# Institut Transdisciplinaire d'Information Quantique (INTRIQ)

### Meeting de INTRIQ (Septembre 2012)

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**Meeting de l'INTRIQ (10 - 11 septembre 2012)**

Location

**Programme**

**Monday, September 10th**

8h30 - Registration (Salle Jean-Paul Lemieux)

9h20 - 9h30 Opening remarks

9h30 - 10h00 Alexandre Blais (UdeS) *Qubit frequency modulations in circuit QED: from gates to noise probe*

10h00 - 10h30 Lilian Childress (McGill) *The nitrogen-vacancy center: controlling quantum registers in diamond*

11h00 - 11h30 Michael Hilke (McGill)

*Double Quantum Dots as Qubits and Nuclear Spin Effects*

11h30 - 12h00 Mark Wilde (McGill)

*The information-theoretic costs of simulating quantum measurements*

12h00 - 13h45 Lunch (Dining room)

15h15 - 15h45 Coffee break (Salle Suzor-Côté)

15h45 - 16h45 Rump session: presentation of the posters

20h00 Student meting (Jean-Paul Lemieux )

**Tuesday, September 11th**

7h00 - 8h30 Breakfast

8h30 - 9h00 Check out

9h00 - 9h30 Guillaume Gervais (McGill) *The information-theoretic costs of simulating quantum measurements*

9h30 - 10h00 Olivier Landon-Cardinal (UdeS) *Adversarial noise defeats 2D self-correcting topological memories*

10h00 - 10h30 Chen-Fu Chiang (UdeS) *Quantum Phase Estimation with an Arbitrary Number of Qubits*

10h30 - 11h00 Coffee break (Salle Suzor-Côté) and check-out

11h00 - 12h00 Propositions de projets INTRIQ à venir

*Simulating many-body systems locally: Markov Entropy Decomposition*

*Property testing in a quantum world*

*From electronic quantum noise to quantum optics*

Note : wiseman meeting from 16h to17h

Note : wiseman meeting from 16h to17h

**Speakers**

*Alexandre Blais*

Département de Physique

Université de Sherbrooke

**Qubit frequency modulations in circuit QED: from gates to noise probe**

Coupling of superconducting qubits to quantized microwave fields stored in electrical circuits has opened new possibilities for quantum optics and quantum information processing in solid-state devices. With the steady improvements of the coherence time of superconducting qubits, and with the large qubit-field coupling that can be achieved, these on-chip realizations of cavity QED, also known as circuit QED, can reach new parameter regimes currently unexplored in traditional quantum optics. After a short introduction to circuit QED, I will propose a new approach, based on modulations of the qubit frequency, to realize two-qubit gates in circuit QED. I will also argue that the very same mechanism, when induced by environmental noise, can also lead to unwanted transitions in the qubit state and that this can be used as a new probe of dephasing noise at high frequencies. First experimental confirmations of these ideas will be presented.

*Lilian Childress*

Département de Physique

Université McGill

**The nitrogen-vacancy center: controlling quantum registers in diamond**

The electronic and nuclear spins associated with the nitrogen-vacancy (NV) center in diamond constitute an exceptional solid state system for investigating quantum phenomena, combining long spin coherence times and fast manipulation with a robust optical interface. Such few-spin quantum registers have been envisioned as building blocks for quantum repeaters, cluster state computation, and distributed quantum computing. By adapting quantum optical techniques pioneered in atomic physics to a solid-state system with microfabricated collection optics, we demonstrate two key steps towards controlling and connecting spin-based quantum registers. We show that spin-selective optical transitions enable high fidelity preparation and projective measurement of both electronic and nuclear spin degrees of freedom. Moreover, we observe quantum interference of photons emitted by different NV centers, opening the door to quantum optical channels between NV-based registers.

