Room‐Temperature Quantum Memories Based on Molecular Electron Spin Ensembles

Abstract

Superconducting quantum bits are addressed by means of microwave radiation and quantum memories in this context thus need to be able to store microwave photon states. To this end, resonant structures for electromagnetic radiation can be used to strongly couple quantum bits and quantum memories to quantized cavity modes of the electromagnetic field. The strong coupling generates a hybrid quantum system that allows mapping the qubit state onto the cavity field state. [6] This strategy has given rise to the field of cavity quantum electrodynamics and has already been used to couple two superconducting qubits together via a cavity bus. [7] A second application of quantum memories is in quantum repeaters for quantum communication. Because telecom wavelengths are in the near-infrared, for this application, optical quantum memories are required that store optical photon states. [8] Considering the choice of material platform for designing microwave quantum memories, electron spins in solids can be easily addressed by microwave pulses and have been shown to possess excellent coherence times up to seconds, for some systems even up to room temperature. [9][10] Unfortunately, the coupling of a single electron spin to a photon is very weak. Although resonant structures can be tailored to improve single-spin coupling, [11][12][13] the weakness of the coupling renders addressing individual spins by means of coupling to cavity modes very challenging. This can be overcome by using an electron spin ensemble rather than single spins, because the coupling strength of a spin ensemble to an electromagnetic resonator mode is proportional to the square root of the number of electron spins in the ensemble. [14] This then leads to the general idea of the implementation of a microwave quantum memory using spin ensembles and microwave resonators (Figure 1). The state to be stored is encoded in a weak microwave pulse and sent to the hybrid quantum system constituted of an electron spin ensemble that is strongly coupled to a microwave resonator, where it is stored. When the quantum state is needed again, a strong microwave pulse is sent to the resonator, which leads to deterministic retrieval of the quantum state that is emitted as a microwave photon to be used as desired.Strong coupling between cavity and electron spin ensembles has been observed in a variety of spin systems, including organic radicals, [15][16][17][18][19][20] phosphorous dopants in silicon, [21,22] P1 and nitrogen vacancy (NV) defects in diamond, [23][24][25][26][27][28][29][30][31][32] N@ C 60 , [33] rare earth dopants in oxides, [34,35] transition metal Whilst quantum computing has recently taken great leaps ahead, the development of quantum memories has decidedly lagged behind. Quantum memories are essential devices in the quantum technology palette and are needed for intermediate storage of quantum bit states and as quantum repeaters in long-distance quantum communication. Current quantum memories operate at cryogenic, mostly sub-Kelvin temperatures and require extens...

Excited State Exchange Control of Photoinduced Electron Spin Polarization in Electronic Ground States

“…One of the most important of these properties is the ability of unpaired electron and nuclear spins to function as molecular qubits . However, a fundamental challenge focuses on how to generate, control, and manipulate these molecular spin qubits with the ultimate goal of developing quantum-based information technologies − that include quantum computing, − metrology, − sensing, − communications, , and cryptography. ,− These quantum information applications can benefit from electron spin polarization (ESP: non-Boltzmann populations of m s levels) to overcome the inherently small population differences between the spin states. , …”

Revealing intrinsic spin coupling in transition metal-doped graphene

“…Compared to conventional electronic devices, spintronic devices use spin as the information carrier and have the advantages of high speed and low power consumption. Hence, they are expected to realize various sophisticated devices for signal transmission, [47][48][49] information storage, [49][50][51] sensors, 52,53 logic gates, [54][55][56] spin valves, 57,58 molecular switches 59 and so on. In particular, theoretical studies can provide insights into the intrinsic properties of various materials.…”

Machine Learning-Assisted Manipulation and Readout of Molecular Spin Qubits

“…Molecular spin qubits have been recently shown to have long coherence times over a wide range of temperature [36][37][38][39][40][41][42][43]. The viability of their integration into hybrid quantum circuits at MW frequency and in solid-state quantum technologies has been also demonstrated both in the Continuous Wave (CW) [44][45][46][47][48][49][50] as well as in the Pulsed Wave (PW) regime [51][52][53] of excitation. For instance, molecular spin qubits were found to reach the coherent spin-photon coupling [44][45][46][47][48]54] and to be suitable as quantum memories for information [51,52].…”

“…The viability of their integration into hybrid quantum circuits at MW frequency and in solid-state quantum technologies has been also demonstrated both in the Continuous Wave (CW) [44][45][46][47][48][49][50] as well as in the Pulsed Wave (PW) regime [51][52][53] of excitation. For instance, molecular spin qubits were found to reach the coherent spin-photon coupling [44][45][46][47][48]54] and to be suitable as quantum memories for information [51,52]. Their readout in the dispersive, non-resonant regime has been also reported [55].…”