Understanding gravity in the framework of quantum mechanics is one of the great challenges in modern physics. However, the lack of empirical evidence has lead to a debate on whether gravity is a quantum entity. Despite varied proposed probes for quantum gravity, it is fair to say that there are no feasible ideas yet to test its quantum coherent behavior directly in a laboratory experiment. Here, we introduce an idea for such a test based on the principle that two objects cannot be entangled without a quantum mediator. We show that despite the weakness of gravity, the phase evolution induced by the gravitational interaction of two micron size test masses in adjacent matter-wave interferometers can detectably entangle them even when they are placed far apart enough to keep Casimir-Polder forces at bay. We provide a prescription for witnessing this entanglement, which certifies gravity as a quantum coherent mediator, through simple spin correlation measurements.

Organic semiconductors are studied intensively for applications in electronics and optics, and even spin-based information technology, or spintronics. Fundamental quantities in spintronics are the population relaxation time (T1) and the phase memory time (T2): T1 measures the lifetime of a classical bit, in this case embodied by a spin oriented either parallel or antiparallel to an external magnetic field, and T2 measures the corresponding lifetime of a quantum bit, encoded in the phase of the quantum state. Here we establish that these times are surprisingly long for a common, low-cost and chemically modifiable organic semiconductor, the blue pigment copper phthalocyanine, in easily processed thin-film form of the type used for device fabrication. At 5 K, a temperature reachable using inexpensive closed-cycle refrigerators, T1 and T2 are respectively 59 ms and 2.6 μs, and at 80 K, which is just above the boiling point of liquid nitrogen, they are respectively 10 μs and 1 μs, demonstrating that the performance of thin-film copper phthalocyanine is superior to that of single-molecule magnets over the same temperature range. T2 is more than two orders of magnitude greater than the duration of the spin manipulation pulses, which suggests that copper phthalocyanine holds promise for quantum information processing, and the long T1 indicates possibilities for medium-term storage of classical bits in all-organic devices on plastic substrates.

Optically active point defects in crystals have gained widespread attention as photonic systems that can find use in quantum information technologies. However challenges remain in the placing of individual defects at desired locations, an essential element of device fabrication. Here we report the controlled generation of single nitrogen-vacancy (NV) centres in diamond using laser writing. The use of aberration correction in the writing optics allows precise positioning of vacancies within the diamond crystal, and subsequent annealing produces single NV centres with up to 45% success probability, within about 200 nm of the desired position. Selected NV centres fabricated by this method display stable, coherent optical transitions at cryogenic temperatures, a pre-requisite for the creation of distributed quantum networks of solid-state qubits. The results illustrate the potential of laser writing as a new tool for defect engineering in quantum technologies.Comment: 21 pages including Supplementary informatio

We show that the two-component model of graphene oxide (GO), that is, composed of highly oxidized carbonaceous debris complexed to oxygen functionalized graphene sheets, is a generic feature of the synthesis of GO, independent of oxidant or protocol used. The debris present, roughly onethird by mass, can be removed by a base wash. A number of techniques, including solid state NMR, demonstrate that the properties of the base-washed material are independent of the base used and that it contains similar functional groups to those present in the debris but at a lower concentration. Removal of the oxidation debris cleans the GO, revealing its true monolayer nature and in the process increases the C/O ratio (i.e., a deoxygenation). By contrast, treating GO with hydrazine both removes the debris and reduces (both deoxygenations) the graphene sheets.

We show how the interference between spatially separated states of the center of mass (COM) of a mesoscopic harmonic oscillator can be evidenced by coupling it to a spin and performing solely spin manipulations and measurements (Ramsey Interferometry). We propose to use an optically levitated diamond bead containing an NV center spin. The nano-scale size of the bead makes the motional decoherence due to levitation negligible. The form of the spin-motion coupling ensures that the scheme works for thermal states so that moderate feedback cooling suffices. No separate control or observation of the COM state is required and thereby one dispenses with cavities, spatially resolved detection and low mass-dispersion ensembles. The controllable relative phase in the Ramsey interferometry stems from a gravitational potential difference so that it uniquely evidences coherence between states which involve the whole nano-crystal being in spatially distinct locations.

