Silicon is one of the most promising semiconductor materials for spin-based information processing devices. Its advanced fabrication technology facilitates the transition from individual devices to large-scale processors, and the availability of a (28)Si form with no magnetic nuclei overcomes a primary source of spin decoherence in many other materials. Nevertheless, the coherence lifetimes of electron spins in the solid state have typically remained several orders of magnitude lower than that achieved in isolated high-vacuum systems such as trapped ions. Here we examine electron spin coherence of donors in pure (28)Si material (residual (29)Si concentration <50 ppm) with donor densities of 10(14)-10(15) cm(-3). We elucidate three mechanisms for spin decoherence, active at different temperatures, and extract a coherence lifetime T(2) up to 2 s. In this regime, we find the electron spin is sensitive to interactions with other donor electron spins separated by ~200 nm. A magnetic field gradient suppresses such interactions, producing an extrapolated electron spin T(2) of 10 s at 1.8 K. These coherence lifetimes are without peer in the solid state and comparable to high-vacuum qubits, making electron spins of donors in silicon ideal components of quantum computers, or quantum memories for systems such as superconducting qubits.

Pulsed electron paramagnetic resonance measurements of donor electron spins in natural phosphorus-doped silicon (Si:P) and isotopically-purified 28 Si:P show a strongly temperaturedependent longitudinal relaxation time, T 1 , due to an Orbach process with ∆E = 126 K. The 2-pulse echo decay is exponential in 28 Si:P, with the transverse relaxation (decoherence) time, T 2 , controlled by the Orbach process above ~12 K and by instantaneous diffusion at lower temperatures. Spin echo experiments with varying pulse turning angles show that the intrinsic T 2 of an isolated spin in 28 Si:P is ~60 ms at 7 K.

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 report advances in nanoimprint lithography, its application in nanogap metal contacts, and related fabrication yield. We have demonstrated 5 nm linewidth and 14 nm linepitch in resist using nanoimprint lithography at room temperature with a pressure less than 15 psi. We fabricated gold contacts (for the application of single macromolecule devices) with 5 nm separation by nanoimprint in resist and lift-off of metal. Finally, the uniformity and manufacturability of nanoimprint over a 4 in. wafer were demonstrated.The field of nanotechnology is advancing rapidly. Applications of nanotechnology include subwavelength optical elements, biochemical analysis devices, photonic crystals, high-density single-domain magnetic storage, and singlemolecule devices, to name a few. Yet, key to the commercial success of these nanotechnology applications are low cost and high throughput manufacturing capabilities. State-of-theart manufacturing photolithography patterning tools are both too expensive and incapable of producing the necessary pitch and feature sizes of these applications. Thus, presently, researchers have been largely constrained to using lowthroughput lithography tools, such as electron-beam lithography (EBL), atomic force microscopy (AFM), and ion-beam lithography. For high-throughput and low-cost lithography, various "nanoprinting" technologies have been developed. 1-3 Here, we report our investigation of the resolution limit of nanoimprint lithography, where we demonstrated a nanoimprint record of 5 nm linewidth features and 14 nm pitch over a large area, its applications in nanogap metal contacts, and a study of fabrication yields.In photocurable nanoimprint lithography (P-NIL) (shown in Fig. 1), a mold is pressed into a low viscosity photocurable resist liquid to physically deform the resist shape such that it conforms to the topology of the mold. The resist is cured with exposure to UV light, crosslinking the various components in the resist liquid, producing a uniform, relatively rigid polymer network. The mold is then separated from the cured resist. Finally, an anisotropic reactive ion etch (RIE) removes the residual resist in the compressed area, exposing the substrate surface.In order to explore the performance of P-NIL, a variety of molds were fabricated to test specific attributes, including minimum pitch (maximum density), minimum feature size, and large-area uniformity patterning. Previously, 10 nm dots and 40 nm pitch have been demonstrated by NIL 1 with the resolution limited by our ability to fabricate the mold, as proximity effects inherent with EBL make sub-35 nm pitch patterning very difficult. To produce a mold with a pitch resolution surpassing EBL capabilities, we fabricated a NIL mold by selectively wet etching Al 0.7 Ga 0.3 As from a cleaved edge of a GaAs/ Al 0.7 Ga 0.3 As superlattice [grown by molecular-beam epitaxy (MBE)] with a dilute solution of hydrofluoric acid (HF). 4,5 This mold fabrication process offers many advantages, specifically very dense sub-50 nm pitch topologies can be ...

A major challenge in using spins in the solid state for quantum technologies is protecting them from sources of decoherence. This is particularly important in nanodevices where the proximity of material interfaces, and their associated defects, can play a limiting role. Spin decoherence can be addressed to varying degrees by improving material purity or isotopic composition, for example, or active error correction methods such as dynamic decoupling (or even combinations of the two). However, a powerful method applied to trapped ions in the context of atomic clocks is the use of particular spin transitions that are inherently robust to external perturbations. Here, we show that such 'clock transitions' can be observed for electron spins in the solid state, in particular using bismuth donors in silicon. This leads to dramatic enhancements in the electron spin coherence time, exceeding seconds. We find that electron spin qubits based on clock transitions become less sensitive to the local magnetic environment, including the presence of (29)Si nuclear spins as found in natural silicon. We expect the use of such clock transitions will be of additional significance for donor spins in nanodevices, mitigating the effects of magnetic or electric field noise arising from nearby interfaces and gates.