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239
H ∞ control of linear quantum stochastic systems
 IEEE Trans. Automat. Contr
, 2008
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Feedback Control of Quantum State Reduction
, 2004
"... Feedback control of quantum mechanical systems must take into account the probabilistic nature of quantum measurement. We formulate quantum feedback control as a problem of stochastic nonlinear control by considering separately a quantum filtering problem and a state feedback control problem for th ..."
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Cited by 72 (6 self)
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Feedback control of quantum mechanical systems must take into account the probabilistic nature of quantum measurement. We formulate quantum feedback control as a problem of stochastic nonlinear control by considering separately a quantum filtering problem and a state feedback control problem for the filter. We explore the use of stochastic Lyapunov techniques for the design of feedback controllers for quantum spin systems and demonstrate the possibility of stabilizing one outcome of a quantum measurement with unit probability.
Indistinguishable photons from a singlephoton device
 Nature
, 2002
"... Singlephoton sources have recently been demonstrated using a variety of devices, including molecules 13 , mesoscopic quantum wells 4 , colour centres 5 , trapped ions 6 and semiconductor quantum dots When identical single photons enter a 5050 beam splitter from opposite sides, quantum mechanics ..."
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Cited by 49 (4 self)
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Singlephoton sources have recently been demonstrated using a variety of devices, including molecules 13 , mesoscopic quantum wells 4 , colour centres 5 , trapped ions 6 and semiconductor quantum dots When identical single photons enter a 5050 beam splitter from opposite sides, quantum mechanics predicts that both photons must leave in the same direction, if their wave packets overlap perfectly. This twophoton interference effect originates from the BoseEinstein statistics of photons. This bunching effect was first observed using pairs of highly correlated photons produced by parametric downcoversion 14 , but it should also occur with single, independently generated photons. Most proposed applications for singlephoton sources in the field of quantum information (with the notable exception of quantum cryptography 15 ) involve twophoton interference. Such applications include quantum teleportation 16 , postselective production of polarizationentangled photons 17 , and linearoptics quantum computation 12 . It is therefore important to demonstrate that consecutive photons emitted by a singlephoton source are identical and exhibit mutual twophoton interference effects. The experiment described here used a semiconductor quantum dot as the photon source. Quantum dots are attractive as singlephoton sources because they are relatively stable, have narrow spectral linewidths and rapid radiative decay rates, and can be integrated into larger fabricated structuressuch as microcavitiesto improve the collection efficiency Our sample contains selfassembled InAs quantum dots (about 25 mm 22 ) embedded in GaAs and sandwiched between distributedBraggreflector (DBR) mirrors, grown by molecularbeam epitaxy By this method, we obtained bright, singlephoton sources with excellent twophoton suppression and negligible background emission. We have chosen three quantum dots for this study, denoted as dots 1, 2 and 3, with emission wavelengths (in nm) of 931, 932 and 937, respectively. A photoncorrelation measurement for dot 2 is shown in
Coherent quantum LQG control
, 2007
"... Based on a recently developed notion of physical realizability for quantum linear stochastic systems, we formulate a quantum LQG optimal control problem for quantum linear stochastic systems where the controller itself may also be a quantum system and the plant output signal can be fully quantum. Su ..."
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Cited by 42 (24 self)
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Based on a recently developed notion of physical realizability for quantum linear stochastic systems, we formulate a quantum LQG optimal control problem for quantum linear stochastic systems where the controller itself may also be a quantum system and the plant output signal can be fully quantum. Such a control scheme is often referred to in the quantum control literature as “coherent control. ” It distinguishes the present work from previous works on the quantum LQG problem where measurement is performed on the plant and the measurement signals are used as input to a fully classical controller with no quantum degrees of freedom. The difference in our formulation is the presence of additional nonlinear and linear constraints on the coefficients of the sought after controller, rendering the problem as a type of constrained controller design problem. Due to the presence of these constraints our problem is inherently computationally hard and this also distinguishes it in an important way from the standard LQG problem. We propose a numerical procedure for solving this problem based on an alternating projections algorithm and, as initial demonstration of the feasibility of this approach, we provide fully quantum controller design examples in which numerical solutions to the problem were successfully obtained. For comparison, we also consider the case of classical controllers which use direct or indirect measurements, and show that fully quantum controllers can offer an improvement in performance over the classical ones.
Quantum data hiding
 IEEE Trans. Inf. Theory
"... Abstract — We expand on our work on Quantum Data Hiding [1] – hiding classical data among parties who are restricted to performing only local quantum operations and classical communication (LOCC). We review our scheme that hides one bit between two parties using Bell states, and we derive upper and ..."
