Periodic Reporting for period 1 - HYSPECQS (Hybrid spin-mechanical quantum systems)
Okres sprawozdawczy: 2022-06-01 do 2024-05-31
HYSPECQS aimed to create a new type of system combining nanomechanical devices and spins, a property inherent to quantum objects that is alike to a small magnet. By using nanomagnets to induce coupling between the mechanical oscillator and the spins, the project sought to study the interaction betwen the two systems and ultimately to generate unique states of motion through the quantum properties of the spin. These new states can be crucial for developing advanced quantum information and sensing technologies, as well as for testing the fundamental principles of quantum mechanics. In order to study the interaction between the two systems, three key parameters are crucial: the strength of the coupling between the two systems, which relies on small separation between spin and oscillator, and on how much the magnetic field generated by the nanomagnet varies over distance-known as magnetic field gradient; the quantum decoherence rate -i.e. the rate at which the two systems lose their quantum properties- of the spin and of the mechanical resonator need to be as small as possible.
The project tackled the latter requirement by choosing appropriately the two platforms. An optically-active defect of the diamond crystal, known as the nitrogen-vacancy centre, which resembles an atom trapped in the solid state, provides a spin with naturally long coherence time, even at room temperature. The mechanical resonator consists of a 5mmx5mm silicon nitride membrane, a ceramic material, only few nanometers thick. The host group at the Niels Bohr Institute optimized the fabrication of such resonators by patterning the membrane with a periodic structure of holes, which confines the mechanical motion at the very centre of the membrane while isolating it from the environment. Using techniques known such as sideband cooling and cold damping, the vibrations can be almost completely removed, making the oscillator a quantum object. Most importantly, owing to its isolation from the environment, such quantum mechanical mode has been proven to survive for more than 100ms at few hundred milliKelvin.
To ensure large coupling rate, the project aimed at growing nanomagnets at the centre of the silicon nitride membrane. The small separation was ensured by using a technique known as atomic force microscopy: a tip with a diameter of ~300nm, containing a single nitrogen-vacancy centre, is pushed against the membrane. By sensing the force building up between the tip and the membrane, their separation can be stabilized using precise motors, and thus allowing for tip-membrane separation below 1nm. Such small value place the spin contained in the tip in the region where the variation of magnetic field generated by the nanomagnet is the largest, thereby ensuring large coupling.
By leveraging on the low decoherece rate of the two systems, combined with the maximization of the coupling rate, HYSPECQS aimed at generating strong spin-mechanical effects both in the classical regime, in the form of sensing of extremely small forces, and in the quantum regime, by generating new quantum states of motion, previously unobserved.
The project tackled the latter requirement by choosing appropriately the two platforms. An optically-active defect of the diamond crystal, known as the nitrogen-vacancy centre, which resembles an atom trapped in the solid state, provides a spin with naturally long coherence time, even at room temperature. The mechanical resonator consists of a 5mmx5mm silicon nitride membrane, a ceramic material, only few nanometers thick. The host group at the Niels Bohr Institute optimized the fabrication of such resonators by patterning the membrane with a periodic structure of holes, which confines the mechanical motion at the very centre of the membrane while isolating it from the environment. Using techniques known such as sideband cooling and cold damping, the vibrations can be almost completely removed, making the oscillator a quantum object. Most importantly, owing to its isolation from the environment, such quantum mechanical mode has been proven to survive for more than 100ms at few hundred milliKelvin.
To ensure large coupling rate, the project aimed at growing nanomagnets at the centre of the silicon nitride membrane. The small separation was ensured by using a technique known as atomic force microscopy: a tip with a diameter of ~300nm, containing a single nitrogen-vacancy centre, is pushed against the membrane. By sensing the force building up between the tip and the membrane, their separation can be stabilized using precise motors, and thus allowing for tip-membrane separation below 1nm. Such small value place the spin contained in the tip in the region where the variation of magnetic field generated by the nanomagnet is the largest, thereby ensuring large coupling.
By leveraging on the low decoherece rate of the two systems, combined with the maximization of the coupling rate, HYSPECQS aimed at generating strong spin-mechanical effects both in the classical regime, in the form of sensing of extremely small forces, and in the quantum regime, by generating new quantum states of motion, previously unobserved.
Part of the work was dedicated to build the entire setup to perform the experiments. A confocal microscope, with single-photon detection capabilities, was built to optically excite the nitrogen-vacancy centres using green light and to to collect their fluorescence, in the red/near infrared spectrum. An atomic force microscope (AFM), employing diamond tips mounted on tuning forks which enable electrical readout of the tip-sample force, was also developed from scratch. Part of the effort was dedicated to successfully testing and deploying a different type of scanning modality. While conventional AFMs scan the sample horizontally, the 15mm x 15mm chip size prevented the tip from reaching its centre. To address this, new mounts were developed, enabling vertical plane scanning and allowing the tip to access the membrane’s centre. A python code, based on the open-source python infrastructure Qudi, was used to control the confocal scanning, synchronize it with the AFM scanning, and to simultaneously acquire photon counts, while also controlling other pieces of of equipment in parallel.
