TianQin is a proposal for a space-borne detector of gravitational waves in the millihertz frequencies. The experiment relies on a constellation of three drag-free spacecraft orbiting the Earth. Inter-spacecraft laser interferometry is used to monitor the distances between the test masses. The experiment is designed to be capable of detecting a signal with high confidence from a single source of gravitational waves within a few months of observing time. We describe the preliminary mission concept for TianQin, including the candidate source and experimental designs. We present estimates for the major constituents of the * experiment's error budget and discuss the project's overall feasibility. Given the current level of technology readiness, we expect TianQin to be flown in the second half of the next decade.

We use a paramagnetic salt TbF 3 with a dc SQUID to search for a possible axionlike s ? r interaction of a rotating copper mass with the salt. We set new limits on the axion coupling constant g s g p ͞hc and the finite-range Leitner -Okubo -Hari Dass coupling constant A. Our limit for range l at 30 mm is 2 orders of magnitude better than previous results. For l . 30 mm, g s g p ͞hc is ͑0.14 6 0.67͒ 3 10 228 , and A is less than 10. The outlook for further improvement is discussed.[S0031-9007(99)08656-1] PACS numbers: 14.80.Mz, 04.80.Cc, 12.20.Fv, 13.10. + q There are a number of groups experimentally searching for spin-dependent (semi-)long-range forces. These works are largely motivated to explore the role of spin in gravitation, and to explore the interaction associated with the exchange of a light or massless pseudoscalar Goldstone boson or similar interactions, e.g., arion interaction or axion interaction. Among the works to search for the spin-dependent (semi-)long-range forces, we can classify them into two categories: those searching for the monopole-dipole interactions [1-9] and those searching for the dipole-dipole interactions [5,[10][11][12][13][14][15][16][17].In connection with P (parity), and T (time reversal) noninvariance, Leitner and Okubo [18], and Hari Dass [19] suggested some time ago the following type of spingravity interaction, H int f͑r͒r ? s , (1) wherer is the unit vector from the massive body to the particle with spinhs . They assumed f͑r͒ 2AUm with U the gravitational potential of the massive body.Fujii [20] proposed finite-range mass-mass interactions. More recently, Fischbach et al. proposed a fifth force which violates the equivalence principle with finite-range monopole-monopole interactions and stimulated many experimental efforts [21].In our previous investigation [6], we used torsion balance with two cylindrical copper test masses and two cylindrical polarized "attracting" Dy 6 Fe 23 masses to search for finite-range mass-spin interactions with the Hamiltonian of the form (1). Our preliminary result showed that for the range of 3-5 cm, the upper limit of this interaction for our test mass and the Dy 6 Fe 23 polarized mass were below 1% of their gravitational interaction. We considered, in particular, the case of f͑r͒ 2Au 2mr mU with U the gravitational potential of the unpolarized body; that is, the finite-range mass-spin interaction is of the following form:H int 2Ae 2mr mUr ? s .(2) Ritter et al. [8], in a recent experiment, used spinpolarized Dy 6 Fe 23 masses acting on unpolarized copper masses in a dynamic-mode torsion pendulum and searched for the interaction of the axion [22][23][24] form,(3) In (3), l is the range of the interaction, g s and g p are the coupling constants of vertices at the polarized and unpolarized particles, and m is the mass of the polarized particle. Constraints on the coupling g s g p ͞hc with respect to the range are plotted in a logarithmic plot (Fig. 1). For l , 0.3 m, Refs. [6] and [8] give more stringent constraints than Refs. [5] and [7], a...

TianQin is a planned space-based gravitational wave (GW) observatory consisting of three Earth-orbiting satellites with an orbital radius of about $10^5 \, {\rm km}$. The satellites will form an equilateral triangle constellation the plane of which is nearly perpendicular to the ecliptic plane. TianQin aims to detect GWs between $10^{-4} \, {\rm Hz}$ and $1 \, {\rm Hz}$ that can be generated by a wide variety of important astrophysical and cosmological sources, including the inspiral of Galactic ultra-compact binaries, the inspiral of stellar-mass black hole binaries, extreme mass ratio inspirals, the merger of massive black hole binaries, and possibly the energetic processes in the very early universe and exotic sources such as cosmic strings. In order to start science operations around 2035, a roadmap called the 0123 plan is being used to bring the key technologies of TianQin to maturity, supported by the construction of a series of research facilities on the ground. Two major projects of the 0123 plan are being carried out. In this process, the team has created a new-generation $17 \, {\rm cm}$ single-body hollow corner-cube retro-reflector which was launched with the QueQiao satellite on 21 May 2018; a new laser-ranging station equipped with a $1.2 \, {\rm m}$ telescope has been constructed and the station has successfully ranged to all five retro-reflectors on the Moon; and the TianQin-1 experimental satellite was launched on 20 December 2019—the first-round result shows that the satellite has exceeded all of its mission requirements.

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A brief status report of an ongoing scientific case study of the Advanced Laser Interferometer Antenna (ALIA) mission is presented. Key technology requirements and primary science objectives of the mission are covered in 0264-9381/11/094012+10$33.00