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.

We show that the electron spin phase memory time, the most important property of a molecular nanomagnet from the perspective of quantum information processing, can be improved dramatically by chemically engineering the molecular structure to optimize the environment of the spin. We vary systematically each structural component of the class of antiferromagnetic Cr(7)Ni rings to identify the sources of decoherence. The optimal structure exhibits a phase memory time exceeding 15  μs.

Electron and nuclear spins have good coherence times and an ensemble of spins is a promising candidate for a quantum memory. By employing holographic techniques via field gradients a single ensemble may be used to store many bits of information. Here we present a coherent memory using a pulsed magnetic field gradient, and demonstrate the storage and retrieval of up to 100 weak 10 GHz coherent excitations in collective states of an electron spin ensemble. We further show that such collective excitations in the electron spin can then be stored in nuclear spin states, which offer coherence times in excess of seconds.Instead of storing information in specific locations as in photography and in conventional computer memory, information can be stored in distributed collective modes, as in holography. Advantages include obviating the need for local manipulations and measurements, enhanced coupling to electromagnetic fields, and robustness against decoherence of individual members of the ensemble. This principle has been applied to different light-matter interfaces such as atoms [1][2][3][4], ion-doped crystals [5][6][7][8][9], polar molecules [10-13], or spins [14,15]. Controlled reversible inhomogeneous broadening (CRIB) [8], or gradient echo memory (GEM) [7] schemes which apply external field gradients to address different storage modes have been proposed and observed in gaseous atomic samples [1,2] and in ion-doped solids [6,7].In this Letter, we demonstrate the storage of multiple microwave excitations in an electron spin ensemble. The spin ensembles used as the storage medium are the electron spin of nitrogen atoms in fullerene cages ( 14 N@C 60 ) and phosphorous donors in silicon. The microwave excitations are phase encoded using a static or pulsed field gradient, with the latter allowing for recall in arbitrary order. We have stored up to 100 weak excitations in a spin ensemble and recalled them sequentially. We also demonstrate the coherent transfer of the stored multiple excitations between electron spin and nuclear spin, which will allow much longer storage times [16]. The multimode storage achieved in this way offers prospects of constructing a long-lived quantum memory which could be used for a hybrid quantum computing architecture with superconducting qubits.A quasistatic magnetic field along the z-axis causes the members of the spin ensemble to precess at an angular frequency B(z, t)µg e / , where µ is the Bohr magneton and g e is the electron gyromagnetic ratio. Applying a magnetic gradient of strength G = ∂B(z, t)/∂z for a time τ consequently leads to a difference in precession angle of δθ = (µg e / )Gτ · δz between two spins with separation δz along the z-axis. A gradient pulse thus maps a spin state with a coherent transverse magnetization (such as that generated by a global resonant microwave tipping pulse) to a spin-wave excitation associated with a wave number k = (µg e / )Gτ · z. Each further application of G for duration τ generates a change in the wavevector of the global spin wave mode, by an a...

Donors in silicon hold considerable promise for emerging quantum technologies, due to their uniquely long electron spin coherence times. Bismuth donors in silicon differ from more widely studied group V donors, such as phosphorous, in several significant respects: They have the strongest binding energy (70.98 meV), a large nuclear spin (I=9/2), and a strong hyperfine coupling constant (A=1475.4  MHz). These larger energy scales allow us to perform a detailed test of theoretical models describing the spectral diffusion mechanism that is known to govern the electron spin decoherence of P donors in natural silicon. We report the electron-nuclear double resonance spectra of the Bi donor, across the range 200 MHz to 1.4 GHz, and confirm that coherence transfer is possible between electron and nuclear spin degrees of freedom at these higher frequencies.

Vagus nerve stimulation (VNS) has demonstrated a significant anticonvulsant effect in preclinical studies, in pilot studies in humans, and in the acute phase of a multicenter, double-blinded, randomized study. After completion of a 14-week, blinded, randomized study, with 31 receiving high (therapeutic) VNS and 36 receiving low (less or noneffective) VNS, 67 patients elected to continue in an open extension phase. During the extension phase, all 67 patients received high VNS. Seizure frequency during the 3-month treatment blocks was compared with a 12-week baseline. For both groups, all periods of high VNS demonstrated a significant decrease in seizure frequency (p < 0.01 level) as compared with baseline. For the 16-18-month period of VNS, data were available for 26 of the 31 patients randomized to high VNS. This group achieved a 52.0% mean seizure frequency percentage reduction as compared with baseline. For those converted from low to high VNS, data were available for 24 of the 36 patients at the 16-18-month time period. This group reported a mean seizure frequency percentage reduction of 38.1% as compared with baseline. No significant change in the safety/side effect profile was reported during long-term follow-up. The previously reported side effects of hoarseness/voice change, coughing, and paresthesia (sensation in neck and jaw) continued to occur during VNS. These side effects were well tolerated. During the follow-up period, 1 patient died of thrombotic thrombocytopenic purpura (TTP) and 5 patients discontinued treatment because of unsatisfactory efficacy.