*Marc Wilde*

Département de Physique

Université McGill

**The information-theoretic costs of simulating quantum measurements**

Winter's measurement compression theorem stands as one of the most penetrating insights of quantum information theory. In addition to making an original and profound statement about measurement in quantum theory, it also underlies several other general protocols used for entanglement distillation or local purity distillation. The theorem provides for an asymptotic decomposition of any quantum measurement into an "extrinsic" source of noise (classical noise in the measurement that does not convey information) and "intrinsic" quantum noise that can be due in part to the nonorthogonality of quantum states. This decomposition leads to an optimal protocol for having a sender simulate many independent instances of a quantum measurement and send the measurement outcomes to a receiver, using as little communication as possible. The protocol assumes that the parties have access to some amount of common randomness, which is a strictly weaker resource than classical communication.

In this paper, we provide a full review of Winter's measurement compression theorem. We then prove an extension of this theorem to the case in which the sender is not required to receive the outcomes of the simulated measurement. The total cost of common randomness and classical communication can be lower for such a "non-feedback" simulation, and we prove a single-letter converse theorem demonstrating optimality. We then review the Devetak-Winter theorem on classical data compression with quantum side information, providing new proofs of its achievability and converse parts. From there, we outline a new protocol that we call "measurement compression with quantum side information." Finally, we prove a single-letter theorem characterizing measurement compression with quantum side information when the sender is not required to obtain the measurement outcome. (This is joint work with Patrick Hayden, Francesco Buscemi, and Min-Hsiu Hsieh.)

*Olivier Landon-Cardinal*

Département de Physique

Université de Sherbrooke

**Adversarial noise defeats 2D self-correcting topological memories**

We put severe constraints on the existence of a self-correcting quantum memory made of a two-dimensional (2D) array of particles. Such a memory would passively protect the encoded information thanks to its dynamics at low temperature. To be robust to perturbation, candidates for such devices encode information in topological degrees of freedom, which are impervious to local errors on a short timescale. However, we show that, for any topologically ordered 2D quantum memory, thermal excitations can accumulate and corrupt the encoded information. We thus prove a no-go theorem, extending the known results to non-stabilizer codes.

*Chen-Fu Chiang*

Département de Physique

Université de Sherbrooke

**Quantum Phase Estimation with an Arbitrary Number of Qubits**

Due to the great difficulty in scalability, quantum computers are limited in the number of qubits during the early stages of the quantum computing regime. In addition to the required qubits for storing the corresponding eigenvector, suppose we have additional k qubits available. Given such a constraint k, we propose an approach for the phase estimation for an eigenphase of exactly n-bit precision. This approach adopts the standard recursive circuit for quantum Fourier transform (QFT) and adopts classical bits to implement such a task. Our algorithm has the complexity of O(n log k), instead of O(n^2) in the conventional QFT, in terms of the total invocation of rotation gates. We also compare with and analyze Kitaev's original approach (Hadamard test) for phase estimation.

*Philippe Lamontage*

Département d'informatique

Université de Montréal

**Property testing in a quantum world**Property testing is the problem of deciding wether a black-box function roughly has a certain property or is far from having this property with as few queries as possible. A function is linear if f(x)+f(y)=f(x+y) and symmetric if it is invariant under permutation of its input bits.

Using standard techniques and some novel work, we improve and generalize the previously best known algorithms for testing both properties of linearity and symmetry, improving the query complexity from O(ε^(-2/3)) to O(ε^(-1/2)) for distance parameter 0<ε<1. This presentation (poster) will introduce the concepts underlying our new linearity testing algorithm.

*Bertrand Reulet*

Département de Physique

Université de Sherbrooke

Noise in condensed matter systems deals with current and voltage fluctuations caused by random motion of electrons. The quantum nature of these fluctuations appears when the frequency at which the fluctuations are measured is greater than temperature and bias voltage. In this regime, the electronic noise can be understood in terms of absorption or emission of photons by the sample. What are the statistical properties of the emitted photons ? An oscillating classical current radiates a coherent state of light. What are the quantum properties of the light emitted by a quantum conductor ?

We will address such questions and show some experimental approaches and results.