Tiger in a Cage: Detecting Single Photons at low GHz Frequencies without Refrigerators, Vacuum Chambers or Magnets

Optically polarizable "spin" molecules trapped within "cage" molecules will be synthesized and grown as molecular crystals. One such system is phenazine, as the spin-active "guest" molecule, enveloped by a cyclophane ExCage as the "host" molecule. It is conjectured that such molecularly engineered systems can provide the long spin relaxation times exhibited by N@C60 whilst being far easier to synthesize (and crystallize out) at high chemical yields. This conjecture will be tested experimentally, the relaxation times being quantified through transient-EPR measurements. It is further speculated that these systems are capable of generating very high spin polarization densities by dint of the guest molecule's high spatial concentration, far exceeding what can be achieved by "dilute" systems such as pentacene-doped para-terphenyl, which cannot be doped (in the bulk) above 0.1 % without compromising crystallinity. As a "back-up", spin-active charge-transfer crystals, such as phenazine:tetracyanobenzene ("TCNB") will also be grown and assessed.

The most promising molecular systems, in the form of suitably shaped crystals, will then be laminated against piezoelectric chips capable of generating, propagating and detecting surface-acoustic waves (SAWs) in acoustic delay line structures. The chips' substrate material will be a selected "cut" of lithium niobate, the generation and detection of SAWs being provided by an opposing pair of metal interdigital transducers (IDTs) lithographically deposited onto each chip's surface. It is speculated that the coupling between the spin-polarized crystal and the SAW (as it propagates between the two IDTs) will either amplify or attenuate (=cool) the SAW depending on whether the crystal is emissively or absorptively spin-polarised. Immune from the forms of electronic noise that limit the performance of semiconductor-based amplifiers, it is further speculated that, with an emissively spin-polarized crystal, the structure will function as an avalanche photo-multiplier capable of detecting very small numbers of microwave photons at room temperature. Alternatively, with an absorptively polarized crystal, the device should be capable of approaching the quantum ground state (zero photons) of the spin-coupled SAW mode in question.

Devices will be accurately simulated using advanced finite-element simulation software capable of representing coupled piezomechanical-microwave SAW modes. Various combinations of spin-polarizable crystal and SAW mode [e.g. Rayleigh versus Bleustein-Gulyaev] will be explored towards maximizing the spin-phonon coupling whilst avoiding excessive "leakiness" (thus loss) from the SAW mode. Both pulsed and CW operation will be explored.