Progress of the Quantum Experiment Science Satellite (QUESS) Micius Project

PAN Jianwei

Progress of the Quantum Experiment Science Satellite (QUESS) Micius Project

PAN Jianwei

(230026) (230026)

The Micius satellite was successfully launched on 16 August 2016, from Jiuquan, China, orbiting at an altitude of about 500 km. The main scientific goals, including satellite-to-ground quantum key distribution, satellite-based quantum entanglement distribution, ground-to-satellite quantum teleportation, and satellite relayed intercontinental quantum network, were achieved in 2017. As a starting point, the Micius satellite has become a platform for quantum science experiments at the space scale. Here, we introduce the latest experimental achievements (in 2018–2020) based on the Micius satellite.

Quantum science satellite, Quantum key distribution, Quantum entanglement, Time-frequency transfer

1 Entanglement-based quantum Key Distribution (QKD) from Satellite to Ground

The entanglement-based QKD is particularly attrac-tive because of its inherent source-independent security where the security can be established without any assumption on the trusted relay.

Following the first satellite-based entanglement distribution[1,2], a later experiment[3]was performed between the ground stations of Delingha and Nanshan with a spatial separation of 1120 km, as shown in Figure 1.

The receiving efficiencies were considerably im-proved using higher efficiency telescope and follow- up optics. Both ground stations used newly built telescopes with diameters of 1.2 m. In each telescope, the main lens was re-coated, and the beam expander was re-designed. In the follow-up optics, the collection efficiency was enhanced by optical pattern matching, in particular, through shortening the optical path by reducing spectral splitting to avoid beam spread. These technical improvements allowed the authors to observe an average two-photon count rate of 2 Hz (corresponding to an increase of the two-photon link efficiency by a factor of 4), which is significant as it increased the obtained key rate and decreased the quantum bit error rate from 8.1% to 4.5%.

A special effort of this work was made to ensure its implementation is practically secure against all known side channels. Thanks to the source-inde-pen-dent nature of the entanglement-based QKD, the system is immune to any loophole in the source, and all left is to ensure the security on the detection sides in the two ground stations.

In general, the side channels, known and to be known, on the detection primarily violate the assumption of fair-sampling. Experimentally, it ensures the validity of the fair-sampling by filtering in different degrees of freedom including frequency, spatial and temporal modes, and countermeasures were taken for the correct operation of the single-photon detectors. All known detection attacks were consi- dered including the detector-related attacks, spatial- mode attack, and other possible side-channels. For example, for the side channels targeting at the opera-tion of detectors such as blinding attacks, additional monitoring circuits were used to monitor the anode of the load resistance in the detection circuit to counter the blinding attack. For the time-shift attack and the dead-time attack, the countermeasure was to operate the detector in free-running mode, in which the detector records all the detection events and post-selects the detection windows such that the detection efficiency is guaranteed to be at a nominal level.

Fig. 1 Overview of the experimental set-up of entanglement-based quantum key distribution. (a) Illustration of the Micius satellite and two ground stations. (b) The spaceborne entangled-photon source. (c) The follow-up optics at the ground station

Consequently, the secret key, generated by our QKD system, is practically secure under realistic devices. By running 1021 trials of the Bell test during an effective collection time of 226 s, Yin. observed the parameterto be 2.56±0.07 with a violation of local realism by 8 standard deviations. Hav-ing violated the Bell’s inequality, we demonstrated the entanglement-based QKD using Bennett-Brassard- Mermin 1992 protocol (BBM92), where both Alice and Bob took measurements randomly along the H/V and basis.

Within 3100 s data collection time, Yinobtained 6208 initial coincidence, which gave 3100 bits of sifted keys with 140 erroneous bits. The quantum bit error rate is 4.5%±0.4%. After error correction and privacy amplification, the secure key rate of 0.43 bit·s-1was obtained in the asymptotic limit of the infinite long key. The secure key rate is 11 orders of magnitude higher than that would be obtained by direct transmission of entangled photons over 1120 km through the best commercial fibers. Note that with the newly developed entangled photon source with 1 GHz generation rate[4], the secure key rate can be increased by about 2 orders of magnitude directly. The results increase the secure distance of practical QKD for ground users by 10 times to the order of thousand kilometers, which represents a key step toward the Holy Grail of cryptography.

2 Satellite-based Quantum-secure Time-frequency Transfer

Today’s time synchronization techniques are vulnerable to sophisticated, malicious adversaries, which require fundamentally new methods of securely distributing high-precision time information. Based on Micius satellite, the recent work is aimed at this direction, with the proposal and demonstration of a satellite-based Quantum-Secure Time Transfer (QSTT) scheme from a two-way quantum key distribution in a free-space link, which is characterized by a quantum signal (., single photon) acting as the carrier for both time transfer and secret-key generation to realize information-theoretic security in the time information transfer[5].

Figure 2 presents a schematic diagram of QSTT. Satellite Alice, which has master Clock A, initiates two-way time transfer with the ground station, Bob, who has slave Clock B. The scheme is then implemented through four basic steps. In Step 1, Alice and Bob mutually transmit single photons over free space for both two-way QKD and two-way transmission of timing signals. In Step 2, Alice and Bob evaluate the Quantum Bit Error Rates (QBERs) in the polarization degree of freedom for the timing signals. In Step 3, Alice transmits the encrypted classical timing data to Bob through a public channel, using the keys generated from the QKD. In Step 4, Alice or Bob, having all the timing data, evaluates the clock offset and the ranging distance with the coincidence time events.

