Cooling the Motion of Diamond Nanocrystals in a Magneto-Gravitational Trap in High Vacuum

Abstract

Levitated diamond nanocrystals with nitrogen-vacancy (NV) centres in high vacuum have been proposed as a unique system for experiments in fundamental quantum mechanics, including the generation of large quantum superposition states and tests of quantum gravity. This system promises extreme isolation from its environment while providing quantum control and sensing through the NV centre spin. While optical trapping has been the most explored method of levitation, recent results indicate that excessive optical heating of the nanodiamonds under vacuum may make the method impractical with currently available materials. Here, we study an alternative magneto-gravitational trap for diamagnetic particles, such as diamond nanocrystals, with stable levitation from atmospheric pressure to high vacuum. Magnetic field gradients from permanent magnets confine the particle in two dimensions, while confinement in the third dimension is gravitational. We demonstrate that feedback cooling of the centre-of-mass motion of a trapped nanodiamond cluster results in cooling of one degree of freedom to less than 1 K.

Pure nanodiamonds for levitated optomechanics in vacuum

“…With the exception of the commercial nanodiamonds used in [12][13][14][15][16][17][18][19][20][21], no significant heating is expected above 1 mbar, as confirmed by our experimental results. Above this pressure, the surrounding gas efficiently dissipates the heat generated by absorption of the trapping laser.…”

“…The concentration of N s 0 in the twenty bulk samples averaged (121 ppb), varying from (95 ppb) to (162 ppb). This signifies an increase in purity of approximately three orders of magnitude compared to the 150 ppm HPHT synthesized starting material used to make the nanodiamonds used for previous work [12][13][14][15][16][17][18][19][20][21].…”

“…Progress with nanodiamonds levitated in optical dipole traps includes the detection of NV − fluorescence [12], optically detected magnetic resonance [13][14][15], and the observation of rotational vibration exceeding 1MHz [16]. Nanodiamonds containing NV − centres have been trapped using ion traps at atmospheric pressure [17,18] and in vacuum [19], and a magneto-gravitational trap has allowed nanodiamond clusters to be held below 10 −2 mbar [20]. However, the latter design requires permanent magnets for the levitation, which is incompatible with the trap-and-release experiments [1,2,6] that reach large distance spatial superpositions of the centre-of-mass as desired for all of the fundamental physics experiments mentioned above.…”

Stern–Gerlach splitting of low-energy ion beams

“…Steep magnetic gradients can be created between magnetised pole pieces with sharp edges, as already used by Gerlach and Stern. The configuration sketched in figures 2(a), (b) was used in [36] for generating very large gradients capable of trapping diamagnetic nano-diamonds. The static magnetic field outside the magnetised structures can be accurately computed from a scalar potential, B=−∇Φ.…”

“…Nevertheless, as noted earlier, our calculations neglect inaccuracies in the multipole expansion (equation (6)), even with the symmetric magnetic pole geometry, and we therefore consider two additional configurations in the following, whereby we benefit from modern chip fabrication techniques to design the magnetic field. [36]. The pole pieces consist ofFeCo (grey) andSmCo (blue) magnets.…”