The spin of the nitrogen-vacancy centre has a resonance at ~2.9GHz. To address it efficiently, a broadband microwave antenna, with proven range from ~100MHz to >4GHz, was designed. The antenna geometry, a gold circuit evaporated on a silicon substrate, was chosen to be compatible with the final spin-mechanical assembly where the membrane chip is glued to the antenna chip, such that the antenna-membrane separation, and thus the antenna-spin separation, is kept small (<2um). The antenna was also capable of delivering fast pulses to flip the spin-a capability which is required to induce spin-mechanical interactions-, which could be as short as 25ns, limited only by the bandwidth of the experimental instrumentation.
At the same time, the code infrastructure required to perform coherent control of the spin state was implemented through python. The low-level programming of the AWG was made into a high-level interface, which allowed to generate sequence of pulses with configurable shape and phsae. The interface was successfully tested to generate spin control sequences such as Rabi oscillation, Ramsey interference, and spin-echo.
A key aspect of the project relied on nanomagnets to generate the spin-mechanical coupling. Importantly, the nanomagnet had to be precisely placed at a point on the membrane where the largest oscillations occur. This presented challenges from the fabrication point of view. The nanomagnets could not deposited before the fabrication of the membrane, since the strong base used to release the membrane from its silicon substrate would remove most of the materials deposited on it. On the other hand, standard growth techniques cannot be used after the release of the membrane, since they would severely degrade its mechanical properties in the best case scenario. The solution was found in a technique called focussed electron beam ion deposition, which is analogous to 3D printing in the nanoscale. In this way, nanomagnets were successfully deposited on the membrane and their magnetic field was imaged, opening the way to spin-mechcanical experiments.
The spin of the nitrogen-vacancy centre has a resonance at ~2.9GHz. To address it efficiently, a broadband microwave antenna, with proven range from ~100MHz to >4GHz, was designed. The antenna geometry, a gold circuit evaporated on a silicon substrate, was chosen to be compatible with the final spin-mechanical assembly where the membrane chip is glued to the antenna chip, such that the antenna-membrane separation, and thus the antenna-spin separation, is kept small (<2um). The antenna was also capable of delivering fast pulses to flip the spin-a capability which is required to induce spin-mechanical interactions-, which could be as short as 25ns, limited only by the bandwidth of the experimental instrumentation.
At the same time, the code infrastructure required to perform coherent control of the spin state was implemented through python. The low-level programming of the AWG was made into a high-level interface, which allowed to generate sequence of pulses with configurable shape and phsae. The interface was successfully tested to generate spin control sequences such as Rabi oscillation, Ramsey interference, and spin-echo.
A key aspect of the project relied on nanomagnets to generate the spin-mechanical coupling. Importantly, the nanomagnet had to be precisely placed at a point on the membrane where the largest oscillations occur. This presented challenges from the fabrication point of view. The nanomagnets could not deposited before the fabrication of the membrane, since the strong base used to release the membrane from its silicon substrate would remove most of the materials deposited on it. On the other hand, standard growth techniques cannot be used after the release of the membrane, since they would severely degrade its mechanical properties in the best case scenario. The solution was found in a technique called focussed electron beam ion deposition, which is analogous to 3D printing in the nanoscale. In this way, nanomagnets were successfully deposited on the membrane and their magnetic field was imaged, opening the way to spin-mechcanical experiments.
The project, as described above, succeeded in overcoming all the technical challenges associated with building the experiment, controlling it, and in finding a way to generate spin-mechanical coupling by growing nanomagnets on the silicon nitride membranes. The experiments stopped short of running the first measurements of spin-mechanical coupling for two reasons. On one hand, the long lead time associated with the cryostat delivery, which was received only two months before the termination of the project, prevented entering the regime in which the coherence times of both the spin and of the membrane were sufficiently long enough to make the coupling observable. On the other hand, furhter engineering is required to improve the magnetic field gradient. The measurements obtained from the nanomagnets revealed gradients which are one order of magnitude lower than the minimum required gradient to overcome the state of the art. However, the gradients are expected to improve in two ways: by finding more precise techniques to apply strong external magnetic fields to the nanomagnets, thereby increasing the magnetic field gradient they generate, while preventing degradation of the spin properties, which can occur at large fields; by improving the growth of the nanomagnet and reducing the amount of impurities in the metal.