We propose an interferometric scheme based on an untrapped nano-object subjected to gravity. The motion of the center of mass (c.m.) of the free object is coupled to its internal spin system magnetically, and a free flight scheme is developed based on coherent spin control. The wave packet of the test object, under a spin-dependent force, may then be delocalized to a macroscopic scale. A gravity induced dynamical phase (accrued solely on the spin state, and measured through a Ramsey scheme) is used to reveal the above spatially delocalized superposition of the spin-nano-object composite system that arises during our scheme. We find a remarkable immunity to the motional noise in the c.m. (initially in a thermal state with moderate cooling), and also a dynamical decoupling nature of the scheme itself. Together they secure a high visibility of the resulting Ramsey fringes. The mass independence of our scheme makes it viable for a nano-object selected from an ensemble with a high mass variability. Given these advantages, a quantum superposition with a 100 nm spatial separation for a massive object of 10^{9}  amu is achievable experimentally, providing a route to test postulated modifications of quantum theory such as continuous spontaneous localization.

A prerequisite for exploiting spins for quantum data storage and processing is long spin coherence times. Phosphorus dopants in silicon (Si:P) have been favoured as hosts for such spins because of measured electron spin coherence times (T2) longer than any other electron spin in the solid state: 14 ms at 7 K with isotopically purified silicon. Heavier impurities such as bismuth in silicon (Si:Bi) could be used in conjunction with Si:P for quantum information proposals that require two separately addressable spin species. However, the question of whether the incorporation of the much less soluble Bi into Si leads to defect species that destroy coherence has not been addressed. Here we show that schemes involving Si:Bi are indeed feasible as the electron spin coherence time T2 is at least as long as for Si:P with non-isotopically purified silicon. We polarized the Si:Bi electrons and hyperpolarized the I=9/2 nuclear spin of (209)Bi, manipulating both with pulsed magnetic resonance. The larger nuclear spin means that a Si:Bi dopant provides a 20-dimensional Hilbert space rather than the four-dimensional Hilbert space of an I=1/2 Si:P dopant.

We investigate electron paramagnetic resonance spectra of bismuth-doped silicon, at intermediate magnetic fields B ' 0:1-0:6 T, theoretically and experimentally (with 9.7 GHz X-band spectra). We identify a previously unexplored regime of ''cancellation resonances,'' where a component of the hyperfine coupling is resonant with the external field. We show that this regime has experimentally accessible consequences for quantum information applications, such as reduction of decoherence, fast manipulation of the coupled electron-nuclear qubits, and spectral line narrowing. DOI: 10.1103/PhysRevLett.105.067602 PACS numbers: 76.30.Àv, 03.67.Lx, 71.55.Cn, 76.90.+d Following Kane's suggestion [1] for using phosphorusdoped silicon as a source of qubits for quantum computing, there has been intense interest in such systems [2]. The phosphorus system ( 31 P) is appealing in its simplicity: It represents a simple electron-spin qubit S ¼ 1 2 coupled to a nuclear-spin qubit I ¼ 1 2 via an isotropic hyperfine interaction AI:S of moderate strength ( A 2 ¼ 117:5 MHz). However, recent developments [3-5] point to Si:Bi (bismuth-doped silicon) as a very promising new alternative. Two recent studies measured spin-dephasing times of over 1 ms at 10 K, which is longer than comparable (nonisotopically purified) materials, including Si:P [3,4]. Another group implemented a scheme for rapid (on a time scale of $100 s) and efficient (of order 90%) hyperpolarization of Si:Bi into a single spin state [5].Bismuth has an atypically large hyperfine constant A 2 ¼ 1:4754 GHz and nuclear spin I ¼ 9 2 . This makes its EPR spectra somewhat more complex than for phosphorus, and there is strong mixing of the eigenstates for external field B & 0:6 T. Mixing of Si:P states was studied experimentally in Ref. [6], by means of electrically detected magnetic resonance, but at much lower fields B & 0:02 T. Residual mixing in Si:Bi for B ¼ 2-6 T, where the eigenstates are *99:9% pure uncoupled eigenstates of bothÎ z andŜ z , was also proposed as important for the hyperpolarization mechanism of illuminated Si:Bi [5]. In Ref.[4] it was found that even a $30% reduction in the effective paramagnetic ratio df dB (where f is the transition frequency) leads to a detectable reduction in decoherence rates.Below, we present an analysis of EPR spectra for Si:Bi, testing this against experimental spectra. We identify the points for which df dB ¼ 0, explaining them in a unified manner in terms of a series of EPR ''cancellation resonances''; some are associated with avoided level crossings while others, such as a maximum shown in ENDOR [7] spectra at B % 0:37 T in Ref.[4], are of a quite different origin. These cancellation resonances represent, to the best of our knowledge, an unexplored regime in EPR spectroscopy, arising in systems with exceptionally high A and I. They are somewhat reminiscent of the so-called ''exact cancellation'' regime, widely used in ESEEM spectroscopy [7,8] but differ in essential ways: For instance, they affect both electronic and nuclear freq...