Quantum computer architectures impose restrictions on qubit interactions. We propose efficient circuit transformations that modify a given quantum circuit to fit an architecture, allowing for any initial and final mapping of circuit qubits to architecture qubits. To achieve this, we first consider the qubit movement subproblem and use the routing via matchings framework to prove tighter bounds on parallel routing. In practice, we only need to perform partial permutations, so we generalize routing via matchings to that setting. We give new routing procedures for common architecture graphs and for the generalized hierarchical product of graphs, which produces subgraphs of the Cartesian product. Secondly, for serial routing, we consider the token swapping framework and extend a 4-approximation algorithm for general graphs to support partial permutations. We apply these routing procedures to give several circuit transformations, using various heuristic qubit placement subroutines. We implement these transformations in software and compare their performance for large quantum circuits on grid and modular architectures, identifying strategies that work well in practice.

VL - 135 UR - https://arxiv.org/abs/1902.09102 CP - 3 U5 - https://doi.org/10.4230/LIPIcs.TQC.2019.3 ER - TY - JOUR T1 - Electro-mechano-optical NMR detection JF - Optica Y1 - 2018 A1 - Kazuyuki Takeda A1 - Kentaro Nagasaka A1 - Atsushi Noguchi A1 - Rekishu Yamazaki A1 - Yasunobu Nakamura A1 - Eiji Iwase A1 - Jacob M. Taylor A1 - Koji Usami AB -Signal reception of nuclear magnetic resonance (NMR) usually relies on electrical amplification of the electromotive force caused by nuclear induction. Here, we report up-conversion of a radio-frequency NMR signal to an optical regime using a high-stress silicon nitride membrane that interfaces the electrical detection circuit and an optical cavity through the electro-mechanical and the opto-mechanical couplings. This enables optical NMR detection without sacrificing the versatility of the traditional nuclear induction approach. While the signal-to-noise ratio is currently limited by the Brownian motion of the membrane as well as additional technical noise, we find it can exceed that of the conventional electrical schemes by increasing the electro-mechanical coupling strength. The electro-mechano-optical NMR detection presented here can even be combined with the laser cooling technique applied to nuclear spins.

VL - 5 U4 - 152-158 UR - https://www.osapublishing.org/optica/abstract.cfm?uri=optica-5-2-152 CP - 2 U5 - 10.1364/OPTICA.5.000152 ER - TY - JOUR T1 - The Power of Quantum Fourier Sampling Y1 - 2015 A1 - Bill Fefferman A1 - Chris Umans AB - A line of work initiated by Terhal and DiVincenzo and Bremner, Jozsa, and Shepherd, shows that quantum computers can efficiently sample from probability distributions that cannot be exactly sampled efficiently on a classical computer, unless the PH collapses. Aaronson and Arkhipov take this further by considering a distribution that can be sampled efficiently by linear optical quantum computation, that under two feasible conjectures, cannot even be approximately sampled classically within bounded total variation distance, unless the PH collapses. In this work we use Quantum Fourier Sampling to construct a class of distributions that can be sampled by a quantum computer. We then argue that these distributions cannot be approximately sampled classically, unless the PH collapses, under variants of the Aaronson and Arkhipov conjectures. In particular, we show a general class of quantumly sampleable distributions each of which is based on an "Efficiently Specifiable" polynomial, for which a classical approximate sampler implies an average-case approximation. This class of polynomials contains the Permanent but also includes, for example, the Hamiltonian Cycle polynomial, and many other familiar #P-hard polynomials. Although our construction, unlike that proposed by Aaronson and Arkhipov, likely requires a universal quantum computer, we are able to use this additional power to weaken the conjectures needed to prove approximate sampling hardness results. UR - http://arxiv.org/abs/1507.05592v1 ER - TY - JOUR T1 - Optical detection of radio waves through a nanomechanical transducer JF - Nature Y1 - 2014 A1 - T. Bagci A1 - A. Simonsen A1 - S. Schmid A1 - L. G. Villanueva A1 - E. Zeuthen A1 - J. Appel A1 - J. M. Taylor A1 - A. Sørensen A1 - K. Usami A1 - A. Schliesser A1 - E. S. Polzik AB - Low-loss transmission and sensitive recovery of weak radio-frequency (rf) and microwave signals is an ubiquitous technological challenge, crucial in fields as diverse as radio astronomy, medical imaging, navigation and communication, including those of quantum states. Efficient upconversion of rf-signals to an optical carrier would allow transmitting them via optical fibers dramatically reducing losses, and give access to the mature toolbox of quantum optical techniques, routinely enabling quantum-limited signal detection. Research in the field of cavity optomechanics has shown that nanomechanical oscillators can couple very strongly to either microwave or optical fields. An oscillator accommodating both functionalities would bear great promise as the intermediate platform in a radio-to-optical transduction cascade. Here, we demonstrate such an opto-electro-mechanical transducer utilizing a high-Q nanomembrane. A moderate voltage bias (<10V) is