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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Room-temperature solid-state quantum processors in diamond

Philip Hemmer, Mikhail D. Lukin

Published Mar 26, 2008• 2 citations
Publisher

Abstract

We are developing solid-state quantum repeaters for long-distance, secure quantum communication networks. These repeaters will combine long-range quantum entanglement, produced by optical measurements, with local few-qubit quantum processing and storage nodes. So far we have demonstrated all the key elements of the local quantum processors using nitrogen-vacancy (NV) color centers in diamond. These include multi-qubit entanglement, long storage times, and robustness against multiple re-initialization steps. Significantly, all these local operations were demonstrated at room temperature. Finally, we estimate that single shot readout should be achievable with optical plasmon wire coupling which would open the door to room-temperature few-qubit quantum computers.

DOI
10.1117/12.778016

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We are developing solid-state quantum repeaters for long-distance, secure quantum communication networks. These repeaters will combine long-range quantum entanglement, produced by optical measurements, with local few-qubit quantum processing and storage nodes. So far we have demonstrated all the key elements of the local quantum processors using nitrogen-vacancy (NV) color centers in diamond. These include multi-qubit entanglement, long storage times, and robustness against multiple re-initialization steps. Significantly, all these local operations were demonstrated at room temperature. Finally, we estimate that single shot readout should be achievable with optical plasmon wire coupling which would open the door to room-temperature few-qubit quantum computers.
Recent progress toward the development of scalable quantum computers based on nitrogen-vacancy (NV) color centers in diamond will be described. Scaling is accomplished through the long-range entanglement of few-qubit processing nodes using photons. Local operations within each processing node will be accomplished using electronically switchable dipole-dipole interactions. Significant progress has been made in the control of the optical transitions, enabling us to reach the level required to attempt long-range entanglement. In the meantime, long-term storage and two-qubit operations have been demonstrated using magnetic dipole-dipole coupling to proximal spins that are not nearest neighbors. Significantly, all the processing node demonstration were been done at room temperature where spin lifetimes were found to be exceptionally long.
The purpose of this project was to develop quantum computing hardware suitable for implementing quantum lattice-gas algorithms, with eventual application to turbulent flow simulations. A number of quantum systems were investigated, including rare-earth dopants in oxide crystals and in II-VI semiconductors. However, in the option year it was decided to concentrate exclusively on nitrogen-vacancy (NV) color centers in diamond. Surprising, it was found that NV diamond exhibited the key elements needed to develop few-qubit room-temperature solid-state quantum processing nodes. This included the ability to optically initialize and readout the NV electron spin state, fractional millisecond lifetimes for electron spins, and few-nanosecond electron spin Rabi flops. We also demonstrated the relatively long distance (few nanometer) coupling of a single NV spin to the electron spin of a single substitutional nitrogen (N). To achieve long range optical interconnections and entanglement between nodes, cryogenic cooling will likely still be required to sufficiently narrow the optical absorption lines. To this end we located diamond samples with unusually high purity, and found NV centers in these crystals that exhibited exceptionally narrowband and stable optical lines. We also demonstrated electric field tuning of the optical transition frequency as required for controlling atom-atom and atom-cavity coupling.
The complex environment of solid-state quantum bits is generally believed to pose a difficult challenge for solid state realizations of quantum information science. Close study of single electronic spins associated with the nitrogen-vacancy (NV) center in diamond has shown how a complex environment -- in this case a spin-bath formed by carbon-13 isotopic impurities -- may be understood and even controlled. Specifically, we show that optical and microwave excitation of the NV center can be used to perform robust initialization, manipulation, and measurement of the two-qubit register formed by the electronic spin and its nearest-neighbor carbon-13 nuclear spin. These few-qubit registers may be used as a basis for scalable quantum information systems provided that one can identify a means to connect different registers. For an optically active system such as the NV center, photons provide a natural medium for inter- register communication, and we discuss initial experiments studying the possibility of quantum optical interconnects. Such an optically-connected few-qubit register offers a flexible hardware for both quantum computation and long-distance quantum communication; in particular, it may allow realization of a solid-state quantum repeater.
Artificial atoms like the nitrogen vacancy (NV) centers in diamond enable the realization of fully functional qubits in a solid at room temperature. The functionalities of all the parts needed to create a quantum computer, such as quantum error correction, couplings, quantum teleportation, and a quantum repeater, have already been experimentally demonstrated. These achievements are expected to influence the industrial development of quantum information technology as well as quantum sensing. Whereas quantum sensing has been established and a large number of organizations are working on new developments in this area, a quantum computer itself remains elusive due to technical reasons and limitations of the available materials. For example, only in recent months has it become possible to electrically readout the NV spin state at the level of a single center and significantly improve the scalability of NV center production. A number of ideas have been proposed to overcome the above-mentioned limitations. This paper summarizes the status of research in the area, details the most promising concepts for development, and discusses factors limiting progress as well as the most recent developments in the field.
