheme for
matching independent systems functioning at different speeds. In order to
optimize two-photon interference from distant sources in a quantum network,
photon buffers having high resolution configurability is needed to store one
photon until the other is transmitted [1]. In addition, entangling quantum
operations in photonics are generally probabilistic, and such short-term
memories play a crucial role to buffer gates of probabilistic nature.
Furthermore, approaching ideal single-photon sources based on parametric
spontaneous pair generation through temporal multiplexing requires low-loss
and controllable photon storage [2, 3].

To date, optical buffers based on delay lines [4], slow light [5], and Bragg
scattering four-wave mixing [6] have been introduced. All these techniques
either have an excessive loss which is not suitable for quantum applications
or are overly sophisticated. Although atomic cloud optical memories are main
contenders, they are difficult to integrate, and only operate at specific
wavelengths. Therefore, to fulfill the requirements of a single-photon buffer,
thin-film lithium niobate (TFLN) based integrated photonics platforms are
ideal candidates, since they offer voltage controlled, low-loss and high-speed
interferometric switching. In this paper, we experimentally demonstrate an on-
chip TFLN single-photon buffer based on recirculating 1 cm-long loop with a
round-trip time of 100 ps, i.e. the overall delay can be controlled with 100
ps time resolution, and storage times of up to 1.4 ns (14 round trips).

##  2 Experimental Setup and Results

The TFLN single-photon buffer was fabricated on a commercial lithium niobate
on insulator (LNOI) platform with top LN thickness of 600 nm. The switch
consists of 4.5 mm long LN phase modulator on push-pull mode, exhibiting
bandwidth more than 40 GHz, as shown in Fig. 1 (b), and the whole chip
insertion loss is less than 6.2 dB (including the coupling loss).

Fig. 1:  (a) Schematics of the experimental setup with a real image of the
TFLN chip consisting several buffers. (b) Electro-optic bandwidth (S21)
measurement. Abbreviations: FPGA: Field-Programmable Gate Array, VOA: Variable
Optical Attenuator, UC: Ultrafast Comparator, EA: Electronic Amplifier, TFLN
S-PB: TFLN Single-Photon Buffer.

The experimental setup is shown in Fig. 1 (a). We conduct the experiments
utilizing heavily-attenuated light from a laser (1550 nm, 40 fs pulse
duration), i.e. weak coherent state, with 100 MHz repetition rate instead of
true single-photon quantum states. The switch control signals are generated
via an FPGA and are fed into an ultrafast comparator to obtain a fast fall-
rise time. Afterwards, the fast signals are amplified to the $V_{\pi}$ of the
TFLN switch, and are applied to the chip through high speed radio frequency
(RF) probes using micropositioners. After storage and read-out for a delay,
photons are detected by a superconducting nanowire single-photon detector(s)
and recorded by a time-tagger which produces a real-time histogram of the
detection event.

Fig. 2:  The experimental results of single-photon storage: (a) Normalized
histogram counts as a function of different storage time. (b) The peak
amplitudes of the normalized histogram counts revealing the loss for different
storage times. (c) Second-order correlation function $g^{(2)}(0)$ of the read-
out single photons for different storage times with the error bars.

The experimental results of single photon storage with our TFLN chip is shown
in Fig. 2. Normalized histogram counts for the first 5 round-trip are depicted
in Fig. 2 (a). The round-trip loss performance of the chip as a function of
time is exhibited in Fig. 2 (b). Each peak value after a round-trip has been
fitted the line with slope 0.74 dB. Accordingly, we measure the second-order
correlation function $g^{(2)}(0)$ after each round-trip by adding a 50/50
fiber optic beam splitter before the detection, see Fig. 2 (c). As expected
$g^{(2)}(0)\approx 1$, since our TFLN photonics chip is illuminated by a weak
coherent state. As a result of constant $g^{(2)}(0)\approx 1$ for every round-
trip, it can be inferred that the statistics do not change significantly as a
function of a storage time and there is no substantial optical background
noise owing to the absence of an optical pump beam [7].

##  3 Conclusion

We present an experimental study of a recirculating on-chip TFLN single-photon
buffer enabling single photons to be captured, stored, and read-out at will
with 100 ps time step resolution in a reliable way. Our promising chip is a
robust and scalable architecture working at room-temperature with low-loss
around 0.74 dB per round-trip.

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