One of the most striking features of quantum mechanics is the profound effect exerted by measurements alone. Sophisticated quantum control is now available in several experimental systems, exposing discrepancies between quantum and classical mechanics whenever measurement induces disturbance of the interrogated system. In practice, such discrepancies may frequently be explained as the back-action required by quantum mechanics adding quantum noise to a classical signal. Here, we implement the "three-box" quantum game [Aharonov Y, et al. (1991) J Phys A Math Gen 24(10):2315-2328] by using state-of-the-art control and measurement of the nitrogen vacancy center in diamond. In this protocol, the back-action of quantum measurements adds no detectable disturbance to the classical description of the game. Quantum and classical mechanics then make contradictory predictions for the same experimental procedure; however, classical observers are unable to invoke measurement-induced disturbance to explain the discrepancy. We quantify the residual disturbance of our measurements and obtain data that rule out any classical model by T7.8 standard deviations, allowing us to exclude the property of macroscopic state definiteness from our system. Our experiment is then equivalent to the test of quantum noncontextuality [Kochen S, Specker E (1967) J Math Mech 17(1):59-87] that successfully addresses the measurement detectability loophole.Leggett-Garg | quantum contextuality | quantum non-demolition measurement C lassical physics describes the nature of systems that are "large" enough to be considered as occupying one definite state in an available state space at any given time. Macrorealism (MR) applies whenever it is possible to perform nondisturbing measurements that identify this state without significantly modifying the system's subsequent behavior (1). MR allows the assignment of a definite history (or probabilities over histories) to classical systems of interest, but the MR condition can break down for systems "small" enough to be quantum mechanical during times "short" enough to be quantum coherent: times and distances that now exceed seconds (2) and millimeters (3) in the solid state. How can we tell whether a particular case is better described by quantum mechanics (QM) or MR? If there is a crossover between these, what does it represent?One explanation for the breakdown of MR is that measurement back-action (either deliberate measurements by an experimenter or effective measurements from the environment) unavoidably change the state in the quantum limit, excluding MR due to a breakdown of nondisturbing measurability. This position is supported by "weak value" experiments (4, 5) that explore the transition from quantum to classical behavior as a measurement coupling is varied. Quantum behavior is found under weak coupling, whereas MR-compatible behavior is recovered when strong projective measurements effectively "impose" a classical value onto the measured quantum system (4).We examine a case in which the back-actions of ...

Cooling nanoelectronic structures to millikelvin temperatures presents extreme challenges in maintaining thermal contact between the electrons in the device and an external cold bath. It is typically found that when nanoscale devices are cooled to ∼10 mK the electrons are significantly overheated. Here we report the cooling of electrons in nanoelectronic Coulomb blockade thermometers below 4 mK. The low operating temperature is attributed to an optimized design that incorporates cooling fins with a high electron–phonon coupling and on-chip electronic filters, combined with low-noise electronic measurements. By immersing a Coulomb blockade thermometer in the 3He/4He refrigerant of a dilution refrigerator, we measure a lowest electron temperature of 3.7 mK and a trend to a saturated electron temperature approaching 3 mK. This work demonstrates how nanoelectronic samples can be cooled further into the low-millikelvin range.

High-spin paramagnetic manganese defects in polar piezoelectric zinc oxide exhibit a simple, almost axial anisotropy and phase coherence times of the order of a millisecond at low temperatures. The anisotropy energy is tunable using an externally applied electric field. This can be used to control electrically the phase of spin superpositions and to drive spin transitions with resonant microwave electric fields. DOI: 10.1103/PhysRevLett.110.027601 PACS numbers: 76.30.Fc, 76.60.Lz, 77.65.Àj Couplings between magnetic and electric degrees of freedom give rise to such fundamental phenomena in condensed matter as multiferroelectricity [1,2], unconventional superconductivity [3], and spin-density waves [4], and are key to proposed future technologies such as high sensitivity metrology [5] and quantum-dot-based quantum information processing [6]. The control of spin states using electric fields rather than magnetic fields is particularly valuable for quantum information processing [7], because electric fields can be applied on short length scales, and down to the scale of qubit separations in a device [8][9][10]. In this Letter, we identify a class of electrically controllable spin qubits: high-spin magnetic defects in polar semiconductor hosts. In one member of this class, the manganese substitutional defect in a crystalline zinc oxide host, we demonstrate coherent oscillations of the spin state driven by dc electric field pulses and spin transitions driven by resonant microwave electric fields.Electrical control has been considered in various candidate physical systems for spin qubits, both experimentally and theoretically. In lithographically defined GaAs quantum dots, spin-orbit coupling permits coherent control of an electron spin by applying a resonant high frequency voltage to one of the confining gates [11]. Stark shifts in spin splittings, originating from hybridization with excited state orbitals, have been observed in self-assembled quantum dots [12] and in bound donors in semiconductors [13,14] and in defects in diamond [15], while frustrated spin triangles exhibit spin-electric couplings via modulation of exchange interactions [16,17]. Here, our approach is to manipulate the electrostatic environment of a high-spin paramagnetic defect, manganese, in a polar piezoelectric host material, zinc oxide, thereby controlling electrically the spin Hamiltonian of the defect [18].The spin Hamiltonian of manganese defects in zinc oxide (ZnO:Mn) is well known from continuous-wave ESR [19]. The polar environment lifts the spin degeneracy of the S ¼ 5=2 Mn 2þ ions which are substitutional on Zn 2þ sites, while the I ¼ 5=2 nuclear spin of 55 Mn is coupled to the electron spin by the hyperfine interaction.The Hamiltonian, including a magnetic field which lifts the remaining Kramer's degeneracies, iswhere the axial anisotropy, or zero-field splitting (ZFS), termHigher order effects, such as fourth order electron spin anisotropy and nuclear quadrupole terms, have been detected [19], but they are small and not relevant f...