This work investigates the applicability of the nitrogen-vacancy (NV) defect center in diamond for quantum computation and metrology purposes. The NV center is a molecule like defect in the diamond lattice which is optically active and possesses an electron spin in its ground state. The spin can be used as a quantum bit and as a field sensor. The NV center has three major properties enabling individual control of single defects. Its optical properties depend on the spin state, the spin can be polarized optically and the spin state has an exceptionally long lifetime. Single NV centers can be controlled by optical and microwave radiation. Generally, the research on individually controllable quantum systems is not only motivated by quantum computation and communication. With the ongoing miniaturization nowadays electronic and electro-optic devices will soon reach the limit of single quantum building blocks. While in these applications the full complexity of single quantum systems might not be needed, small sensors based on single atoms or molecules might very well take advantage of the susceptibility of quantum coherences to environmental conditions. Major challenges for all approaches towards single quantum system control are the individual addressing, the measurement of the quantum state and the protection of quantum coherence from environmental noise. Today, there is a variety of systems (including the NV center) that allow control on the single quantum level. These are for example single photons, single ions and atoms in traps, single spins in semiconductors or single molecules in solid state systems. Rather mesoscopic examples are superconducting devices, nanomechanical oscillators or surface plasmon polaritons. Regarding the NV center it has been shown that the magnetic moment of its electron spin allows individual sensing and coupling to other surrounding electron and nuclear spins. Especially single nuclear spins were shown to be exceptional quantum memories. In this work small quantum registers of up to 5 spins have been used to test major requirements for a feasible quantum processor. Therefore, mainly artificially created high purity diamond samples with controlled spin and impurity concentration were used. The NV centers were either grown in or created via ion implantation techniques in a controlled fashion. As a first requirement for quantum computation entanglement among up to three spins has been generated and was verified by quantum state tomography. This proves the access of the full Hilbert space. Especially, entanglement among nuclear spins was very long-living. In further experiments this processor was used for first quantum algorithms like super-dense coding and the Deutsch algorithm. In addition, this work demonstrates new methods to initialize nuclear spin qubits into a desired quantum state. This is particularly important as improper initialization inhibits quantum computation. One of these methods is related to dynamic nuclear spin polarization and the other to measurement based state preparation. Nuclear spins in the diamond lattice originate mainly from the C-13 isotope. By artificially altering its content the influence on the NV center spin was investigated. The coherence times of the NV center were increasing inversely with the nuclear spin density. Even increasing nuclear spin density to offer more nuclear spin qubits to an operational quantum register was possible. As an alternative route of scaling up the quantum register pairs of closely spaced NV centers were investigated. For the first time two qubit gates among two NV centers mediated by magnetic dipolar coupling have been realized. Furthermore the sensing ability was demonstrated by measuring the relative distance of the two centers (10 nm) with sub-nm precision. Eventually, this work contains new insight into the electronically excited state of the NV center which allowed a more advanced control of the electron and nuclear spins. A comparably high magnetic field was used to decouple electron and nuclear spins. Finally, this allowed quantum nondemolition measurements on single nuclear spins which manifests in the observation of quantum jumps. The new insight was used to drastically improve the performance of quantum state readout. Summarizing, it was shown that single NV centers and proximal nuclear spins form versatile small quantum registers which can be interconnected with neighboring such NV quantum registers. Although the number of qubits used in this work is still small the experimental results propose a way of how to proceed when building a quantum computer. Regarding quantum metrology, however, the use of such a small register which can sense fields and store and process the results would already outperform a single NV electron spin by orders of magnitude.
Abstract Nuclear spins in semiconductors are leading candidates for future quantum technologies, including quantum computation, communication, and sensing. Nuclear spins in diamond are particularly attractive due to their long coherence time. With the nitrogen-vacancy (NV) centre, such nuclear qubits benefit from an auxiliary electronic qubit, which, at cryogenic temperatures, enables probabilistic entanglement mediated optically by photonic links. Here, we demonstrate a concept of a microelectronic quantum device at ambient conditions using diamond as wide bandgap semiconductor. The basic quantum processor unit – a single 14 N nuclear spin coupled to the NV electron – is read photoelectrically and thus operates in a manner compatible with nanoscale electronics. The underlying theory provides the key ingredients for photoelectric quantum gate operations and readout of nuclear qubit registers. This demonstration is, therefore, a step towards diamond quantum devices with a readout area limited by inter-electrode distance rather than by the diffraction limit. Such scalability could enable the development of electronic quantum processors based on the dipolar interaction of spin-qubits placed at nanoscopic proximity.