sufficient to induce strong coupling between the voltage fluctuations in a rf resonance circuit and the membrane's displacement, which is simultaneously coupled to light reflected off its metallized surface. The circuit acts as an antenna; the voltage signals it induces are detected as an optical phase shift with quantum-limited sensitivity. The half-wave voltage is in the microvolt range, orders of magnitude below that of standard optical modulators. The noise added by the membrane is suppressed by the electro-mechanical cooperativity C~6800 and has a temperature of 40mK, far below 300K where the entire device is operated. This corresponds to a sensitivity limit as low as 5 pV/Hz^1/2, or -210dBm/Hz in a narrow band around 1 MHz. Our work introduces an entirely new approach to all-optical, ultralow-noise detection of classical electronic signals, and sets the stage for coherent upconversion of low-frequency quantum signals to the optical domain. VL - 507 U4 - 81 - 85 UR - http://arxiv.org/abs/1307.3467v2 CP - 7490 J1 - Nature U5 - 10.1038/nature13029 ER - TY - JOUR T1 - On Beating the Hybrid Argument JF - Proceedings, ITCS Y1 - 2012 A1 - Bill Fefferman A1 - Ronen Shaltiel A1 - Christopher Umans A1 - Emanuele Viola AB - The hybrid argument allows one to relate the distinguishability of a distribution (from uniform) to the predictability of individual bits given a prefix. The argument incurs a loss of a factor k equal to the bit-length of the distributions: -distinguishability implies only /k-predictability. This paper studies the consequences of avoiding this loss – what we call “beating the hybrid argument” – and develops new proof techniques that circumvent the loss in certain natural settings. Specifically, we obtain the following results: 1. We give an instantiation of the Nisan-Wigderson generator (JCSS ’94) that can be broken by quantum computers, and that is o(1)-unpredictable against AC0 . This is not enough to imply indistinguishability via the hybrid argument because of the hybrid-argument loss; nevertheless, we conjecture that this generator indeed fools AC0 , and we prove this statement for a simplified version of the problem. Our conjecture implies the existence of an oracle relative to which BQP is not in the PH, a longstanding open problem. 2. We show that the “INW” generator by Impagliazzo, Nisan, and Wigderson (STOC ’94) with seed length O(log n log log n) produces a distribution that is 1/ log n-unpredictable against poly-logarithmic width (general) read-once oblivious branching programs. Thus avoiding the hybrid-argument loss would lead to a breakthrough in generators against small space. 3. We study pseudorandom generators obtained from a hard function by repeated sampling. We identify a property of functions, “resamplability,” that allows us to beat the hybrid argument, leading to new pseudorandom generators for AC0 [p] and similar classes. Although the generators have sub-linear stretch, they represent the best-known generators for these classes. Thus we establish that “beating” or bypassing the hybrid argument would have two significant consequences in complexity, and we take steps toward that goal by developing techniques that indeed beat the hybrid argument in related (but simpler) settings, leading to best-known PRGs for certain complexity classes. VL - 9 U4 - 809-843 UR - http://users.cms.caltech.edu/~umans/papers/FSUV10.pdf ER - TY - JOUR T1 - Pseudorandom generators and the BQP vs. PH problem Y1 - 2010 A1 - Bill Fefferman A1 - Christopher Umans AB - It is a longstanding open problem to devise an oracle relative to which BQP does not lie in the Polynomial-Time Hierarchy (PH). We advance a natural conjecture about the capacity of the Nisan-Wigderson pseudorandom generator [NW94] to fool AC_0, with MAJORITY as its hard function. Our conjecture is essentially that the loss due to the hybrid argument (which is a component of the standard proof from [NW94]) can be avoided in this setting. This is a question that has been asked previously in the pseudorandomness literature [BSW03]. We then make three main contributions: (1) We show that our conjecture implies the existence of an oracle relative to which BQP is not in the PH. This entails giving an explicit construction of unitary matrices, realizable by small quantum circuits, whose row-supports are "nearly-disjoint." (2) We give a simple framework (generalizing the setting of Aaronson [A10]) in which any efficiently quantumly computable unitary gives rise to a distribution that can be distinguished from the uniform distribution by an efficient quantum algorithm. When applied to the unitaries we construct, this framework yields a problem that can be solved quantumly, and which forms the basis for the desired oracle. (3) We prove that Aaronson's "GLN conjecture" [A10] implies our conjecture; our conjecture is thus formally easier to prove. The GLN conjecture was recently proved false for depth greater than 2 [A10a], but it remains open for depth 2. If true, the depth-2 version of either conjecture would imply an oracle relative to which BQP is not in AM, which is itself an outstanding open problem. Taken together, our results have the following interesting interpretation: they give an instantiation of the Nisan-Wigderson generator that can be broken by quantum computers, but not by the relevant modes of classical computation, if our conjecture is true. UR - http://arxiv.org/abs/1007.0305v3 ER -