The transfer of information between different physical forms is a central theme in communication and computation, for example between processing entities and memory. Nowhere is this more crucial than in quantum computation [1], where great effort must be taken to protect the integrity of a fragile quantum bit (qubit) [2]. However, transfer of quantum information is particularly challenging, as the process must remain coherent at all times to preserve the quantum nature of the information [3]. Here we demonstrate the coherent transfer of a superposition state in an electron spin 'processing' qubit to a nuclear spin 'memory' qubit, using a combination of microwave and radiofrequency pulses applied to 31 P donors in an isotopically pure 28 Si crystal [4,5]. The state is left in the nuclear spin on a timescale that is long compared with the electron decoherence time and then coherently transferred back to the electron spin, thus demonstrating the 31 P nuclear spin as a solid-state quantum memory. The overall store/readout fidelity is about 90%, attributed to imperfect rotations which can be improved through the use of composite pulses [6]. The coherence lifetime of the quantum memory element at 5.5 K exceeds one second.Classically, transfer of information can include a copying step, facilitating the identification and correction of errors. However, the no-cloning theorem limits the ability to faithfully copy quantum states across different degrees of freedom [7]; thus error correction becomes more challenging than for classical information and the transfer of information must take place directly. Experimental demonstrations of such transfer include moving a trapped ion qubit in and out of a decoherence-free subspace for storage purposes [8] and optical measurements of NV centres in diamond [9].Nuclear spins are known to benefit from long coherence times compared to electron spins, but are slow to manipulate and suffer from weak thermal polarisation. A powerful model for quantum computation is thus one in which electron spins are used for processing and readout while nuclear spins are used for storage. The storage element can be a single, well-defined nuclear spin, or perhaps a bath of nearby nuclear spins [10]. 31 P donors in silicon provide an ideal combination of long-lived spin-1/2 electron [11] and nuclear spins [12], with the additional advantage of integration with existing technologies [4] and the possibility of single spin detection by electrical measurement [13,14,15]. Direct measurement of the 31 P nuclear spin by NMR has only been possible at very high doping levels (e.g. near the metal insulator transition [16]). Instead, electron-nuclear double resonance (ENDOR) can be used to excite both the electron and nuclear spin associated with the donor site, and measure the nuclear spin via the electron [17]. This was recently used to measure the nuclear spin-lattice relaxation time T 1n , which was found to follow the electron relaxation time T 1e over the range 6 to 12 K with the relationship T 1n ≈ 250T 1e [5,...

Entanglement is the quintessential quantum phenomenon. It is a necessary ingredient in most emerging quantum technologies, including quantum repeaters, quantum information processing and the strongest forms of quantum cryptography. Spin ensembles, such as those used in liquid-state nuclear magnetic resonance, have been important for the development of quantum control methods. However, these demonstrations contain no entanglement and ultimately constitute classical simulations of quantum algorithms. Here we report the on-demand generation of entanglement between an ensemble of electron and nuclear spins in isotopically engineered, phosphorus-doped silicon. We combined high-field (3.4 T), low-temperature (2.9 K) electron spin resonance with hyperpolarization of the (31)P nuclear spin to obtain an initial state of sufficient purity to create a non-classical, inseparable state. The state was verified using density matrix tomography based on geometric phase gates, and had a fidelity of 98% relative to the ideal state at this field and temperature. The entanglement operation was performed simultaneously, with high fidelity, on 10(10) spin pairs; this fulfils one of the essential requirements for a silicon-based quantum information processor.