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Cited by 38 (3 self)
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Abstract — We expand on our work on Quantum Data Hiding [1] – hiding classical data among parties who are restricted to performing only local quantum operations and classical communication (LOCC). We review our scheme that hides one bit between two parties using Bell states, and we derive upper and lower bounds on the secrecy of the hiding scheme. We provide an explicit bound showing that multiple bits can be hidden bitwise with our scheme. We give a preparation of the hiding states as an efficient quantum computation that uses at most one ebit of entanglement. A candidate data hiding scheme that does not use entanglement is presented. We show how our scheme for quantum data hiding can be used in a conditionally secure quantum bit commitment scheme.
The Computational Complexity of Linear Optics
 in Proceedings of STOC 2011
"... We give new evidence that quantum computers—moreover, rudimentary quantumcomputers built entirely out of linearoptical elements—cannotbeefficientlysimulatedbyclassical computers. In particular, we define a model of computation in which identical photons are generated, sent through a linearoptical n ..."
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Cited by 33 (8 self)
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We give new evidence that quantum computers—moreover, rudimentary quantumcomputers built entirely out of linearoptical elements—cannotbeefficientlysimulatedbyclassical computers. In particular, we define a model of computation in which identical photons are generated, sent through a linearoptical network, then nonadaptively measured to count the number of photons in each mode. This model is not known or believed to be universal for quantum computation, and indeed, we discuss the prospects for realizing the model using current technology. On the other hand, we prove that the model is able to solve sampling problems and search problems that are classically intractable under plausible assumptions. Our first result says that, if there exists a polynomialtime classical algorithm that samples from the same probability distribution as a linearoptical network, then P #P = BPP NP, and hence the polynomial hierarchy collapses to the third level. Unfortunately, this result assumes an extremely accurate simulation. Our main result suggests that even an approximate or noisy classical simulation would already imply a collapse of the polynomial hierarchy. For this, we need two unproven conjectures: the PermanentofGaussians Conjecture, which says that it is #Phard to approximate the permanent of a matrixAofindependentN (0,1)Gaussianentries, withhigh probability over A; and the Permanent AntiConcentration Conjecture, which says that Per(A)  ≥ √ n!/poly(n) with high probability over A. We present evidence for these conjectures, both of which seem interesting even apart from our application. For the 96page full version, see www.scottaaronson.com/papers/optics.pdf
Architectural implications of quantum computing technologies
 ACM Journal on Emerging Technologies in Computing Systems (JETC
, 2006
"... In this article we present a classification scheme for quantum computing technologies that is based on the characteristics most relevant to computer systems architecture. The engineering tradeoffs of execution speed, decoherence of the quantum states, and size of systems are described. Concurrency, ..."
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Cited by 27 (4 self)
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In this article we present a classification scheme for quantum computing technologies that is based on the characteristics most relevant to computer systems architecture. The engineering tradeoffs of execution speed, decoherence of the quantum states, and size of systems are described. Concurrency, storage capacity, and interconnection network topology influence algorithmic efficiency, while quantum error correction and necessary quantum state measurement are the ultimate drivers of logical clock speed. We discuss several proposed technologies. Finally, we use our taxonomy to explore architectural implications for common arithmetic circuits, examine the implementation of quantum error correction, and discuss clusterstate quantum computation.
Classical simulation of noninteractingfermion quantum circuits
 Phys. Rev. A
"... We show that a class of quantum computations that was recently shown to be efficiently simulatable on a classical computer by Valiant [1] corresponds to a physical model of noninteracting fermions in one dimension. We give an alternative proof of his result using the language of fermions and extend ..."
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Cited by 26 (2 self)
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We show that a class of quantum computations that was recently shown to be efficiently simulatable on a classical computer by Valiant [1] corresponds to a physical model of noninteracting fermions in one dimension. We give an alternative proof of his result using the language of fermions and extend the result to noninteracting fermions with arbitrary pairwise interactions, where gates can be conditioned on outcomes of complete von Neumann measurements in the computational basis on other fermionic modes in the circuit. This last result is in remarkable contrast with the case of noninteracting bosons where universal quantum computation can be achieved by allowing gates to be conditioned on classical bits [2].
Synthesis of Reversible Sequential Elements*
"... Abstract – To construct a reversible sequential circuit, reversible sequential elements are required. This work presents novel designs of reversible sequential elements such as D latch, JK latch, and T latch. Based on these reversible latches, we also construct the designs of the corresponding flip ..."
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Cited by 17 (0 self)
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Abstract – To construct a reversible sequential circuit, reversible sequential elements are required. This work presents novel designs of reversible sequential elements such as D latch, JK latch, and T latch. Based on these reversible latches, we also construct the designs of the corresponding flipflops. Comparing with previous work, the implementation cost of our new designs, including the number of gates and the number of garbage outputs is considerably reduced. I.