To verify the key technologies and show their feasibility, we performed an experimental study of satellite-based QSTT between the Micius satellite and the Nanshan ground observatory in China. In the downlink, we demonstrated satellite-based QKD by using single photons as carriers of time transfer. In the uplink, we performed standard optical time transfer using classical laser pulses. The experimental set-up is illustrated in Figure 3.

Fig. 2 Schematic of satellite-based QSTT

Fig. 3 Experimental set-up. (a) An overview of satellite-based QSTT. A two-way optical link is established between the Micius satellite and the Nanshan ground observatory. (b) Schematic of the transceiver on the satellite. (c) Schematic of the transceiver on the ground

Finally, the authors perform satellite-to-ground time synchronization using single-photon-level signals and achieve a quantum bit error rate of less than 1%, a time data rate of 9 kHz and a time-transfer precision of 30 ps. These results offer possibilities towards an enhanced infrastructure for a time- transfer network, whose security stems from quantum physics. We also anticipate that the findings of this study will generate new possibilities for a revolutionary quantum time-transfer network at a global scale.

3 Probing Gravity-induced Decoherence

The satellite Micius can also provide the feasibility for testing the entanglement decoherence induced by the gravitation of the Earth. Quantum mechanics and relativity form the bedrock of modern physics. General theory of relativity predicts a kind of exotic spacetime structure called Closed Time Curve (CTC). CTC is interesting because it violates causality and in principle can be formed from the quantum fluctuations of spacetime itself.

To theoretically describe the quantum fields in both exotic spacetime containing CTCs and ordinary spacetime, in recent years, scientists reported the event formalism of quantum fields. This theory predicts that the different evolutions of quantum fields may probabilistically induce time decorrelation of two entangled photons passed through different regions of curved spacetime, which are able to keep the entanglement in standard quantum theory. Considering about the curved spacetime brought by the Earth’s gravitation, the decoherence effect can be tested via distributing entanglement between ground station and satellite.

Recently, Xuimplemented a quantum op-tical experimental test of event formalism of quantum fields using the Micius satellite[6]. We experimentally test a prediction of the theory that a pair of time-energy entangled particles probabilistically de-c-o-rrelate passing through different regions of the gravitational potential of Earth.

In the implementation, polarization entangled photon pair is prepared in Ngari ground station, as shown in Figure 4. Photon in Path 2 is detected on the ground after passing through the ordinary spacetime, while its twin is received by the Micius satellite after propagating in the curved spacetime. Because the gravitation cannot induce the decoherence of classical correlation, the coherent laser is possible to be used as a reference. Before transmitting, the entangled photons are combined with the faint coherent laser pulses in Path 1. The transmitted coherent photons are classically correlated with the photons in Path 3 on the ground. Two trains of entangled and coherent photons are shifted by half a pulse interval about 6 ns, which make the satellite to distinguish the photons by their arrival times.

Fig. 4 Schematics of an experimental test of event formalism in Earth’s gravitational field. (a) The ground station sends both entangled single photons and faint coherent laser pulses to the Micius satellite. (b) Preparation of entangled photon pairs and faint coherent laser pulses at the ground station. (c) Single photons received by the satellite are passed through polarization analysis and detected by single photon detectors

Fig. 5 Experimentally estimated decorrelation factors in different altitude angle of the satellite without (a) and with (b) fulfilling the non-signaling condition

The experiment is implemented with and without fulfilling the no-signaling condition to account for the quantum collapse models. By using 1 km fiber to delay the photons in the ground station, the detection events of entangled photons on the ground and satellite are separated spacelike. By collecting data when the altitude angle of satellite varying from 40° to 60°, the estimated decoherence factors for both spacetime settings are shown in Figure 5. We then conclude that our experimental results are consistent with the descriptions of standard quantum theory and do not support the predictions of event formalism. The future testing of such model may be performed based on a satellite in a higher orbit.

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[2] YIN J, CAO Y, LI Y H,. Satellite-to-ground entanglement-based quantum key distribution [J].., 2017, 119:200501

[3] YIN J, LI Y H, LIAO S K,. Entanglement-based secure quantum cryptography over 1120 kilometers [J]., 2020, 582:501-505

[4] CAO Y, LI Y H, ZOU W J,. Bell test over extremely high-loss channel: towards distributing entangled photon pairs between Earth and the Moon [J].., 2018,120:140405

[5] DAI H, SHEN Q, WANG C Z,. Towards satellite-based quantum-secure time transfer [J]., 2020, 16:848- 852

[6] XU P, MA Y, REN J G,. Satellite testing of a gravitationally induced quantum decoherence model [J]., 2019, 366:132-135

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PAN Jianwei. Progress of the Quantum Experiment Science Satellite (QUESS) Micius Project., 2020, 40(5): 643-647. DOI:10.11728/cjss2020.05.643

June 28, 2020

E-mail: pan@ustc.edu.cn