Endohedral fullerenes encapsulating a spin-active atom or ion within a carbon cage offer a route to self-assembled arrays such as spin chains. In the case of metallofullerenes the charge transfer between the atom and the fullerene cage has been thought to limit the electron spin phase coherence time (T2) to the order of a few microseconds. We study electron spin relaxation in several species of metallofullerene as a function of temperature and solvent environment, yielding a maximum T2 in deuterated o-terphenyl greater than 200µs for Y, Sc and La@C82. The mechanisms governing relaxation (T1, T2) arise from metal-cage vibrational modes, spin-orbit coupling and the nuclear spin environment. The T2 times are over 2 orders of magnitude longer than previously reported and consequently make metallofullerenes of interest in areas such as spin-labelling, spintronics and quantum computing.The possibility of encapsulating an atom inside a fullerene cage was discovered by Heath et al. [1] in 1985 and has led to widespread research into these novel materials. Encapsulation of a nitrogen atom within a C 60 cage (N@C 60 ) has been the subject of particular interest due to the remarkably long electron spin coherence times (T 2 ) reported from 80 to 250 µs [2,3]. Metallofullerenes, containing metal ions encased in a similar way, benefit from faster purification and higher production yields than N@C 60 . However, they have not shown particularly long coherence times (< 1.5 µs), attributed to much greater spin density on the fullerene cage [4,5].The prospect of exploiting the self-assembly of spinactive fullerene molecules into larger structures [6] has stimulated interest in spintronics [7] and quantum information processing (QIP) [8,9]. In particular, metallofullerenes can self-assemble within carbon nanotubes in a 'peapod' structure, creating a 1-D spin chain [6,[10][11][12]. For the potential of such structures to be fully realised, longer electron spin coherence times are required of the constituent metallofullerenes.Several electron paramagnetic resonance (EPR) studies have been conducted on the metallofullerenes Sc-, Y-and La@C 82 focusing on geometric and electronic properties of the molecules [4,5,[13][14][15][16][17][18]. These, along with x-ray diffraction measurements, have shown the metal atom to be off centre in the cage [19][20][21], with charge transfer to the cage dependent on the metal ion species [22]. These previous EPR studies have primarily used CW spectroscopy and few T 2 times have have been accurately extracted. Using pulsed EPR, Knorr et al. report a T 2 of 600 ns for Sc@C 82 (in trichlorobenzene solvent at 2.5 K) but a measured T 2 of 4.1 µs in a Y@C 82 sample was attributed to a 'background' signal [4]. Okabe et al. report a temperature and m I dependence of T 1 and T 2 for La@C 82 (in CS 2 ) arising from motional effects of anisotropic interactions and coherence times < 1.5 µs, in the range 183-283 K [5].In this letter we report pulsed EPR studies of spin relaxation times over a range of temperatures ...

Electron spin qubits in molecular systems offer high reproducibility and the ability to self assemble into larger architectures. However, interactions between neighbouring qubits are 'always-on' and although the electron spin coherence times can be several hundred microseconds, these are still much shorter than typical times for nuclear spins. Here we implement an electron-nuclear hybrid scheme which uses coherent transfer between electron and nuclear spin degrees of freedom in order to both controllably turn on/off dipolar interactions between neighbouring spins and benefit from the long nuclear spin decoherence times (T2n). We transfer qubit states between the electron and 15 N nuclear spin in 15 N@C60 with a two-way process fidelity of 88%, using a series of tuned microwave and radiofrequency pulses and measure a nuclear spin coherence lifetime of over 100 ms.Hybrid quantum computing schemes aim to harness the benefits of multiple quantum degrees of freedom through the coherent transfer of quantum information between them. Such transfer has previously been shown between light and atomic ensembles [1,2], as well as electron to nuclear spin states in nitrogen vacancies [3] and 31 P donors [4], and progress is being made towards coupling electron spin ensembles to superconducting qubits [5, 6]. Common motivations for state transfer between electron to nuclear spin qubits include the much longer decoherence times typically exhibited by the nuclear spin, and also the weaker dipolar interaction between nuclear spins which allows interactions between neighbouring qubits to be effectively turned off [3, 4,[7][8][9]. Both effects can be attributed to the relatively weak nuclear magnet moment, typically 3 orders of magnitude smaller than an electron spin. Thus, a powerful hybrid model for quantum computing is one where the electron spin qubit (which is more readily polarised and more quickly manipulated) is used for initialisation and processing, while the nuclear spin is used as a memory. The presence of the electron spin also offers considerable advantages for the readout of a single qubit, either of the electron spin state directly [10, 11], or a quantum nondemolition measurement of the nuclear spin [12,13].Endohedral fullerenes (atoms held within a carbon cage) offer promise as a molecular qubits due to their exceptionally long electron decoherence times [14][15][16] and convenient coupling to a local nuclear spin. In particular, N@C 60 has been used to demonstrate polarisation transfer from the electron to the nuclear spin and subsequent 'bang-bang' decoupling [17,18], as well as generation of pseudo-entanglement between the electron and nuclear spin [19]. The advantages of a molecular system include the ability to produce larger arrays that can be engineered to control electron dipolar interaction [9,[20][21][22]. Molecular approaches offer self assembly of highly reproducible structures [23,24], but are limited by the 'always-on' nature of dipolar interactions between neighbouring electron spins. In this Lette...

The ESR properties and crystal structures of solid-state La@C(82) in empty fullerene matrices were investigated by changing the concentration of La@C(82) and the species of an empty fullerene matrix: C(60), C(70), C(78)(C(2v)(3)), C(82)(C(2)) and C(84)(D(2d)(4)). The rotational correlation time of La@C(82) molecules tended to be shorter when La@C(82) is dispersed in larger fullerene matrices because large C(2n) molecules provide more space for La@C(82) molecules for rotating. La@C(82) dispersed in a hcp-C(82) matrix showed the narrowest hyperfine structure (hfs) due to the ordered nature of La@C(82) molecules in the C(82) crystal. On the other hand, in a C(60) matrix, La@C(82) molecules formed clusters because of the large different solubility, which leads to the ESR spectra being broad sloping features due to strong dipole-dipole and exchange interactions.