Charles D. Hill

Sensing of fluctuating nanoscale magnetic fields using nitrogen-vacancy centers in diamond

New magnetometry techniques based on nitrogen-vacancy (NV) defects in diamond allow for the detection of static (dc) and oscillatory (ac) nanoscopic magnetic fields, yet are limited in their ability to detect fields arising from randomly fluctuating (FC) environments. We show here that FC fields restrict dc and ac sensitivities and that probing the NV dephasing rate in a FC environment should permit the characterization of FC fields inaccessible to dc and ac techniques. FC sensitivities are shown to be comparable to those of ac magnetometry and require no additional experimental overhead or sample control.

Monitoring ion-channel function in real time through quantum decoherence

In drug discovery, there is a clear and urgent need for detection of cell-membrane ion-channel operation with wide-field capability. Existing techniques are generally invasive or require specialized nanostructures. We show that quantum nanotechnology could provide a solution. The nitrogen-vacancy (NV) center in nanodiamond is of great interest as a single-atom quantum probe for nanoscale processes. However, until now nothing was known about the quantum behavior of a NV probe in a complex biological environment. We explore the quantum dynamics of a NV probe in proximity to the ion channel, lipid bilayer, and surrounding aqueous environment. Our theoretical results indicate that real-time detection of ion-channel operation at millisecond resolution is possible by directly monitoring the quantum decoherence of the NV probe. With the potential to scan and scale up to an array-based system, this conclusion may have wide-ranging implications for nanoscale biology and drug discovery.

A surface code quantum computer in silicon

A scalable shared-control architecture for silicon-based quantum computing using topological quantum error correction. The exceptionally long quantum coherence times of phosphorus donor nuclear spin qubits in silicon, coupled with the proven scalability of silicon-based nano-electronics, make them attractive candidates for large-scale quantum computing. However, the high threshold of topological quantum error correction can only be captured in a two-dimensional array of qubits operating synchronously and in parallel—posing formidable fabrication and control challenges. We present an architecture that addresses these problems through a novel shared-control paradigm that is particularly suited to the natural uniformity of the phosphorus donor nuclear spin qubit states and electronic confinement. The architecture comprises a two-dimensional lattice of donor qubits sandwiched between two vertically separated control layers forming a mutually perpendicular crisscross gate array. Shared-control lines facilitate loading/unloading of single electrons to specific donors, thereby activating multiple qubits in parallel across the array on which the required operations for surface code quantum error correction are carried out by global spin control. The complexities of independent qubit control, wave function engineering, and ad hoc quantum interconnects are explicitly avoided. With many of the basic elements of fabrication and control based on demonstrated techniques and with simulated quantum operation below the surface code error threshold, the architecture represents a new pathway for large-scale quantum information processing in silicon and potentially in other qubit systems where uniformity can be exploited.

Global control and fast solid-state donor electron spin quantum computing

We propose a scheme for quantum information processing based on donor electron spins in semiconductors, with an architecture complementary to the original Kane proposal. We show that a naive implementation of electron spin qubits provides only modest improvement over the Kane scheme, however through the introduction of global gate control we are able to take full advantage of the fast electron evolution timescales. We estimate that the latent clock speed is 100-1000 times that of the nuclear spin quantum computer with the ratio T-2/T-ops approaching the 10(6) level.

Surface code quantum communication

Quantum communication typically involves a linear chain of repeater stations, each capable of reliable local quantum computation and connected to their nearest neighbors by unreliable communication links. The communication rate of existing protocols is low as two-way classical communication is used. By using a surface code across the repeater chain and generating Bell pairs between neighboring stations with probability of heralded success greater than 0.65 and fidelity greater than 0.96, we show that two-way communication can be avoided and quantum information can be sent over arbitrary distances with arbitrarily low error at a rate limited only by the local gate speed. This is achieved by using the unreliable Bell pairs to measure nonlocal stabilizers and feeding heralded failure information into post-transmission error correction. Our scheme also applies when the probability of heralded success is arbitrarily low.

