The field of superconducting quantum computing has made sufficient progress that we can begin to investigate challenges in scaling quantum processors to systems with larger numbers of qubits. Our group studies circuit design for extended coherence, develops methods for verification and validation of quantum states and processes, and builds custom room temperture and cryogenic control hardware.
Our group is spearheading work on evaluating the fundamental limits to the efficiency with which information--either classical, quantum, or a combination thereof--can be encoded in the photon and sent reliably over an optical channel (free-space or fiber), and finding explicit means--codes, transmitter modulation, joint-detection measurements, optical realizations of receivers to implement such measurements, and decoders--to attain performance that is close to the quantum-information-theoretic limits.
We are investigating both fundamental quantum limits to imaging and sensing and how quantum effects can enhance the precision of such measurements. Among our recent accomplishments are the quantification and modeling of enhancements to remote optical sensing (e.g. LADAR) using phase-sensitive amplfication to overcome detection inefficiencies, proposal of an optical parametric amplfier (OPA) based receiver to realize the target detection advantage of quantum illumination (entangled state interrogation) in high noise, high loss environements, and the W-state transmitter to greatly enhance the photon efficiency of particular imaging tasks.
High-speed and low power memories for cryogenic computing systems
We are exploring new memory technologies for supercondcuting digital logic and well as for quantum computing. Recently we started investigating integration of spintronics with superconducting circuits. Spin Torque Transfer MRAM can operate at superconducting circuit temperatures while providing high-density, fast and low power RAM located near cryogenic processors.
Quantum optics with microwave photons
Our group was the first to experimentally demonstrate Coherent Population Trapping (CPT) in superconducting quantum circuits. This advance demsontrated the underlying effect associated with electromagnetically induced transparency (EIT) and slow light in a new medium and in a new frequecny regime (RF as opposed to optical).
Resource estimation for fault-tolerant quantum computing
We are developing tools and methods that integrate all hardware and software components of the quantum computer in a common framework. As part of this work, we have focused on developing representations and computational models of the quantum processing domain, as well as methods to estimate the resources required to solve real-world problems on processing architectures of matching scale. These resource and performance estimates include all levels of the computational architecture, such as machine control, fault tolerance, and communications. Eventually, these tools will enable researchers to explore the comparative merits of different solutions by comparing tradeoffs across computational processing techniques, such as fault tolerant protocols, control protocols, and compiler optimization.
Quantum Key Distribution (QKD) and Quantum Communication
Quantum communication is the transmission of quantum bits (or qubits) over large distances via optical photons. One of the most mature applications of quantum information is the use of quantum communication to perform quantum key distribution (QKD), in which secret key with physically based security can be exchanged between parties over remote optical links. BBN pioneered work in demonstrating this over metropolitan scale (see below). More generally quantum communication can be used for sophisticated protocols which physically guarantee privacy of information from eavesdroppers and other third parties as well as enable distributed quantum computing among distant processors. We are currently working on technologies and methods to extend the maximum distance of quantum communication and generalize it beyond simple "point-to-point" links to large, scalable quantum networks.
The DARPA Quantum Network - World's First Quantum Cryptographic Network
Under DARPA sponsorship, and together with our academic colleagues Harvard University and Boston University, BBN Technologies built and begun and operated the world's first Quantum Key Distribution (QKD) network. The DARPA Quantum Network employed 24x7 quantum cryptography to provide unprecedented levels of security for standard Internet traffic flows such as web-browsing, e-commerce, and streaming video.
The DARPA Quantum Network became fully operational on October 23, 2003 in BBN's laboratories, and has run continuously since. It currently consists of two BBN-built, interoperable weak-coherent QKD systems running at a 5 MHz pulse rate (0.1 mean photons per pulse) through telecommunications fiber, and inter-connected via a photonic switch, together with a full suite of production-quality QKD protocols. In the near future, we plan to roll out this network into dark fiber between our campuses through the Cambridge, Massachusetts metropolitan area, introduce a series of new quantum cryptographic links based on a variety of physical phenomena, and start testing the resulting network against sophisticated attacks.
The principles underlying a quantum cryptographic network have already been proven on a limited scale. Using lasers and photo detectors, light is sent, in a manner in which eavesdropping is always detectable, through either fiber optic cable or the atmosphere to distribute cryptographic keys that are used to scramble (encrypt) and de-scramble (decrypt) a message. The DARPA Quantum Network has improved upon these techniques to create an extremely secure, highly robust network protected by quantum cryptography. This secure network technology is 100% compatible with conventional Internet technology.