Using X-band pulsed electron spin resonance, we report the intrinsic spin-lattice (T1) and phase coherence (T2) relaxation times in molecular nanomagnets for the first time. In Cr7M heterometallic wheels, with M = Ni and Mn, phase coherence relaxation is dominated by the coupling of the electron spin to protons within the molecule. In deuterated samples T2 reaches 3 µs at low temperatures, which is several orders of magnitude longer than the duration of spin manipulations, satisfying a prerequisite for the deployment of molecular nanomagnets in quantum information applications.Certain computational tasks can be efficiently implemented using quantum logic, in which the informationcarrying elements are permitted to exist in quantum superpositions [1]. To achieve this in practice, a physical system that is suitable for embodying quantum bits (qubits) must be identified. Some proposed scenarios employ electron spins in the solid state, for example phosphorous donors in silicon [2], quantum dots [3], heterostructures [4] and endohedral fullerenes [5,6], motivated by the long electron-spin relaxation times exhibited by these systems. An alternative electron-spin based proposal exploits the large number of quantum states and the non-degenerate transitions available in high spin molecular magnets [7,8]. Although these advantages have stimulated vigorous research in molecular magnets [9,10,11], the key question of whether the intrinsic spin relaxation times are long enough has hitherto remained unaddressed. Here we show, using pulsed electron spin resonance experiments on heterometallic wheels, that the relaxation times in molecular magnets can significantly exceed the duration of coherent manipulations, a prerequisite for the deployment of these systems in quantum information applications.Molecular magnets comprising clusters of exchanged coupled transition metal ions have been studied extensively in recent years [12]. They can exhibit a substantial ground state spin with a large and negative zerofield splitting (ZFS), leading to a spontaneous magnetic moment parallel to the easy axis. In the absence of a magnetic field, the configurations in which the moment is 'up' or 'down' relative to the easy axis are degenerate, and this bistable nature has stimulated interest in the application of magnetic clusters as classical [13] or quantum [7,8,9, 11] information elements.Molecules in this class have been synthesised with widely varying properties, from the S = 10 highly axial Mn 12 -acetate [14], to the diamagnetic ring Cr 8 F 8 Piv 16 [15,16]. A key recent chemical advance is the the development of procedures for magnetically 'doping' a diamagnetic cluster to synthesise paramagnetic molecules in a systematic and controllable way [17]. Thus, substituting a Cr 3+ (s = 3/2) by a Mn 2+ (s = 5/2) or a Ni 2+ (s = 1) generates the S = 1 Cr 7 Mn or the S = 1/2 Cr 7 Ni respectively.Many clusters have been investigated using thermodynamic probes such as magnetization [18] and heat capacity [19], and spectroscopic probes such as neutr...

We combined the high-energy resolution of conventional spin resonance (here ~10 nano-electron volts) with scanning tunneling microscopy to measure electron paramagnetic resonance of individual iron (Fe) atoms placed on a magnesium oxide film. We drove the spin resonance with an oscillating electric field (20 to 30 gigahertz) between tip and sample. The readout of the Fe atom's quantum state was performed by spin-polarized detection of the atomic-scale tunneling magnetoresistance. We determine an energy relaxation time of T1 ≈ 100 microseconds and a phase-coherence time of T2 ≈ 210 nanoseconds. The spin resonance signals of different Fe atoms differ by much more than their resonance linewidth; in a traditional ensemble measurement, this difference would appear as inhomogeneous broadening.

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,...

We propose to encode a register of quantum bits in different collective electron spin wave excitations in a solid medium. Coupling to spins is enabled by locating them in the vicinity of a superconducting transmission line cavity, and making use of their strong collective coupling to the quantized radiation field. The transformation between different spin waves is achieved by applying gradient magnetic fields across the sample, while a Cooper pair box, resonant with the cavity field, may be used to carry out one- and two-qubit gate operations.

Quantum entangled states can be very delicate and easily perturbed by their external environment. This sensitivity can be harnessed in measurement technology to create a quantum sensor with a capability of outperforming conventional devices at a fundamental level. We compare the magnetic field sensitivity of a classical (unentangled) system with that of a 10-qubit entangled state, realised by nuclei in a highly symmetric molecule. We observe a 9.4-fold quantum enhancement in the sensitivity to an applied field for the entangled system and show that this spin-based approach can scale favorably compared to approaches where qubit loss is prevalent. This result demonstrates a method for practical quantum field sensing technology.The concept of entanglement, in which coherent quantum states become inextricably correlated [1], has evolved from one of the most startling and controversial outcomes of quantum mechanics to the enabling principle of emerging technologies such as quantum computation [2] and quantum sensors [3,4]. The use of entangled particles in measurement permits the transcendence of the standard quantum limit in sensitivity, which scales as √ N for N particles, to the Heisenberg limit, which scales as N . This approach has been applied to optical interferometry using entangled photons [5] and using up to six trapped ions for the measurement of magnetic fields and improvements in atomic clocks [6,7,8]. Spinsqueezing has been investigated as an alternative mode of entanglement generation, being proposed for sensitive phase detection [9] and demonstrated using four 9 BeA single spin will precess in the presence of a magnetic field. In the rotating frame used to describe magnetic resonance, this precession occurs at a rate governed by the detuning δ of the magnetic field from resonance (expressed in frequency units), such that the state |0 + |1 evolves as |0 + e iδt |1 (for clarity, normalisation constants are omitted throughout). This principle forms the basis of several kinds of magnetic field sensor, where the externally applied field δ is detected as a phase shift. States possessing many-qubit entanglement can acquire phase at a greater rate and thus offer an enhanced sensitivity to the applied field.The requirements for constructing the resource of a large number of entangled spins are less severe than those for a complete NMR quantum computer [11,12,13]. Indeed, rather than striving towards individual addressability of each consitutent nuclear spin, global addressing is advantageous in quickly and efficiently growing the state. For example, we consider a star topology with one central spin, A, and N peripheral B spins which cannot be * Electronic address: john.morton@materials.ox.ac.uk separately addressed (see Fig 1A).The B spins cannot be distinguished by any NMR observable and their behaviour is well described by number states, as used to describe photon occupation in one of two modes. Many-body entanglement in such states has been referred to as the NOON state [14,15,16], and has received much att...