Ambient nanoscale sensing with single spins using quantum decoherence

Magnetic resonance detection is one of the most important tools used in life-sciences today. However, as the technique detects the magnetization of large ensembles of spins it is fundamentally limited in spatial resolution to mesoscopic scales. Here we detect the natural fluctuations of nanoscale spin ensembles at ambient temperatures by measuring the decoherence rate of a single quantum spin in response to introduced extrinsic target spins. In our experiments 45?nm nanodiamonds with single nitrogen?vacancy (NV) spins were immersed in solution containing spin 5/2 Mn2+ ions and the NV decoherence rate measured though optically detected magnetic resonance. The presence of both freely moving and accreted Mn spins in solution were detected via significant changes in measured NV decoherence rates. Analysis of the data using a quantum cluster expansion treatment of the NV-target system found the measurements to be consistent with the detection of ?2500 motionally diffusing Mn spins over an effective volume of (16?nm)3 in 4.2?s, representing a reduction in target ensemble size and acquisition time of several orders of magnitude over conventional, magnetic induction approaches to electron spin resonance detection. These measurements provide the basis for the detection of nanovolume spins in solution, such as in the internal compartments of living cells, and are directly applicable to scanning probe architectures.

Quantum-error correction on linear-nearest-neighbor qubit arrays

A quantum circuit implementing 5-qubit quantum-error correction on a linear-nearest-neighbor architecture is described. The canonical decomposition is used to construct fast and simple gates that incorporate the necessary swap operations allowing the circuit to achieve the same depth as the current least depth circuit. Simulations of the circuits performance when subjected to discrete and continuous errors are presented. The relationship between the error rate of a physical qubit and that of a logical qubit is investigated with emphasis on determining the concatenated error correction threshold.

Ultrasensitive diamond magnetometry using optimal dynamic decoupling

Magnetometry techniques based on nitrogen-vacancy (NV) defects in diamond have received much attention of late as a means to probe nanoscale magnetic environments. The sensitivity of a single NV magnetometer is primarily determined by the transverse spin-relaxation time, T(2). Current approaches to improving the sensitivity employ crystals with a high NV density at the cost of spatial resolution or extend T(2) via the manufacture of novel isotopically pure diamond crystals. We adopt a complementary approach in which optimal dynamic decoupling techniques extend coherence times out to the self-correlation time of the spin bath. This suggests single spin, room-temperature magnetometer sensitivities as low as 5 pT Hz-(1/2) may be possible with current technology.

Robust controlled-NOT gates from almost any interaction

There are many cases where the interaction between two qubits is not precisely known, but single-qubit operations are available. In this Letter we show how, regardless of an incomplete knowledge of the strength or form of the interaction between two qubits, it is often possible to construct a controlled-NOT gate which has arbitrarily high fidelity. In particular, we show that oscillations in the strength of the exchange interaction in solid state Si and Ge structures are correctable.

Towards single-molecule NMR detection and spectroscopy using single spins in diamond

Magnetic resonance (MR) based detection and imaging is an important tool across many areas of nanoscience. From a bio-medical perspective, the need to better understand cellular processes at the nanoscale, occurring both naturally and as a result of introduced nanoparticles and/or molecular species, poses a significant and a constant question. The long tradition of magnetometry techniques in bio-imaging, such as in electron spin resonance (ESR), nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI), have been successful in detecting the bulk properties of cells and their reactions [1, 2]. However, all of these methods rely on detecting very large numbers of electronic or nuclear spins and hence are limited fundamentally in their resolution. New methods such as magnetic resonance force microscopy (MRFM) are capable of detecting single electron or nuclear spins, yet have the additional requirements of vacuum conditions and low temperatures (< 2K) [3–5]. We focus on how to perform NMR for the task of detecting individual molecules under ambient conditions. The ability to selectively detect molecular species has important implications in a range of fields, including nanomedicine. The MR detector we consider is the nitrogen-vacancy (NV) centre in diamond, in which a number of fortuitous properties converge making it a very promising sensor: it is bio-compatible, exhibits sustained fluorescence over arbitrarily long timescales, and is inherently a nanoscale magnetic sensor with high sensitivity. Recent experiments demonstrate that this centre in diamond is capable of identifying the presence of relatively modest number of nuclear spins (between 10 4 and 10 6 actual protons) external to the diamond lattice under controlled conditions, both by passive observation [6] and by manipulation of proton spins states [7]. The NV centre is a spin 1 system which ground state can be optically initialised and read out [8–12]. The coherence of a centre